Service TrainingMeeting Guide 682 SESV1682

March 1997

TECHNICAL PRESENTATION

793C OFF-HIGHWAY TRUCK

793C OFF-HIGHWAY TRUCKMEETING GUIDE 682 SLIDES AND SCRIPT

AUDIENCE

Level II - Service personnel who understand the principles of machine systems operation, diagnosticequipment, and procedures for testing and adjusting.

CONTENT

This presentation provides basic maintenance information and describes the systems operation of theengine, power train, steering, hoist and the air system and brakes for the 793C Off-highway Truck. TheAutomatic Retarder Control (ARC) and the Traction Control System (TCS) are also discussed.

OBJECTIVES

After learning the information in this meeting guide, the serviceman will be able to:1. locate and identify the major components in the engine, power train, steering, hoist and the air

system and brakes;

2. explain the operation of the major components in the systems; and

3. trace the flow of oil or air through the systems.

REFERENCES

793C Off-highway Truck Service Manual SENR1440793C Off-highway Truck Parts Book SEBP2503Vital Information Management System (VIMS) Service Manual SENR6059Fluid Power Graphic Symbols User's Guide SENR3981

PREREQUISITES

Interactive Video Course "Fundamentals of Mobile Hydraulics" TEVR9001Interactive Video Course "Fundamentals of Electrical Systems" TEVR9002STMG 546 "Graphic Fluid Power Symbols" SESV1546

Estimated Time: 8 HoursVisuals: 184 (2 X 2) SlidesServiceman Handouts: 4 Data SheetsForm: SESV1682

© 1997 Caterpillar Inc. Date: 3/97

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SUPPLEMENTAL MATERIAL

Specification Sheets

793B Off-highway Truck AEHQ5061793C Off-highway Truck AEHQ5186

Salesgrams

Vital Information Management System (VIMS) TELQ4478793B Rear Axle Improvements TELQ3736785B/789B/793B Introduction TELQ3725789B/793B Feature Status TELQ4477

Video Tapes

793C Off-highway Truck--Service Introduction SEVN4016793C Marketing Introduction AEVN37423500 EUI Service Introduction SEVN2241Intelligence of Powerful Connections AEVN2974Suspension Cylinder Charging TEVN2155TPMS Management/Technical Information AEVN2211TPMS Operating Tips AEVN2212Automatic Electronic Traction Aid (AETA) Introduction SEVN9187

Service Training Meeting Guides

STMG 625 "793 Off-highway Truck" SESV1625STMG 660 "785B/789B/793B Off-highway Trucks--Maintenance" SESV1660STMG 681 "3500B Engine Controls--Electronic Unit Injection (EUI)" SESV1681

Technical Instruction Modules

Vital Information Management System--785B/789B/793B Off-highway Trucks SEGV2610Vital Information Management System--Introduction SEGV25973500 Electronic Engine Controls--Introduction SEGV25883500 Electronic Engine Controls--Off-highway Trucks SEGV2589Electronic Programmable Transmission Control (EPTC II) SEGV2584769C - 793B Off-highway Trucks--Torque Converterand Transmission Hydraulic Systems SEGV2591785B/789B/793B Off-highway Trucks--Steering System SEGV2587769C - 793B Off-highway Trucks--Hoist System SEGV2594769C - 793B Off-highway Trucks--Air System and Brakes SEGV2595Automatic Retarder Control System SEGV2593Automatic Electronic Traction Aid SEGV2585769C - 793B Off-highway Trucks--Suspension System SEGV2599

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SUPPLEMENTAL MATERIAL (Continued)

Booklets

Know Your Cooling System SEBD0518Diesel Fuels and Your Engine SEBD0717Oil and Your Engine SEBD0640

Special Instructions

Using the ECAP NEXG4521 Machine Functions Service Program Module SEHS9343Using the 8T8697 Electronic Control Analyzer Programmer (ECAP) SEHS8742Using the 7X1700 Communication Adapter Group SEHS9264Use of 6V3000 Sure-Seal Repair Kit SMHS7531Use of CE Connector Tools SEHS9065Servicing DT Connectors SEHS9615Use of 8T5200 Signal Generator/Counter Group SEHS8579Repair of 4T8719 Bladder Accumulator Group SEHS8757Suspension Cylinder Servicing SEHS9411Using 1U5000 Auxiliary Power Unit (APU) SEHS8715Using the 1U5525 Attachment Group SEHS8880

Brochures

Caterpillar Vital Information Management System (VIMS) AEDK2946Caterpillar Electronic Technician NEHP5614Caterpillar DataView NEHP5622Diesel Engine Oil (CG4) Product Data Sheet PEHP5026How to Take a Good Oil Sample PEHP6001Air Filter Service Indicator PEHP9013Intelligence of Powerful Connections AEDK2966Caterpillar Fully Automatic Transmission AEDQ0066Caterpillar Oil-cooled Multiple Disc Brakes AEDK2546Caterpillar Automatic Retarder Control AEDK0075

Miscellaneous

Electronic Diagnostic Code Pocket Card NEEG2500Pressure Conversion Chart SEES5677793B Transmission Assembly Wall Chart SENR6834793B Final Drive Assembly Wall Chart SENR8602Improved Transmission/Drive Train Oil (IRM) PELE0179

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TABLE OF CONTENTS

INTRODUCTION ........................................................................................................................7

WALK AROUND INSPECTION...............................................................................................11

OPERATOR'S STATION............................................................................................................36

ENGINE......................................................................................................................................49Cooling System.....................................................................................................................66Lubrication System ...............................................................................................................77Fuel System...........................................................................................................................80Air Induction and Exhaust System .......................................................................................85

POWER TRAIN .........................................................................................................................90Power Train Components......................................................................................................91Power Train Hydraulic System...........................................................................................103Electronic Programmable Transmission Control (EPTC II)...............................................120

STEERING SYSTEM ..............................................................................................................128

HOIST SYSTEM......................................................................................................................157

AIR SYSTEM AND BRAKES ................................................................................................177Operator Controls................................................................................................................179Air Charging System...........................................................................................................182Parking and Secondary Brake System ................................................................................188Service and Retarder Brake System....................................................................................195

AUTOMATIC RETARDER CONTROL (ARC) .....................................................................206

TRACTION CONTROL SYSTEM (TCS)...............................................................................211

CONCLUSION.........................................................................................................................221

SLIDE LIST..............................................................................................................................222

SERVICEMAN'S HANDOUTS...............................................................................................224

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INSTRUCTOR NOTES

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• 3516B DITA engine

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793C OFF-HIGHWAY TRUCK

C 1997 Caterpillar Inc.

INTRODUCTION

This presentation provides an introduction to the Caterpillar 793C Off-highway Truck. Included in this package is a walk aroundinspection which provides information about daily service requirementsand identifies the locations of the major components. The major systemsof the truck will also be discussed. The major systems include the engine,power train, steering, hoist, and the air system and brakes.

The 793C is the largest rigid frame truck produced by Caterpillar. The793C is equipped with a Caterpillar 3516B engine rated at 1716 kW(2300 gross hp) and 1616 kW (2166 flywheel hp). The load carryingcapacity is 218 Metric tons (240 tons) at a Gross Machine Weight (GMW)of 376488 kg (830000 lbs.).

This slide shows a view of the left side of the truck. Notice that the bodycanopy is extended over the cab to protect the front of the truck fromfalling objects.

The fuel tank is located on the left side of the truck.• Fuel tank

• Extended bodycanopy

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Shown is the right side of the truck. The large air tank on the rightplatform supplies air for starting the truck and for the service and retarderbrake system.

The main hydraulic tank is also visible. The hydraulic tank supplies oilfor the hoist system and the brake system.

On the 793B truck, torque converter oil is also supplied from the mainhydraulic tank. A transmission oil supply tank is located in front of themain hydraulic tank.

The 793C now uses the torque converter case as the supply tank for thetorque converter and the transmission.

• Main system air tank:

- Air starting

- Service/retarderbrakes

• Main hydraulic tank:

- Hoist system

- Brake system

• Torque converter caseused as sump forconverter andtransmission

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• 789B and 793C aresimilar

• 793C has four airfilters

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The 793C is similar in appearance to the 789B and may be difficult torecognize from a distance. The 793C can be recognized by the four airfilters and the diagonal access ladder. The 789B has only two air filtersmounted in the same locations and is equipped with two vertical ladders.

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• Truck body

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The truck body has a dual-slope main floor and a "vee" bottom to centerthe load and reduce spills. The steel used to construct the body has ayield strength of 6205 bar (90000 psi).

The rear suspension cylinders absorb bending and twisting stresses ratherthan transmitting them to the main frame.

• Rear suspensioncylinders

5

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• Read the Operationand MaintenanceManual

793C MAINTENANCE

793C Service

Procedure

WALK AROUND INSPECTION

WALK AROUND INSPECTION

Before working on or operating the truck, read the Operation andMaintenance Manual (Form SEBU6995) thoroughly for information onsafety, maintenance and operating techniques.

Safety precautions and Warnings are provided in the manual and on thetruck. Be sure to identify and understand all symbols before starting thetruck.

The first step to perform when approaching the truck is to make athorough walk around inspection. Look around and under the truck forloose or missing bolts, trash build-up and for coolant, fuel or oil leaks.Look for indications of cracks. Pay close attention to high stress areas asshown in the Operation and Maintenance Manual.

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• Front wheel bearinginspection plug(arrow)

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The front wheel bearing oil level is checked and filled by removing theplug (arrow) in the center of the wheel bearing cover. The oil should belevel with the bottom of the plug hole.

Check the tire inflation pressure. Operating the truck with the wrong tireinflation pressure can cause heat build-up in the tire and accelerate tirewear.

NOTE: Care must be taken to ensure that fluids are contained whileperforming any inspection, maintenance, testing, adjusting andrepair of the machine. Be prepared to collect the fluid in suitablecontainers before opening any compartment or disassembling anycomponent containing fluids. Refer to the "Tools and Shop ProductsGuide" (Form NENG2500) for tools and supplies suitable to collectand contain fluids in Caterpillar machines. Dispose of fluidsaccording to local regulations and mandates.

• Tire inflation

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1. Front wheel bearingaxle housingbreather

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1

2

Inspect the condition of the front wheel bearing axle housing breather (1).The breather prevents pressure from building up in the axle housing.Pressure in the axle housing may cause brake cooling oil to leak throughthe Duo-Cone seals in the wheel brake assemblies.

Two grease outlet fittings (2) are located on the front of each suspensioncylinder. The grease supply line for the Auto Lubrication System islocated at the rear of the suspension cylinder. No grease outlet fittingsshould be located on the same side of the suspension cylinder as thegrease fill location. Having an outlet fitting on the same side of thesuspension cylinder as the grease fill location will prevent properlubrication of the cylinder.

Make sure that grease is flowing from the outlet fittings to verify that thesuspension cylinders are being lubricated and that the pressure in thecylinders is not excessive.

2. Suspension cylindergrease outlet fittings

• Make sure greaseflows from outletfittings

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1. Rear brake oilcoolers

2. Parking brakerelease filter

3. Torque convertercharging filter

4. Automaticlubrication injectorbank

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4

2

1

3

Located behind the right front tire are the rear brake oil coolers (1), theparking brake release filter (2), and the torque converter charging filter (3).

One of the three injector banks (4) for the automatic lubrication system isalso in this location. These injectors are adjustable and regulate thequantity of grease that is injected during each cycle (approximately onceper hour).

A solenoid air valve provides a controlled air supply for the automaticlubrication system. The solenoid air valve is controlled by the VitalInformation Management System (VIMS), which energizes the solenoidten minutes after the machine is started. The VIMS energizes thesolenoid for 75 seconds before it is de-energized. Every 60 minutesthereafter, the VIMS energizes the solenoid for 75 seconds until themachine is stopped (shut down). These settings are adjustable through theVIMS keypad in the cab.

INSTRUCTOR NOTE: For more detailed information on servicingthe automatic lubrication system, refer to the Service Manual Module"Automatic Lubrication System" (Form SENR4724).

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• Hoist and brakehydraulic tank

• Oil level sight gauges(arrows)

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Shown is the hoist and brake hydraulic tank and the oil level sight gauges(arrows). The oil level is normally checked with the upper sight gauge.The oil level should first be checked with cold oil and the engine stopped.The level should again be checked with warm oil and the engine running.

The lower sight gauge can be used to fill the hydraulic tank when thehoist cylinders are in the RAISED position. When the hoist cylinders arelowered, the hydraulic oil level will increase. After the hoist cylinders arelowered, check the hydraulic tank oil level with the upper sight gauge asexplained above.

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• Final drives

• Check magnetic plugs(arrow) for metal

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The rear axles are equipped with double reduction planetary final drives.The magnetic plug (arrow) should be removed from the final drives atregular intervals and checked for metal particles. For some conditions,checking the magnetic plug is the only way to identify a problem whichmay exist.

The rear axle is a common sump for the differential and both final drives.If a final drive or the differential fails, the other final drive componentsmust also be checked for contamination and then flushed. Failure tocompletely flush the rear axle after a failure can cause a repeat failurewithin a short time.

NOTICE

The rear axle is a common sump for the differential and both finaldrives. If a final drive or the differential fails, the other final drivecomponents must also be checked for contamination and thenflushed. Failure to completely flush the rear axle after a failure cancause a repeat failure within a short time.

• Flush all axlecomponents after afailure

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1. Differential oil levelsight glass

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22

4

1

5

3

The differential oil level is checked by viewing the oil level sight glass (1). The oil should be level with the bottom of the inspection hole.

Two oil level sensors (2) provide input signals to the VIMS whichinforms the operator of the rear axle oil level.

A rear axle oil filter (3) is used to remove contaminants from the rear axlehousing.

Check the charge condition of the rear suspension cylinders when thetruck is empty and on level ground.

The second of three injector banks (4) for the automatic lubricationsystem is mounted on the top rear of the differential housing.

Above the lubrication injectors is a breather (5) for the rear axle. Inspectthe condition of the breather at regular intervals. The breather preventspressure from building up in the axle housing. Excessive pressure in theaxle housing can cause brake cooling oil to leak through the Duo-Coneseals in the wheel brake assemblies.

INSTRUCTOR NOTE: For more detailed information on servicingthe suspension system, refer to the Special Instruction "SuspensionCylinder Servicing" (Form SEHS9411).

2. Rear axle oil levelsensors

3. Rear axle housing oilfilter

• Rear suspensioncylinders

4. Automaticlubrication injectorbank

5. Rear axle breather

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• Cable holds body up

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The cable that holds the body up is stored below the rear of the body.Whenever work is to be performed while the body is raised, the safetycable must be connected between the body and the rear hitch to hold thebody in the raised position.

The space between the body and the frame becomes a zero clearancearea when the body is lowered. Failure to install the cable can resultin injury or death to personnel working in this area.

WARNING

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• Fuel tank

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The fuel tank is located on the left side of the truck. The fuel level sightgauge (arrow) is used to check the fuel level during the walk aroundinspection.

• Fuel level sight gauge(arrow)

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1. Primary fuel filter

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2

3

1

The primary fuel filter (1) is located on the inner surface of the fuel tank.

Open the drain valve (2) to remove condensation from the fuel tank.

A fuel level sensor (3) is also located on the fuel tank. The fuel levelsensor provides input signals to the VIMS which informs the operator ofthe fuel level.

3. Fuel level sensor

2. Condensation drainvalve

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1

2

Supply oil for the torque converter and the transmission is contained inthe torque converter case. Sight gauges (1) are used to check the oil levelfor the torque converter and the transmission.

Torque converter and transmission oil is added at the fill tube (2).

When filling the torque converter and transmission oil sump after an oilchange, fill the sump with oil to the top of the upper sight gauge. Turn offthe engine manual shutdown switch (see slide No. 23) so the engine willnot start. Crank the engine for approximately 15 seconds. The oil levelwill decrease as oil fills the torque converter and transmission system.Add more oil to the sump to raise the oil level to the FULL COLD mark.Crank the engine for an additional 15 seconds. Repeat this step asrequired until the oil level stabilizes.

Turn off the engine manual shutdown switch and start the engine. Warmthe torque converter and transmission oil. Add more oil to the sump asrequired to raise the torque converter and transmission oil level to theFULL WARM mark.

NOTICE

Failure to correctly fill the torque converter and transmission oilsump after an oil change may cause transmission clutch damage.

1. Torque converterand transmission oillevel sight gauges

2. Torque converterand transmission oilfill tube

• Torque converter andtransmission oil fillprocedure

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1

4

2

3

Shown is the location of the torque converter outlet screen (1). Oil flowsfrom the torque converter outlet relief valve through the torque converteroutlet screen to the torque converter and transmission oil cooler locatedon the right side of the engine. Oil from the torque converter andtransmission oil cooler returns to the torque converter housing.

Shown is the location of the transmission charging filter (2).Transmission charging oil flows through the transmission charging filterto the transmission control valves on top of the transmission and to thetorque converter lockup clutch valve located on top of the torqueconverter.

The scavenge screen for torque converter and transmission oil is locatedbehind the cover (3).

Torque converter and transmission oil samples can be taken at theScheduled Oil Sampling (S•O•S) tap (4).

1. Torque converteroutlet screen

2. Transmissioncharging filter

3. TC/Transmissionscavenge screen

4. TC/TransmissionS•O•S tap

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• Brake cylinderbreather (arrow)

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Inspect the condition of the two breathers (arrow, one visible) for thebrake cylinders. The second breather is located behind the cross tube.Oil should not leak from the breathers. Oil leaking from the breathers isan indication that the oil piston seals in the brake cylinder needreplacement. Air flow from the breathers during a brake application is anindication that the brake cylinder air piston seals need replacement.

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• Front brake oil coolerfilters (arrow)

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Located in front of the fuel tank are the front brake oil cooler filters(arrow). Oil not used to raise or lower the hoist cylinders flows from thehoist valve through the front brake oil filters to the front brake oil coolerlocated above the torque converter.

The third injector bank for the automatic lubrication system is alsolocated in this area.

• Automatic lubricationinjector bank

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• Front suspensioncylinder

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1

2

Check the charge condition of the front suspension cylinders when thetruck is empty and on level ground.

The air dryer (1) is located in front of the left front suspension cylinder.

The air system can be charged from a remote air supply through a groundlevel connector (2) inside the left frame.

INSTRUCTOR NOTE: For more detailed information on servicingthe suspension system, refer to the Special Instruction "SuspensionCylinder Servicing" (Form SEHS9411).

1. Air dryer

2. Remote air supplyconnector

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• Engine oil filters

1. Engine oil fill tube

2. Engine oil dipstick

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2

1

3

4

The engine oil filters are located on the left side of the engine.

Engine oil should be added at the fill tube (1) and checked with thedipstick (2).

Engine oil samples can be taken at the Scheduled Oil Sampling (S•O•S) tap (3).

The engine lubrication system is equipped with two oil pressure sensors (4). A sensor is located on each end of the oil filter base. Onesensor measures engine oil pressure before the filters. The other sensormeasures oil pressure after the filters. The sensors provide input signalsto the second generation Advanced Diesel Engine Management (ADEM II) engine Electronic Control Module (ECM). The ECMprovides input signals to the VIMS which informs the operator of theengine oil pressure. Together, these sensors inform the operator if theengine oil filters are restricted.

4. Engine oil pressuresensor

3. Engine oil S•O•S tap

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1. High speed oilchange connector

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1 3

2

Engine oil can be added through the high speed oil change connector (1)located in the left front corner of the oil pan.

Two engine oil level switches (2 and 3) provide input signals to theengine ECM. The engine ECM provides an input signal to the VIMS,which informs the operator of the engine oil level.

If the truck is equipped with the engine oil renewal system attachment,the upper oil level switch (2) tells the operator when engine oil must beadded. The ADD ENG OIL message is a Category 1 Warning.

The lower oil level switch (3) tells the operator when the engine oil levelis low and it is unsafe to operate the truck without causing damage to theengine. The ENG OIL LEVEL LOW message is a Category 2 or 3Warning.

2. Add engine oil levelswitch

3. Engine oil level lowswitch

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• Secondary fuel filters

• Fuel priming pump(arrow)

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The secondary fuel filters and the fuel priming pump (arrow) are locatedabove the engine oil filters on the left side of the engine.

NOTE: If the fuel system requires priming, it may be necessary toblock the fuel return line during priming to force the fuel into theinjectors.

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1. Manual engineshutdown switch

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3 5

6

1

2

4

Before climbing the truck ladder, make sure that the manual engineshutdown switch (1) is OFF. The engine will not start if the manualshutdown switch is ON. If necessary, the switch can be used to stop theengine from the ground level.

The toggle switches (2) control the lights in the engine compartment andabove the access ladder.

The RS-232 service connector (3) is used to connect a laptop computerwith VIMS PC software to upload new source and configuration files,view real time data or download logged information from the VIMS.

The battery disconnect switch (4) and VIMS service connector key switch(5) must be in the ON position before the laptop computer with VIMSsoftware will communicate with the VIMS.

The blue service lamp (6) is part of the VIMS. The service lamp will turnon to notify service personnel that the VIMS has an active machine orsystem event. The service lamp flashes to indicate when an event isconsidered abusive to the machine.

2. Engine and accessladder light switches

3. RS-232 connector forVIMS

4. Battery disconnectswitch

5. Key switch for VIMSservice connector

6. VIMS service lamp

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• Inspect radiator

• Check air cleanerindicators (arrow)

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While climbing the ladder, make a thorough inspection of the radiator. Besure that no debris or dirt is trapped in the cores. Check the air cleanerindicators (arrow) located on both sides of the truck. If the yellow pistonsare in the red zone (indicating that the filters are plugged), the air cleanersmust be serviced.

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• Engine coolingsystems:

- Jacket water coolingsystem

- Aftercooler coolingsystem

1. Engine coolantshunt tank

2. Coolant level gauges

3. Coolant level sensor

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3

2

1

The cooling system on the 793C is divided into two systems. The twosystems are the jacket water cooling system and the aftercooler coolingsystem. These two systems are not connected. When servicing thecooling systems, be sure to drain and fill both systems separately.

The engine cooling system shunt tank (1) is located on the hood above theradiator. The coolant levels are checked at the shunt tank. Use the gauges (2) on top of the shunt tank to check the two coolant levels.

A coolant level sensor (3) is located on each side of the shunt tank tomonitor the coolant level of both cooling systems (guard removed forviewing sensor). The coolant level sensors provide input signals to theVIMS which informs the operator of the engine coolant levels.

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23

1

Located on the right platform are the automatic lubrication system greasetank (1), the main air system tank (2) and the steering system tank (3).

Check the level of the grease in the automatic lubrication system tankwith the grease level indicator located on top of the tank.

A drain valve is located at the bottom right of the main air system tank. Drain the condensation from the air tank each morning.

1. Automaticlubrication tank

2. Main air system tank

3. Steering system tank

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6

3

7

1

2

5

4

The oil level for the steering system tank is checked at the upper sight gauge (1) when the oil is cold and the engine is stopped. After the engineis started, the oil level will decrease as the oil fills the steeringaccumulators.

After the accumulators are filled, the oil level should be checked again atthe lower sight gauge (2). When the engine is running and theaccumulators are fully charged, the oil level should not be below theENGINE RUNNING marking of the lower gauge. If the ENGINERUNNING level is not correct, check the nitrogen charge in eachaccumulator. A low nitrogen charge will allow excess oil to be stored inthe accumulators and will reduce the secondary steering capacity.

Before removing the cap to add oil to the steering system, be sure that theengine was shut off with the key start switch, and the steering oil hasreturned to the tank from the accumulators. Then, depress the pressurerelease button (3) on the breather to release any remaining pressure fromthe tank.

Also located on the tank are the main steering oil filter (4) and thesteering pump case drain filter (5).

1. Upper sight gauge

2. Lower sight gauge

4. Main steering oilfilter

5. Steering pump casedrain filter

3. Steering tankpressure releasebutton

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If the steering pump fails or if the engine cannot be started, the connector (6) is used to attach an Auxiliary Power Unit (APU). The APUwill provide supply oil from the steering tank at the connector (6) tocharge the steering accumulators. Steering capability is then available totow the truck.

The steering oil temperature sensor (7) provides an input signal to theVIMS which informs the operator of the steering system oil temperature.

INSTRUCTOR NOTE: For more detailed information on servicingthe steering accumulators, refer to the Service Manual Module "793COff-highway Truck Steering System" (Form SENR1452) and theSpecial Instruction "Repair of 4T8719 Bladder Accumulator Group" (Form SEHS8757). For more information on using the APU, refer tothe Special Instructions "Using 1U5000 Auxiliary Power Unit (APU)"(Form SEHS8715) and "Using the 1U5525 Attachment Group"(Form SEHS8880).

6. APU supplementalsteering connector

7. Steering oiltemperature sensor

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1. Parking/secondarybrake air tank drainvalve (arrow)

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2

1

Another small air tank (not visible) is located behind the cab (see SlideNo. 156). The air tank behind the cab supplies air to the parking andsecondary brakes. Drain the moisture from the tank daily with the drainvalve (1).

Check the fluid level of the windshield washer reservoir (2).2. Windshield washer

fluid reservoir

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• Transmission shiftlever

- Six speedsFORWARD

- One speed REVERSE

• Back-up light switch(arrow)

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OPERATOR’S STATION

At the front of the center console is the transmission shift lever. The793C transmission has six speeds FORWARD and one speed REVERSE.

To the right of the shift lever is the back-up light switch (arrow).

INSTRUCTOR NOTE: In this section of the presentation,component locations inside the operator’s station will be shown.Many of the components shown in this section will be furtherexplained in the sections that follow.

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• Center consolecomponents:

1. Throttle back-upswitch

2. Manual ether startaid switch

3. Key start switch

4. TCS switch

5. Parking brakeswitch

6. Windshield washerand wiper switch

7. Cigarette lighter

8. Brake retractionswitch

- Service hourmeter

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5 6 7 8

1 2 3 4

Shown is the center console. In the center of the console are the throttleback-up switch (1), manual ether start aid switch (2), key start switch (3)and the Traction Control System (TCS) switch (4).

The throttle back-up switch (1) increases the engine speed to 1300 rpm ifthe engine ECM detects that the throttle sensor signal is invalid.

The manual ether start aid switch (2) allows the operator to manuallyinject ether when the coolant temperature is below 10°C (50°F) andengine speed is below 1200 rpm.

The Traction Control System (TCS) switch (4) is used to test theoperation of the TCS (formerly referred to as the "Automatic ElectronicTraction Aid").

Shown below these components are the parking brake switch (5), thewindshield washer and wiper switch (6), the cigarette lighter (7) and thebrake retraction switch (8).

The brake retraction switch (8) is used to release the parking brakes whentowing the truck.

The service hourmeter is located toward the rear of the center console.

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• Overhead lightswitches:

1. Headlights andparking/taillights

2. Panel lights

3. Interior cab light

4. Front flood/ladderlights

5. Fog lights

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2 34 51

Located above the operator's head are several light switches:

1. Headlights and parking/taillights2. Panel lights3. Interior cab light4. Front flood/ladder lights5. Fog lights

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1. Gauge clustermodule:

- Engine coolanttemperature

- Brake oiltemperature

- System air pressure

- Fuel level

2. Speed/tach module:

- Analog tachometer

- Ground speed

- Transmission actualgear

3. Dash backlitindicators:

- Left and right turnsignals

- High beam indicator

- Action light

- Service/retarderbrakes ENGAGEDlight

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213

Located on the front dash are two of the VIMS output components. Theyare the gauge cluster module (1) and the speedometer/tachometer module (2).

The four gauges in the gauge cluster module (from left to right and top tobottom) are:

- Engine coolant temperature- Brake oil temperature- System air pressure- Fuel level

The speedometer/tachometer module consists of an analog tachometerand a display window which shows the ground speed and the transmissionactual gear.

Several backlit indicators will appear in the upper area (3) of the displaywhen they are active. The backlit indicators are:

- Left and right turn signals- High beam indicator- Action light- Service/retarder brakes ENGAGED light

- 40 -

1. Automatic RetarderControl (ARC)ON/OFF switch

2. Message centermodule:

- Alert indicator

- Universal gauge

- Message displaywindow

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3

2

1

To the right of the steering column is the Automatic Retarder Control(ARC) ON/OFF switch (1).

To the right of the ARC switch are two more components of the VIMS.They are the message center module (2) and the keypad module (3).

The message center module consists of an alert indicator, a universalgauge and a message display window. The alert indicator flashes when aCategory 1 Warning is present. The universal gauge displays the status ofthe sensor selected for viewing by depressing the GAUGE key on thekeypad. The message display window shows various types of textinformation to the operator.

- 41 -

3. Keypad module

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The keypad module allows the operator or a service technician to interactwith the VIMS. Some of the functions that can be performed by thekeypad are:

- Scroll parameters monitored by VIMS by depressing the GAUGE key.

- Payload Monitor ON/OFF PAYLOAD 7295623

- Calibrate Payload Monitor PAYCAL 729225

- Payload Resettable Totals TOT 868

- Reset Displayed Data RESET 73738

- Display Self Test TEST 8378

- Reset Service Light SVCLIT 782548

- Set Lube Cycle Times LUBSET 582738

- Manual Lube LUBMAN 582626

- Show Acknowledged Events EACK 3225

- Show Event Statistics ESTAT 37828

- Show Event List ELIST 35478

- Start Event Recorder EREC 3732

- Start/Stop Data Logger DLOG 3564

- Reset Data Logger DLRES 35737

- Odometer Set/Reset ODO 636(requires VIMS PC connection)

- Machine Status MSTAT 67828

- Change Language LA 52

- Change Units UN 86

- Change Backlight BLT 258

- Change Display Contrast CON 266(requires Updated Message Center)

INSTRUCTOR NOTE: For more detailed information on the VIMS,refer to the Service Manual Module "Vital Information ManagementSystem (VIMS)" (Form SENR6059).

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• VIMS

SENSORS

ADEM IICONTROL

SERVICELAMP

MESSAGE CENTERMODULE

GAUGE CLUSTERMODULE

KEYPADMODULE

SENSORS

VIMSINTERFACE

MODULE

VIMSINTERFACE

MODULE

SENSORS

VIMSSERVICE TOOL

ANDSOFTWARE

CAT DATA LINK

SERVICEKEYSWITCH

ACTIONLAMP

ACTIONALARM

ELECTRONICTECHNICIAN/ECAP

VIMS MAIN MODULE

DISPLAY DATA LINK

VIMSRS-232PORT

AUTO RETARDERCONTROL

CAT DATA LINK

TRANSMISSIONCONTROL

VITAL INFORMATIONMANAGEMENT SYSTEM

(VIMS)

SPEEDOMETER/TACHOMETER

MODULE

3F12 MPHkm/h

KEYPADDATA LINK

As shown in some of the previous slides, the 793C is equipped with theVIMS which receives input signals from many sensors and alsocommunicates with other electronic controls on the machine. The VIMSprovides the operator and the service technician with a complete look atthe current and past conditions of all the systems on the truck.

- 43 -

• Behind the operator'sseat are:

- Fuse panel

- ECAP/ET diagnosticconnector (arrow)

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Behind the operator’s seat are the fuse panel and the ECAP/ET diagnosticconnector (arrow). The ECAP/ET diagnostic connector is used to connectthe Electronic Control Analyzer Programmer (ECAP) or a laptopcomputer with the Electronic Technician (ET) software installed.

While VIMS monitors all of the systems on the truck, the ECAP or ET isused for programming, running diagnostic tests and retrieving loggedinformation from the engine, transmission and automatic retardercontrols.

- 44 -

• Electronic Technician(ET)

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Shown is the communication adapter and a laptop computer with theElectronic Technician (ET) diagnostic software installed. Thecommunication adapter is connected to the diagnostic connector shown inthe previous slide.

- 45 -

• VIMS connector(arrow)

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Shown is a laptop computer with the VIMS PC diagnostic softwareinstalled. The laptop computer is connected to the VIMS diagnosticconnector (arrow).

- 46 -

• Hoist control lever(arrow)

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The operator controls include the hoist lever (arrow) which is located tothe left of the operator’s seat. The four positions are RAISE, HOLD,FLOAT and LOWER.

The truck should normally be operated with the hoist lever in the FLOATposition. Operating with the hoist lever in the FLOAT position allows thehoist valve to provide some downward hydraulic pressure on the hoistcylinders and prevents an empty body from bouncing on rough haulroads.

The 793C hoist system is different from previous trucks. The hoistsystem is electronically controlled.

INSTRUCTOR NOTE: The hoist system will be explained in moredetail in the HOIST SYSTEM section of this presentation.

• Electronicallycontrolled hoistsystem

• Hoist lever in FLOATfor normal operation

- 47 -

• Operator controls:

- Secondary brakelever (red)

- Retarder lever(black)

1. Tilt steering lock

2. Turn signal andhazard switch

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2

1

The operator controls on the steering column are the secondary brakelever (red), the retarder lever (black), the tilt steering lock (1) and the turnsignal and hazard switch (2).

- 48 -

1. Service brake pedal

2. Throttle pedal

3. Throttle positionsensor

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2

1

3

On the floor to the right of the steering column are the service brake pedal (1) and the throttle pedal (2). A throttle position sensor (3) isattached to the throttle pedal. The throttle position sensor provides thethrottle position input signals to the engine ECM.

The engine ECM provides an elevated engine idle speed of 1300 rpmwhen the coolant temperature is below 60°C (140°F). Above 60°C(140°F), the elevated idle rpm is gradually reduced until the coolanttemperature reaches 71°C (160°F). Above 71°C (160°F), the engine willidle at 700 rpm.

Increasing the low idle speed helps prevent incomplete combustion andovercooling. To temporarily reduce the elevated idle speed, the operatorcan depress the throttle momentarily, and the idle speed will decrease to700 rpm for 10 minutes.

On the floor to the left of the steering column are the horn button and thehigh beam switch (not shown).

• Horn button and highbeam switch (notshown)

• Elevated low idlereduced with throttlepedal

- 49 -

• 793C uses 3516Bengine

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ENGINE

The 793C is equipped with the Caterpillar 3516B engine with a grosspower rating of 1715 kW (2300 hp) and a net flywheel power rating of1615 kW (2166 hp) at 1750 rpm.

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• 3516B electroniccontrol systemcomponent diagram

A/C PRESSURESWITCH

CRANKCASEPRESSURE

GROUND LEVELSHUTDOWN SWITCH

FUEL FILTERSWITCH

PRE-LUBRICATIONRELAY

OIL LEVELSWITCH (LOW)

OIL LEVELSWITCH (ADD)

FANFAN SPEED SENSOR

FAN CLUTCHSOLENOID

SERVICE TOOLEPTC II

ARCVIMS

CAT DATA LINK

ENGINE COOLANT TEMPERATURE

ADEM IICONTROLMODULE

GROUNDBOLT

15 AMPBREAKER

MAINPOWER RELAY

KEY STARTSWITCH

SPEED/TIMING SENSOR

ENGINE OIL PRESSURE(UNFILTERED)

COOLANT FLOW SWITCH

TIMING PROBECONNECTOR

ETHER SOLENOID

DISCONNECT SWITCH

3516B ELECTRONIC CONTROLSYSTEM COMPONENT DIAGRAM

ELECTRONIC UNITINJECTORS

TURBO OUTLET PRESSURE (BOOST)

RIGHT TURBO INLET PRESSURE

ATMOSPHERIC PRESSURE

ENGINE OIL PRESSURE (FILTERED)

THROTTLE

ENGINE OILRENEWAL SOLENOID

SHUTTER SOLENOID

REAR AFTERCOOLER TEMPERATURE

LEFT TURBO INLET PRESSURE

RIGHT TURBO EXHAUST

LEFT TURBO EXHAUST

THROTTLE OVERRIDESWITCH

MANUAL ETHERSWITCH

EXHAUSTWASTEGATE

SOLENOID

24 V

Shown is the electronic control system component diagram for the 3516Bengine used in the 793C. Fuel injection is controlled by the secondgeneration Advanced Diesel Engine Management (ADEM II) engineElectronic Control Module (ECM).

Many electronic signals are sent to the ADEM II ECM by sensors,switches and senders. The engine ECM analyzes these signals anddetermines when and for how long to energize the injector solenoids.

When the injector solenoids are energized determines the timing of theengine. How long the solenoids are energized determines the enginespeed.

- 51 -

• ECM (arrow)

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Fuel injection is controlled by the ADEM II ECM (arrow) located on theright front of the engine.

The previous ECM had one 70-pin connector. The ADEM II ECM hastwo 40-pin connectors.

The engine ECM is cooled by fuel. Fuel flows from the fuel transferpump through the ECM to the secondary fuel filters.

• ECM has two 40-pinconnectors

• ECM cooled by fuel

- 52 -

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The atmospheric pressure sensor (arrow) is located adjacent to the engineECM. Formerly, this sensor was located in the compartment behind thecab. The engine ECM uses the atmospheric pressure sensor as a referencefor calculating boost and air filter restriction and for derating the engine athigh altitudes.

The engine ECM also uses the atmospheric pressure sensor as a referencewhen calibrating all the pressure sensors.

• Atmospheric pressuresensor (arrow)

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• 3516B improvements

3516B IMPROVEMENTSINPUT SWITCHES AND SENSORS

• COOLANT FLOW

• REAR AFTERCOOLER TEMPERATURE

• ENGINE OIL LEVEL

• TURBOCHARGER TEMPERATURE

• ENGINE OIL FILTER PRESSURE/RESTRICTION

• ENGINE FAN SPEED

• FUEL FILTER RESTRICTION

• AIR CONDITIONER COMPRESSOR PRESSURE

• CRANKCASE PRESSURE

The 3516B engine has many improvements over the original 3516 engine.Some of the improvements are accomplished by adding additional switchand sensor inputs to the engine ECM. Adding additional inputs to theengine ECM allows the ECM to control the engine more precisely.Additional inputs to the 3516B ECM are:

- Coolant flow is monitored.

- Rear aftercooler temperature is measured.

- Engine oil level is monitored.

- Two turbocharger temperature sensors measure exhaust temperatures.

- Two engine oil pressure sensors are located on the oil filter base tomeasure oil pressure and oil filter restriction.

- Engine fan speed is measured (with variable fan speed attachment).

INSTRUCTOR NOTE: The following slides will show some of theengine ECM input components. The remaining inputs to the engineECM will be discussed when the systems they monitor are shown.

• Additional inputs

- 54 -

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2

1

Fuel filter restriction is monitored with a fuel filter bypass switch (1)located on the fuel filter base. The fuel filter bypass switch provides aninput signal to the engine ECM. The ECM provides a signal to the VIMSwhich informs the operator if the secondary fuel filters are restricted.

If the fuel filter restriction exceeds 138 kPa (20 psi), a fuel filterrestriction event is logged. No factory password is required to clear thisevent.

An air conditioner compressor switch (2) is located at the rear of the airconditioner compressor. If the truck is equipped with the variable fanspeed attachment, the air conditioner compressor switch informs theengine ECM when the air conditioner system is ON. When the airconditioner system is ON, the ECM sets the variable speed fan atMAXIMUM rpm.

Disconnecting the air conditioner compressor switch will also signal theECM to set the fan speed at MAXIMUM rpm.

1. Fuel filter bypassswitch

2. Air conditionercompressor switch

• Fuel filter restrictionevent

- 55 -

• Crankcase pressuresensor (arrow)

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The crankcase pressure sensor (arrow) is located on the right side of theengine above the engine oil cooler. The crankcase pressure sensorprovides an input signal to the engine ECM. The ECM provides thesignal to the VIMS which informs the operator of the crankcase pressure.

High crankcase pressure may be caused by worn piston rings or cylinderliners.

If crankcase pressure exceeds 3.6 kPa (.5 psi), a high crankcase pressureevent will be logged. No factory password is required to clear this event.

• Crankcase pressureevent

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3516B IMPROVEMENTSPREVIOUS LOGGED EVENTS

• AIR FILTER RESTRICTION

• LOW OIL PRESSURE

• HIGH COOLANT TEMPERATURE

• ENGINE OVERSPEED

The 3500B ECM logs the four events of the previous 3500 engine plussome additional events. The four events logged by the 3500 ECM and the3500B ECM are:

Air filter restriction: Greater than 6.25 kPa (25 in. of water). Maximumderate of 20%.

Low oil pressure: From less than 100 kPa (15 psi) at LOW IDLE to lessthan 300 kPa (44 psi) at HIGH IDLE.

High coolant temperature: Greater than 107°C (226°F).

Engine overspeed: Greater than 2200 rpm.

NOTE: Factory passwords are required to clear all the events listedabove.

• Events logged by 3500ECM and 3500B ECM

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• Additional loggedevents

3516B IMPROVEMENTSADDITIONAL LOGGED EVENTS

• OIL FILTER RESTRICTION • HIGH CRANKCASE PRESSURE

• FUEL FILTER RESTRICTION • LOW COOLANT FLOW

• HIGH EXHAUST TEMPERATURE • USER DEFINED SHUTDOWN

• HIGH AFTERCOOLER TEMPERATURE • LOW BOOST PRESSURE

• ENGINE OIL LEVEL LOW • HIGH BOOST PRESSURE

Additional events logged by the 3500B ECM are:

Oil filter restriction: Greater than 70 kPa (10 psi). No factory passwordrequired. Greater than 200 kPa (29 psi). Factory password required.

Fuel filter restriction: Greater than 138 kPa (20 psi). No factorypassword required.

Exhaust temperature high: Greater than 760°C (1400°F). Maximumderate of 20%. Factory password required.

Aftercooler coolant temperature high: Greater than 107°C (226°F).Factory password required.

Engine oil level low: No factory password required.

Crankcase pressure high: Greater than 3.6 kPa (.5 psi). No factorypassword required.

Coolant flow low: Factory password required.

User defined shutdown: Parameters determined by the user.

Boost pressure high: 20 kPa (3 psi) greater than desired. Maximumderate of 10%. No factory password required.

Boost pressure low: 30 kPa (4 psi) lower than desired. Maximum derateof 10%. No factory password required.

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3516B IMPROVEMENTSSYSTEMS CONTROLLED BY ECM

• ETHER INJECTION

• RADIATOR SHUTTER CONTROL

• COLD CYLINDER CUTOUT

• ENGINE START FUNCTION

• ENGINE OIL PRE-LUBRICATION

• VARIABLE SPEED FAN CONTROL

• ENGINE OIL RENEWAL SYSTEM

• EXHAUST BYPASS AT HIGH BOOST

The engine ECM also regulates other systems by energizing solenoids orrelays. Some of the other systems controlled by the ECM are:

Ether Injection: Ether injection is controlled by the engine ECM ormanually. The engine ECM will energize the ether injection relay only if:

- The coolant temperature is below 10°C (50°F).

- Engine speed is below 1200 rpm.

Radiator Shutter Control: On trucks that operate in cold weather,shutters can be added in front of the radiator. Installing shutters in front ofthe radiator allows the engine to warm up to operating temperaturequicker. If a truck is equipped with the attachment radiator shuttercontrol, the shutters are controlled by the engine ECM.

• Engine ECM controlsother systems

• Ether injection

• Radiator shuttercontrol

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Cold Cylinder Cut-out: The 3508B engine uses a cold cylinder cut-outfunction to reduce white exhaust smoke after start-up and during extendedidling in cold weather.

After the engine is started and the automatic ether injection system hasstopped injecting ether, the engine ECM will cut out one cylinder at atime to determine which cylinders are firing. The ECM will disable someof the cylinders that are not firing.

The ECM can identify a cylinder which is not firing by monitoring thefuel rate and engine speed during a cylinder cut-out. The ECM averagesthe fuel delivery and analyzes the fuel rate change during a cylinder cut-out to determine if the cylinder is firing.

Disabling some of the cylinders during Cold Mode operation will causethe engine to run rough until the temperature increases above the ColdMode temperature. This condition is normal, but the operator should beaware it exists to prevent unnecessary complaints.

Engine Start Function: The Engine Start function is controlled byADEM II and the Electronic Programmable Transmission Control (EPTC II). The engine ECM provides signals to the EPTC II regardingthe engine speed and the condition of the engine pre-lubrication system.The EPTC II will energize the starter relay only when:

- The shift lever is in NEUTRAL.

- The parking brake is ENGAGED.

- The engine speed is 0 rpm.

- The engine pre-lubrication cycle is complete or turned OFF.

NOTE: To protect the starter, the starter is disengaged when theengine rpm is above 300 rpm.

INSTRUCTOR NOTE: The remaining improvements are describedin the slides that follow.

• Cold cylinder cut-out

• Engine runs roughduring cold mode

• Engine start function

- 60 -

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2

1

Engine Oil Pre-lubrication: Engine oil pre-lubrication is controlled bythe ADEM II and EPTC II. The EPTC II signals the ADEM II when toenergize the pre-lubrication pump relay (1). The ADEM II signals EPTC II to crank the engine when:

- Engine oil pressure is 27 kPa (5 psi) or higher.

- The pre-lubrication pump (2) has run for 15 seconds. (If thesystem times out after 15 seconds, a pre-lubrication fault islogged in the ADEM II.)

- The engine has been running in the last 2 minutes.

- Coolant temperature is above 50°C (122°F).

NOTE: The ECAP and ET can enable or disable the pre-lubricationfeature in the ADEM II. On some trucks, the pre-lubrication pump islocated near the right front of the engine.

• Engine oilpre-lubrication

1. Pre-lubrication pumprelay

2. Pre-lubrication pump

- 61 -

• Variable speed fancontrol:

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1

3

2

Variable Speed Fan Control: If the engine is equipped with a variablespeed fan, the engine ECM regulates the fan speed. Fan speed variesaccording to the temperature of the engine. The ECM sends a signal tothe variable speed fan control solenoid valve (1) and engine oil pressureengages a clutch as needed to change the speed of the fan.

The jacket water coolant temperature sensor (2) is located in the jacketwater temperature regulator (thermostat) housing. The ECM uses thecoolant temperature sensor information as the main parameter to controlthe fan speed. The aftercooler temperature sensors, air conditionerpressure sensor and brake cooling oil temperature sensors are also used asinputs to determine the required fan speed. A speed sensor (not shown) islocated behind the fan pulley and informs the ECM of the current fanspeed.

The variable speed fan feature can be turned off using the ECAP or ETservice tool. Turning off the variable speed fan feature will set the fanspeed at MAXIMUM rpm. Disconnecting the air conditioningcompressor switch will also signal the ECM to set the fan speed atMAXIMUM rpm.

The turbocharger outlet pressure sensor (3) sends an input signal to theECM. The ECM compares the value of the turbo outlet pressure sensorwith the value of the atmospheric pressure sensor and calculates boostpressure.

INSTRUCTOR NOTE: For more information on the variable speedfan, refer to the Service Manual "Variable Speed Fan Clutch" (Form SENR8603).

1. Fan control solenoidvalve

• Fan speed sensor(not shown)

• Fan speed overrides

3. Turbo outletpressure sensor

2. Jacket water coolanttemperature sensor

- 62 -

• Engine oil renewalsystem components:

1. Oil filter

2. Oil renewalsolenoid

3. Fuel pressureregulator

• Oil mixes with fuel infuel tank

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1

3

2

Engine Oil Renewal System: Located on the right side of the engine arethe components of the engine oil renewal system. Engine oil flows fromthe engine block through an oil filter (1) to the engine oil renewalsolenoid (2). A small amount of oil flows from the engine oil renewalsolenoid into the return side of the fuel pressure regulator (3). The engineoil returns to the fuel tank with the return fuel.

The engine oil mixes with the fuel in the tank and flows with the fuel tothe EUI injectors to be burned.

When the engine oil renewal system is used, the operator must pay closeattention to the ADD OIL message that the VIMS provides to the operatorwhen makeup oil must be added (see Slide No. 54).

The oil does not have to be changed when using the engine oil renewalsystem. When the engine oil renewal system is used, the engine oilfilters, the engine oil renewal system filter, the primary fuel filter and thesecondary fuel filters must all be changed at 500 hour intervals.

Engine oil samples must be taken regularly to ensure that the soot level ofthe engine oil is in a safe operating range.

• Sample engine oil tocheck soot level

- 63 -

• Oil injection controlledby engine ECM

• Engine oil renewalsystem parameters

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The ECM regulates the amount of oil that is injected by the engine oilrenewal solenoid. Several parameters must be met before the ECM willallow the injection of oil through the engine oil renewal system. Theparameters that must be met are:

- Fuel position is greater than 10.

- Engine rpm is between 1300 and 1850 rpm.

- Jacket water temperature is between 63°C (145°F) and107°C (225°F).

- Oil filter differential pressure at high idle with warm oil is less than70 kPa (10 psi).

- Fuel filter differential pressure is less than 140 kPa (20 psi).

- Engine oil level switches are sending a valid signal to the ADEM IIcontrol.

- Engine has been running more than five minutes.

The engine oil renewal system can be turned ON or OFF with the ECAPor ET service tool. The amount of oil injected can also be adjusted byprogramming the ECM with the ECAP or ET service tool. The factorysetting shown in the service tool is "0" and is equivalent to a 0.5% oil tofuel ratio. The ratio can be changed with the service tool from minus 50(-50) to plus 50 (+50), which is equivalent to 0.25% to 0.75% oil to fuelratios.

• Oil renewal adjustedwith ECAP or ET

- 64 -

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1

2

The engine oil level switches (1 and 2) provide input signals to the engineECM. The ECM provides an input signal to the VIMS which informs theoperator of the engine oil level.

If the truck is equipped with the engine oil renewal system attachment,the upper oil level switch (1) will tell the operator when makeup oil mustbe added. The ADD ENG OIL message is a Category 1 Warning.

The lower oil level switch (2) will tell the operator when the engine oillevel is low and it is unsafe to operate the truck without causing damageto the engine. The ENG OIL LEVEL LOW message is a Category 2 or 3Warning.

If the engine ECM detects a low oil level condition (oil level below thelower switch), the ECM will log a low oil level event. No factorypassword is required to clear this event.

1. Add engine oil levelswitch

2. Engine oil level lowswitch

• Low oil level event

- 65 -

1. Exhaust bypassvalve

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1

2

Exhaust Bypass Control: An exhaust bypass (wastegate) valve (1)prevents excessive boost pressure by diverting exhaust gasses away fromthe turbochargers. The bypass valve is controlled by the engine ECM.

Brake system air pressure is reduced to 380 kPa (55 psi) by a valvelocated outside the right rear of the cab and is supplied to the wastegatesolenoid valve (2). If boost pressure exceeds a predetermined value, theECM will open the wastegate solenoid and send air pressure to open theexhaust bypass valve. When the exhaust bypass valve is open, exhaust atthe turbine side of the turbochargers is diverted through the muffler.Diverting the turbine exhaust pressure decreases the speed of theturbochargers which reduces the boost pressure to the cylinders.

The wastegate solenoid valve can be controlled with the ECAP or ETservice tool for diagnostic purposes. Connect a multimeter to thewastegate solenoid and set the meter to read DUTY CYCLE. Using theservice tool, override the wastegate solenoid valve and use the multimeterto measure the corresponding duty cycle.

If the actual boost pressure is 20 kPa (3 psi) higher than the desired boostpressure calculated by the ECM, a high boost pressure event will belogged. If the actual boost pressure is 30 kPa (4 psi) lower than thedesired boost pressure calculated by the ECM, a low boost pressure eventwill be logged. If the ECM detects a high or low boost condition, theECM will derate the fuel delivery (maximum derating of 10%) to preventdamage to the engine.

2. Wastegate solenoidvalve

- Controlled by engineECM

• Engine wastegatesolenoid checked withECAP or ET

• Boost pressure events

- 66 -

1. Cooling systemshunt tank

• Engine coolingsystems:

- Jacket water coolingsystem

- Aftercooler coolingsystem

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3

1

4

2

Cooling System

The 793C is equipped with a shunt tank (1) to increase the coolingcapacity. The shunt tank provides a positive pressure at the coolant pumpinlets to prevent cavitation during high flow conditions.

The cooling system is divided into two systems. The two systems are thejacket water cooling system and the aftercooler cooling system. The onlyconnection between these two systems is a small hole in the separatorplate in the shunt tank. The small hole in the shunt tank prevents areduction of coolant from either of the two systems if leakage occurs inone of the separator plates in the radiator top or bottom tank. Whenservicing the cooling systems, be sure to drain and fill both systemsseparately.

The coolant levels are checked at the shunt tank. Use the gauges (2) ontop of the shunt tank to check the coolant level.

A coolant level sensor (3) is located on each side of the shunt tank tomonitor the coolant level of both cooling systems (guard removed forviewing sensor). The coolant level sensors provide input signals to theVIMS which informs the operator of the engine coolant levels.

Pressure relief valves (4) prevent the cooling systems from becoming overpressurized.

4. Pressure reliefvalves

3. Coolant level sensor

2. Coolant level gauges

- 67 -

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The jacket water cooling system uses 17 of the 30 cores on the right sideof the radiator (approximately 60% of the total capacity). The jacketwater cooling system temperature is controlled by temperature regulators(thermostats).

The aftercooler cooling system uses 13 of the 30 cores on the left side ofthe radiator (approximately 40% of the total capacity). The aftercoolercooling system does not have thermostats in the circuit. The coolantflows through the radiator at all times to keep the turbocharged inlet aircool for increased horsepower.

• Aftercooler coolingsystem

• Jacket water coolingsystem

- 68 -

1. Jacket water pump

2. Bypass tube

3. Jacket waterthermostat housing

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2

1

3

The jacket water pump (1) is located on the right side of the engine. Thepump draws coolant from the bypass tube (2) until the temperatureregulators (thermostats) open. The thermostats are located in the housing (3) at the top of the bypass tube. When the thermostats are open,coolant flows through the radiator to the water pump inlet.

If the jacket water cooling system temperature increases above 107°C(226°F), the engine ECM will log an event that requires a factorypassword to clear.

• High coolanttemperature event

- 69 -

• Coolant flow warningswitch (arrow)

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Coolant flows from the jacket water pump, past the coolant flow warningswitch (arrow), and through the various system oil coolers (engine, torqueconverter/transmission and rear brake).

The coolant flow switch sends an input signal to the engine ECM. TheECM provides the input signal to the VIMS which informs the operator ofthe coolant flow status.

If the ECM detects a low coolant flow condition, a low coolant flow eventwill be logged. A factory password is required to clear this event.

• Low coolant flowevent

- 70 -

1. Engine oil cooler

2. Torque converter/transmission oilcooler

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1

2

Shown is the right side of the engine. The engine oil cooler (1) and thetorque converter and transmission oil cooler (2) are visible in this view.

The coolant flows through these coolers to the rear brake oil coolerslocated on the outside right frame.

- 71 -

• Rear brake oil coolers(arrow)

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Jacket water coolant flows from the rear brake oil coolers (arrow) to bothsides of the engine cylinder block. Coolant flows through the engineblock and through the cylinder heads. From the cylinder heads, thecoolant returns to the temperature regulators and either goes directly tothe water pump through the bypass tube or to the radiator (depending onthe temperature of the coolant).

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• Jacket water coolingsystem circuit

ENGINE OIL COOLER

TORQUE CONVERTER/TRANSMISSION OIL COOLER

ENGINEBLOCK

JACKET WATER COOLANT FLOW

REAR BRAKEOIL COOLERS

THERMOSTATHOUSING

RADIATOR

JACKETWATER PUMP

SHUNTTANK

Shown is the jacket water cooling system circuit. Coolant flows from thejacket water pump through the coolers to the engine block. Coolant flowsthrough the engine block and the cylinder heads. From the cylinderheads, the coolant returns to the temperature regulators (thermostats) andeither goes directly to the water pump through the bypass tube or to theradiator (depending on the temperature of the coolant).

The shunt tank increases the cooling capacity and provides a positivepressure at the coolant pump inlet to prevent cavitation during high flowconditions.

In this illustration and those that follow, the colors used to identify thevarious pressures in the systems are:

Red - Supply oil/water pressureGreen - Drain or reservoir oil/waterRed and White Stripes - Reduced supply oil pressureBrown - Lubrication or cooling pressureOrange - Pilot or load sensing signal pressureBlue - Blocked oilYellow - Moving componentsPurple - Air pressure

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1. Aftercooler waterpump

2. Shunt tank supplytube

3. Aftercooler circuitcoolant tubes

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12

The auxiliary (aftercooler) water pump (1) for the aftercooler coolingsystem is located on the left side of the engine. Coolant enters theaftercooler water pump from the radiator or the shunt tank supply tube (2). Coolant flows from the pump to the aftercooler cores throughthe large tubes (3)

- 74 -

• Rear aftercoolertemperature sensor(arrow)

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Located in a tube at the rear of the aftercooler is the rear aftercoolertemperature sensor (arrow). The rear aftercooler temperature sensorprovides an input signal to the engine ECM. The engine ECM uses therear aftercooler temperature sensor signal with the jacket watertemperature sensor signal to control the variable speed fan attachment.

The ECM also provides the input signal to the VIMS which informs theoperator of the aftercooler coolant temperature.

If the rear aftercooler temperature increases above 107°C (226°F), theengine ECM will log an event that requires a factory password to clear.

Another aftercooler temperature sensor is located in a tube at the front ofthe aftercooler. The front aftercooler temperature sensor does not send aninput signal to the engine ECM. The front aftercooler temperature sensorprovides an input signal directly to the VIMS.

• Front aftercoolertemperature sensor

• Rear aftercoolertemperature event

- 75 -

1. Front brake oilcooler

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1

Coolant flows through the aftercooler cores to the front brake oil cooler (1) located at the rear of the engine.

Coolant flows through the front brake oil cooler to the aftercooler sectionof the radiator. The aftercooler cooling system does not have temperatureregulators (thermostats) in the circuit.

When the service or retarder brakes are ENGAGED, the front brake oilcooler diverter valve (2) allows brake cooling oil to flow through the frontbrake oil cooler.

Normally, front brake cooling oil is diverted around the cooler and goesdirectly to the front brakes. Diverting oil around the cooler provideslower temperature aftercooler air during high power demands (whenclimbing a grade with the brakes RELEASED, for example).

• Aftercooler coolingcircuit does not havethermostats

2. Front brake oilcooler diverter valve

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• Aftercooler coolingsystem circuit

AFTERCOOLER COOLANT FLOW

FRONT BRAKEOIL COOLER

RADIATOR

AFTERCOOLERWATER PUMP

SHUNTTANK

AFTERCOOLER

Shown is the aftercooler cooling system circuit. Coolant flows from theaftercooler water pump through the aftercooler.

Coolant flows through the aftercooler cores to the front brake oil cooler located at the rear of the engine.

Coolant then flows through the front brake oil cooler to the aftercoolersection of the radiator. The aftercooler cooling circuit does not havetemperature regulators (thermostats) in the circuit.

The shunt tank increases the cooling capacity and provides a positivepressure at the aftercooler water pump inlet to prevent cavitation duringhigh flow conditions.

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1. Engine oil pump

2. Engine oil pumprelief valve

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4

1

2

Lubrication System

The engine oil pump (1) is located behind the jacket water pump on theright side of the engine. The pump draws oil from the oil pan through ascreen. The relief valve (2) for the lubrication system is located on thepump.

The engine also has a scavenge pump at the rear of the engine to transferoil from the rear of the oil pan to the main sump.

Oil flows from the pump through an engine oil cooler bypass valve (3) tothe engine oil cooler (4). The bypass valve for the engine oil coolerpermits oil flow to the system during cold starts when the oil is thick or ifthe cooler is plugged.

3. Engine oil coolerbypass valve

4. Engine oil cooler

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• Engine oil filters

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3

1

Oil flows from the engine oil cooler to the oil filters on the left side of theengine. The oil flows through the filters and enters the engine cylinderblock to clean, cool and lubricate the internal components and theturbochargers.

Engine oil is added at the fill tube (1) and checked with the dipstick (2). A bypass valve for each filter is located in each oil filterbase.

Engine oil samples can be taken at the Scheduled Oil Sample (S•O•S) tap (3).

The engine has two oil pressure sensors. One sensor is located on eachend of the oil filter base. The front sensor measures engine oil pressurebefore the filters. The rear sensor (4) measures oil pressure after thefilters. The sensors send input signals to the engine ECM. The ECMprovides the input signal to the VIMS which informs the operator of theengine oil pressure. Used together, the two engine oil pressure sensorsinform the operator if the engine oil filters are restricted.

If the engine oil pressure is less than 100 kPa (15 psi) at low idle to lessthan 300 kPa (44 psi) at high idle, the engine ECM will log an event thatrequires a factory password to clear.

If the oil filter restriction exceeds 70 kPa (10 psi), a low oil filterrestriction event will be logged. No factory password is required to clearthis event. If the oil filter restriction exceeds 200 kPa (29 psi), a high oilfilter restriction event will be logged. A factory password is required toclear this event.

4. Engine oil pressuresensors

• Engine oil filterrestriction events

• Engine oil pressureevent

3. Engine oil S•O•S tap

1. Engine oil fill tube

2. Engine oil dipstick

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• Engine oil system

ENGINEBLOCK

ENGINE OIL SYSTEM

ENGINEOIL COOLER

ENGINEOIL FILTERS

ENGINEOIL PUMP

SCAVENGEPUMP

BYPASSVALVE

ENGINEOIL RENEWAL

SYSTEM SOLENOID

TO FUELSYSTEM

The engine oil pump draws oil from the oil pan through a screen.

The engine also has a scavenge pump at the rear of the engine to transferoil from the rear of the oil pan to the main sump.

Oil flows from the pump through an engine oil cooler bypass valve to theengine oil cooler. The bypass valve for the engine oil cooler permits oilflow to the system during cold starts when the oil is thick or if the cooleris plugged.

Oil flows from the engine oil cooler to the oil filters. The oil flowsthrough the filters and enters the engine cylinder block to clean, cool andlubricate the internal components and the turbochargers.

Some trucks are equipped with an engine oil renewal system. Engine oilflows from the engine block through an oil filter to an engine oil renewalsystem manifold. A small amount of oil flows from the engine oilrenewal system manifold into the return side of the fuel pressure regulator.The engine oil returns to the fuel tank with the return fuel (see Slides No. 53 and 74).

• Engine oil renewalsystem

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• Fuel heater(not shown)

• Primary fuel filter(arrow)

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Fuel System

The fuel tank is located on the left side of the truck. Fuel is pulled fromthe tank through the fuel heater (not shown), if equipped, and through theprimary fuel filter (arrow) by the fuel transfer pump located on the rightside of the engine behind the engine oil pump.

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1. Fuel transfer pump

2. Fuel transfer pumpbypass valve

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The fuel transfer pump (1) contains a bypass valve (2) to protect the fuelsystem components from excessive pressure. The bypass valve setting ishigher than the setting of the fuel pressure regulator which will be shownlater. Fuel flows from the transfer pump through the engine ECM to thesecondary fuel filters located on the left side of the engine.

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• Secondary fuel filters

1. Fuel priming pump

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1

2

The secondary fuel filters and the fuel priming pump (1) are locatedabove the engine oil filters on the left side of the engine.

The fuel priming pump is used to fill the filters after they are changed.

A fuel filter bypass switch (2) is located on the fuel filter base. The fuelfilter bypass switch sends an input signal to the engine ECM. The ECMprovides the input signal to the VIMS which informs the operator if thesecondary fuel filters are restricted.

If fuel filter restriction exceeds 138 kPa (20 psi), a fuel filter restrictionevent will be logged. No factory password is required to clear this event.

Fuel flows from the fuel filter base through the Electronic Unit Injection(EUI) fuel injectors and the fuel pressure regulator and then returns to thefuel tank. The injectors receive 4 1/2 times the amount of fuel needed forinjection. The extra fuel is used for cooling.

NOTE: If the fuel system requires priming, it may be necessary toblock the fuel return line during priming to force the fuel into theinjectors.

2. Fuel filter bypassswitch

• Fuel flows to EUIinjectors

• Extra fuel used to coolinjectors

• Fuel filter restrictionevent

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1. Fuel pressure tubesto injectors

2. Fuel pressureregulator

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Fuel flows from the fuel filter base through the steel tubes (1) to the EUIfuel injectors. Return fuel from the injectors flows through the fuelpressure regulator (2) before returning to the fuel tank. Fuel pressure iscontrolled by the fuel pressure regulator.

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• Fuel system circuit

FUELTANK

PRIMARYFUEL

FILTER

SECONDARYFUEL FILTERS

ENGINEBLOCK

ENGINE OILRENEWAL

SYSTEM SOLENOID

FUELPRESSURE

REGULATOR

FUELTRANSFER

PUMP

FUELHEATER

ADEM IICONTROL

CYLINDERHEAD

FUEL SYSTEM

Fuel is pulled from the tank through a fuel heater, if equipped, andthrough the primary fuel filter by the fuel transfer pump. Fuel flows fromthe transfer pump through the ADEM II control to the secondary fuelfilters.

Fuel flows from the fuel filter base through the fuel injectors in thecylinder heads. Return fuel from the injectors flows through the fuelpressure regulator before returning through the fuel heater to the fuel tank.

Engine oil flows from the engine block through an oil filter to the engineoil renewal system manifold. A small amount of oil flows from theengine oil renewal system manifold into the return side of the fuelpressure regulator. The engine oil returns to the fuel tank with the returnfuel.

The engine oil mixes with the fuel in the tank and flows with the fuel tothe injectors to be burned.

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• Air filter restrictionindicators (arrow)

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Air Induction and Exhaust System

The engine receives clean air through the four air filters located on thefront of the truck. Any restriction caused by plugged filters can bechecked at the filter restriction indicators (arrow). If the yellow piston isin the red zone, the filters must be cleaned or replaced.

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1. Turbocharger inletpressure sensor

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1

The turbocharger inlet pressure sensor (1) is located in a tube between theair cleaners and the turbochargers. The engine ECM uses theturbocharger inlet pressure sensor in combination with the atmosphericpressure sensor to determine air filter restriction. The ECM provides theinput signal to the VIMS which informs the operator of the air filterrestriction.

If air filter restriction exceeds 6.25 kPa (25 in. of water), an air filterrestriction event will be logged, and the ECM will derate the fuel delivery(maximum derating of 20%) to prevent excessive exhaust temperatures.A factory password is required to clear this event.

If the truck is equipped with an ether start system, the ECM willautomatically inject ether from the ether cylinders (2) during cranking.The operator can also inject ether manually with the ether switch in thecab on the center console (see Slide No. 30). Ether will be injected onlyif the engine coolant temperature is below 10°C (50°F) and engine speedis below 1200 rpm.

2. Ether cylinders

• Air filter restrictionevent

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• Series turbochargersystem

1. Low pressureturbochargers

2. High pressureturbochargers

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2

The 793C engine is equipped with a series turbocharger system. Theclean air from the filters enters the larger low pressure turbochargers (1).The compressed air from the low pressure turbochargers flows to the inletof the smaller high pressure turbochargers (2). After additionalcompression by the high pressure turbochargers, the air flows to theaftercoolers. After the air is cooled by the aftercoolers, the air flows tothe cylinders and combines with the fuel for combustion.

The turbochargers are driven by the exhaust gasses from the cylinders.The exhaust gasses first enter the smaller high pressure turbochargers.The exhaust from the high pressure turbochargers flows to the larger lowpressure turbochargers. The exhaust gasses then flow through the lowpressure turbochargers, the exhaust piping, and the mufflers.

- 88 -

• Exhaust temperaturesensor (arrow)

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An exhaust temperature sensor (arrow) is located in each exhaustmanifold before the turbochargers. The two exhaust temperature sensorsprovide input signals to the engine ECM. The ECM provides the inputsignal to the VIMS which informs the operator of the exhausttemperature.

Some causes of high exhaust temperature may be faulty injectors, pluggedair filters, or a restriction in the turbochargers or the muffler.

If the exhaust temperature is above 760°C (1400°F), the engine ECM willderate the fuel delivery (maximum derate of 20%) to prevent excessiveexhaust temperatures. The ECM will also log an event that requires afactory password to clear.

• Causes of highexhaust temperature

• High exhausttemperature deratesengine and logs event

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• Turbocharger speedreduced when exhaustbypass valve opens

EXHAUST BYPASS VALVE

MUFFLER

HIGH PRESSURETURBOCHARGER

LOW PRESSURETURBOCHARGER

FROM AIRFILTER

AFTERCOOLER

FROM BRAKEAIR SYSTEM

WASTEGATESOLENOID

VALVE

EXHAUST SYSTEM

PRESSUREREDUCING

VALVE

This schematic shows the air flow through the air induction system. Ifboost pressure exceeds a predetermined value programmed in the engineECM, the ECM will open the wastegate solenoid valve and send brake airpressure to open the exhaust bypass valve. The exhaust bypass valve willvent the exhaust gasses before they reach the turbochargers. Less exhaustgasses will flow through the turbochargers, and the turbocharger speedwill decrease. The slower turbochargers reduce the boost pressure untilthe bypass valve closes and the exhaust gasses are again directed throughthe turbochargers.

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• Power traincomponents:

1. Torque converter

2. Transfer gears

3. Transmission

4. Differential

5. Final drives

793C

POWER TRAIN

POWER TRAIN

Power flows from the engine to the rear wheels through the power train.The components of the power train are:

1. Torque converter2. Transfer gears3. Transmission4. Differential5. Final drives

INSTRUCTOR NOTE: In this section of the presentation, componentlocations and a brief description of the component functions areprovided. For more detailed information on the ElectronicProgrammable Transmission Control (EPTC II), torque converter andICM (Individual Clutch Modulation) transmission, refer to theTechnical Instruction Modules "Electronic ProgrammableTransmission Control (EPTC II)" (Form SEGV2584-01) and "769C - 793B Off-highway Trucks--Torque Converter and TransmissionHydraulic System" (Form SEGV2591).

- 91 -

• Torque converter:

- Provides a fluidcoupling

- Multiplies torque

- Provides direct driveoperation

1. Inlet relief valve

2. Outlet relief valve

3. Lockup clutchcontrol valve

4. Outlet temperaturesensor

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Power Train Components

The first component in the power train is the torque converter. The torqueconverter provides a fluid coupling that permits the engine to continuerunning with the truck stopped. In converter drive, the torque convertermultiplies torque to the transmission. At higher ground speeds, a lockupclutch engages to provide direct drive. The NEUTRAL and REVERSEranges are converter drive only. FIRST SPEED is converter drive at lowground speed and direct drive at high ground speed. SECOND throughSIXTH SPEEDS are direct drive only. The torque converter goes toconverter drive between each shift (during clutch engagement) to providesmooth shifts.

Mounted on the torque converter are the inlet relief valve (1), the outletrelief valve (2) and the torque converter lockup clutch control valve (3).

A torque converter outlet temperature sensor (4) provides an input signalto the VIMS which informs the operator of the torque converter outlettemperature.

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• CONVERTER DRIVE

- Output shaft rotatesslower than enginerpm

- Torque is increased

• Torque convertercomponents:

- Lockup clutch

- Impeller

- Turbine

- Stator

STATOR

TORQUE CONVERTERCONVERTER DRIVE

LOCKUP PISTON

TORQUE CONVERTERLOCKUP OIL PASSAGE

TURBINE IMPELLER

FREEWHEELASSEMBLY

TORQUE CONVERTERINLET OIL

This sectional view shows a torque converter in CONVERTER DRIVE.The lockup clutch (yellow piston and blue discs) is not engaged. Duringoperation, the rotating housing and impeller (red) can rotate faster than theturbine (blue). The stator (green) remains stationary and multiplies thetorque transfer between the impeller and the turbine. The output shaftrotates slower than the engine crankshaft, but with increased torque.

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• DIRECT DRIVE

- Lockup clutchengaged

- Output shaft rotatesat engine rpm

- Stator freewheels

STATOR

TORQUE CONVERTERDIRECT DRIVE

LOCKUP PISTON

TORQUE CONVERTERLOCKUP OIL PASSAGE

TURBINE IMPELLER

STATOR

FREEWHEELASSEMBLY

TORQUE CONVERTERINLET OIL

In DIRECT DRIVE, the lockup clutch is engaged by hydraulic pressureand locks the turbine to the impeller. The housing, impeller, turbine, andoutput shaft then rotate as a unit at engine rpm. The stator, which ismounted on a freewheel assembly, is driven by the force of the oil in thehousing and will freewheel at approximately the same rpm.

- 94 -

1. Transfer gears

2. Transmission

3. Differential

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2

1

3

Power flows from the torque converter through a drive shaft to thetransfer gears (1). The transfer gears are splined to the transmission.

The transmission (2) is located between the transfer gears and thedifferential (3). The transmission is electronically controlled andhydraulically operated like all other ICM (Individual Clutch Modulation)transmissions in Caterpillar rigid frame trucks.

The differential is located in the rear axle housing behind thetransmission. Power from the transmission flows through the differentialand is divided equally to the final drives in the rear wheels. The finaldrives are double reduction planetaries.

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• Transmission is powershift planetary design

1 2 3

45 6

POWER SHIFT PLANETARY TRANSMISSION

The transmission is a power shift planetary design which contains sixhydraulically engaged clutches. The transmission provides sixFORWARD speeds and one REVERSE speed.

- 96 -

1. Rear axle oil pump

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1

2

3

4

Shown is the differential removed from the rear axle housing. The rearaxle cooling and filter system starts with a rear axle oil pump (1) that isdriven by the differential. Since the pump rotates only when the machineis moving, no oil flow is produced when the machine is stationary.Cooling oil flow increases with ground speed to provide cooling when itis most needed.

The rear axle pump pulls oil from the bottom of the rear axle housingthrough a suction screen (2). Oil flows from the pump through atemperature and flow control valve located on top of the differentialhousing to a filter mounted on the rear of the axle housing. Oil then flowsfrom the filter back to the valve located on top of the differential housing.Oil then flows from the valve to the rear wheel bearings and thedifferential bearings.

Oil flows through tubes (3) to the differential bearings.

The fiberglass shroud (4) reduces the temperature of the rear axle oil onlong hauls by reducing the oil being splashed by the bevel gear.

2. Rear axle suctionscreen

3. Differential bearingoil tubes

4. Fiberglass shroud

- 97 -

1. Differential oiltemperature sensor

2. Rear axletemperature andflow control valve

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2

4

1

3

Oil flows from the pump past the differential oil temperature sensor (1) tothe rear axle temperature and flow control valve (2). The differential oiltemperature sensor provides an input signal to the VIMS.

The temperature sensor input signal is used to warn the operator of a highrear axle oil temperature condition or to turn on the attachment rear axlecooling fan (if equipped).

Oil flows from the temperature and flow control valve to the differentialoil filter (3) mounted on the rear of the axle housing. Oil then flows fromthe filter back to the temperature and flow control valve. Some of the oilthat flows from the temperature and flow control valve flows through thesmall supply hose (4) to the differential bearings.

3. Differential oil filter

4. Differential bearingoil supply hose

- 98 -

1. Differential oil filterrestriction switch

2. Rear axle oil levelswitches

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32

The differential oil filter restriction switch (1) and the two rear axle oillevel switches (2) provide input signals to the VIMS.

The differential oil filter restriction switch signal is used to warn theoperator when the differential oil filter is plugged.

The rear axle oil level switch input signals are used to warn the operatorwhen the rear axle oil level is LOW.

When the truck is initially put into operation, a 1R0719 (40 micron) filteris installed. This filter removes the rust inhibitor used duringmanufacturing. The 40 micron filter should be changed after the first 50 hours of operation and replaced with a 4T3131 (13 micron) filter. The13 micron filter should be changed every 500 hours.

A differential carrier thrust pin is located behind the small cover (3). Thethrust pin prevents movement of the differential carrier during high thrustload conditions.

3. Differential carrierthrust pin cover

• Differential oil filterservice information

- 99 -

1. Differential oilpressure sensor

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The differential oil pressure sensor (1) provides an input signal to theVIMS. The differential oil pressure sensor signal is used to warn theoperator of a HIGH or LOW rear axle oil pressure condition.

A LOW oil pressure warning is provided if the pressure is below35 kPa (5 psi) when the differential oil temperature is above 52°C (125°F) and the ground speed is higher than 24 km/h (15 mph).

A HIGH oil pressure warning is provided if the pressure is above 690 kPa(100 psi) when the differential oil temperature is above 52°C (125°F).

The temperature and pressure control valve (2) prevents high oil pressurewhen the rear axle oil is cold. When the oil temperature is below 43°C(110°F), the valve is OPEN and allows oil to flow to the rear axlehousing. When the oil temperature is above 43°C (110°F), the valve isCLOSED and all the oil flows through the filter to a flow control valvelocated in the temperature and flow control valve. The temperature andpressure control valve is also the system main relief valve. If the pressureexceeds 690 kPa (100 psi), the temperature and pressure control valvewill open to prevent high oil pressure to the rear axle oil filter.

The flow control valve distributes the oil flow to the rear wheel bearingsand the differential bearings.

2. Temperature andpressure controlvalve

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• Rear axle oil coolingand filter system

OIL COOLER OILFILTER

TEMPERATURE/PRESSURE

CONTROL VALVE

REAR AXLEOIL COOLING AND FILTER SYSTEM

FLOW CONTROL VALVE

DIFFERENTIALOIL PUMP

SUCTIONSCREEN

REAR AXLE

Shown is a schematic of the rear axle oil cooling and filter system. Thedifferential oil pump pulls oil from the bottom of the rear axle housingthrough a suction screen. Oil flows from the pump through a temperatureand flow control valve located on top of the differential housing.

The temperature and pressure control valve, which is part of thetemperature and flow control valve, prevents high oil pressure when therear axle oil is cold. When the oil temperature is below 43°C (110°F), thevalve is OPEN and allows oil to flow to the rear axle housing. When theoil temperature is above 43°C (110°F), the valve is CLOSED and all theoil flows through the differential oil filter and the oil cooler (if equipped)to a flow control valve, which is also part of the temperature and flowcontrol valve.

• Temperature andpressure control valve

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• Temperature andpressure control valveis main relief

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The temperature and pressure control valve is also the system main reliefvalve. If the pressure exceeds 690 kPa (100 psi), the temperature andpressure control valve will open to prevent high oil pressure to the rearaxle oil filter.

The flow control valve distributes the oil flow to the rear wheel bearingsand the differential bearings. At high ground speeds, excess oil flow isdiverted to the axle housing to prevent overfilling the wheel bearing andfinal drive compartments.

• Flow control valveprevents overfillingwheel bearingcompartment

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• Double reductionplanetary gear finaldrive

FIRST REDUCTIONRING GEAR

SECOND REDUCTIONRING GEAR SECOND REDUCTI

CARRIER

SECOND REDUCSUN GEAR

SECOND REDUCTPLANETARY GE

FIRST REDUCTIONPLANETARY GEAR

FIRST REDUCTIONSUN GEAR

FIRST REDUCTIONCARRIER

FINAL DRIVE

Shown is a sectional view of the double reduction planetary gear finaldrive. Power flows from the differential through axles to the sun gear ofthe first reduction planetary set. The ring gears of the first reductionplanetary set and the second reduction planetary set cannot rotate. Sincethe ring gears cannot rotate, the first reduction sun gear causes rotation ofthe first reduction planetary gears and the first reduction carrier.

The first reduction carrier is splined to the second reduction sun gear. Thesecond reduction sun gear causes rotation of the second reductionplanetary gears and the second reduction carrier. Since the secondreduction carrier is connected to the wheel assembly, the wheel assemblyalso rotates.

The wheel assembly rotates much slower than the axle shaft but withincreased torque.

- 103 -

• Torque converterhousing is oil sump

• Four section pump:

1. Transmissionscavenge

2. Torque convertercharging

3. Transmissioncharging

4. Transmission lube

5. Transmission oilreturn screen

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14 3 2

5

Power Train Hydraulic System

The torque converter housing is the oil sump for the torque converter andtransmission oil supply.

A four section torque converter and transmission pump is located at therear of the torque converter. The four sections (from front to rear) are:

1. Transmission scavenge2. Torque converter charging3. Transmission charging4. Transmission lube

The transmission scavenge section pulls oil through the magnetic screenslocated at the bottom of the transmission. The scavenged oil from thetransmission is transferred into the torque converter housing through thetransmission oil return screen located behind the cover (5).

- 104 -

• Transmissionmagnetic scavengescreens (arrow)

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Shown is the location of the transmission magnetic scavenge screens (arrow). These screens should always be checked for debris if aproblem with the transmission is suspected.

Oil is scavenged from the transmission by the first section of the pump(shown in Slide No. 92).

- 105 -

• Torque converter/transmission suctionscreen cover (arrow)

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The three rear sections of the torque converter and transmission pumppull oil from the torque converter housing sump. Most of the required oilsupply is pulled directly from the torque converter and transmission oilcooler return oil. The remaining required oil supply is drawn through asuction screen located behind the cover (arrow).

- 106 -

1. Torque convertercharging filter

2. Torque converterinlet relief valve

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2

1

Oil flows from the torque converter charging section of the torqueconverter and transmission pump to the torque converter charging filter(1) located on the front of the hydraulic tank.

Oil flows from the torque converter charging filter to the torque converterinlet relief valve (2).

- 107 -

• Torque converter inletrelief valve (arrow)

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Oil flows from the torque converter charging filter to the inlet relief valve (arrow) mounted on the torque converter. The inlet relief valvecontrols the maximum pressure of the supply oil to the torque converter.The torque converter inlet relief pressure can be measured at this valve byremoving a plug and installing a pressure tap.

Oil flows through the inlet relief valve and enters the torque converter.

- 108 -

1. Torque converteroutlet relief valve

2. Outlet relief valvepressure tap

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1

5

2

4

3

Torque converter charging oil either drops to the bottom of the housing orflows through the torque converter outlet relief valve (1). The outletrelief valve limits the pressure inside the torque converter. The outletrelief pressure can be measured at the tap (2) on the outlet relief valve.

All the oil from the torque converter outlet relief valve flows through thetorque converter outlet screen (3) to the torque converter and transmissionoil cooler located on the right side of the engine (see Slide No. 60). Oilflows from the torque converter and transmission oil cooler back to thetorque converter housing.

A torque converter outlet screen bypass switch (4) provides an inputsignal to the VIMS which informs the operator if the torque converteroutlet screen is restricted.

A torque converter outlet temperature sensor (5) provides an input signalto the VIMS which informs the operator of the torque converter outlettemperature.

3. Torque converteroutlet screen

4. Torque converteroutlet screen bypassswitch

5. Torque converteroutlet temperaturesensor

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1. Transmissioncharging filter

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1

Oil flows from the transmission charging section of the torque converterand transmission pump to the transmission charging filter (1).

A transmission charging filter bypass switch (2) sends an input signal tothe VIMS which informs the operator if the transmission charging filter isrestricted.

Transmission charging oil flows in two directions from the transmissioncharging filter:

- Transmission charging oil flows to the torque converter lockupclutch valve located on top of the torque converter.

- Transmission charging oil also flows to the transmission controlvalves located on top of the transmission.

Torque converter and transmission oil samples can be taken at theScheduled Oil Sample (S•O•S) tap (3).

2. Filter bypass switch

• Transmissioncharging oil flows intwo directions:

- To torque converterlockup clutch valve

- To transmissioncontrol valves

3. S•O•S tap

- 110 -

1. Torque converterlockup clutch valvesupply port

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3

The transmission charging pump supplies oil to the torque converterlockup clutch valve through the inlet port (1). When the lockup clutchsolenoid (located on the transmission housing) is energized by thetransmission control, the lockup clutch valve supplies oil to ENGAGE thelockup clutch in the torque converter.

Torque converter lockup clutch pressure can be measured at the tap (2).Torque converter lockup clutch pressure should be 2205 ± 70 kPa (320 ± 10 psi) at 1300 rpm or higher. Do not check the torque converterlockup clutch pressure below 1300 rpm.

The transmission control uses a dual stage relief valve for clutch supplypressure. At high idle in torque converter drive, transmission chargingpressure should be 3065 kPa (445 psi) maximum. At low idle in torqueconverter drive, transmission charging pressure should be 2480 kPa (360 psi) minimum.

During torque converter lockup (DIRECT DRIVE), clutch supplypressure is reduced to extend the life of the transmission clutch seals. At high idle in direct drive, the clutch supply pressure should be 1620 + 240 - 100 kPa (235 + 35 - 15 psi). The correspondingtransmission charge pressure is reduced to 2205 ± 70 kPa (320 ± 10 psi).

2. Torque converterlockup clutchpressure tap

• Do not test converterlockup pressurebelow 1300 rpm

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The torque converter output speed (TCO) sensor (3) sends an input signalto the Electronic Programmable Transmission Control (EPTC II). TheEPTC II memory also contains the engine rpm and the TransmissionOutput Speed (TOS) for each gear of the transmission. The EPTC IIprovides all these input signals to the VIMS.

Using the information from the EPTC II, the VIMS calculates if anyslippage exists in the torque converter lockup clutch or any of thetransmission clutches and stores this information in the VIMS mainmodule. This information can be downloaded from the VIMS with alaptop computer.

3. Torque converteroutput speed (TCO)sensor

• Clutch slippage isrecorded in VIMS

100

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• Lockup clutch valveoperation

LOCKUPSOLENOID

ON

FROMTRANSMISSION

CHARGEPUMP

RELAY VALVE

LOCKUPREDUCING

VALVE

LOCKUPMODULATION

VALVE

LOCKUP CLUTCHPILOT OIL

PRESSURE

TO LOCKUPCLUTCH

FROMTRANSMISSION

CHARGEPUMP

TORQUE CONVERTER LOCKUP CLUTCH CONTROLDIRECT DRIVE

SHUTTLEVALVE

SELECTORPISTON

TOTRANSMISSION

LUBE PUMP

TOSTATION

"D"

Shown is a sectional view of the torque converter lockup clutch valve inDIRECT DRIVE. Supply oil from the transmission charging pump isused to provide lockup clutch oil and has two functions:

1. Supply pressure is reduced to provide pilot pressure.

2. When the solenoid is energized, supply pressure is reduced by themodulation reduction valve to provide lockup clutch pressure.

The lockup solenoid has been energized and directs pump supply pressureto the relay valve. Before moving the selector piston, pilot oil moves ashuttle valve to the right which closes the drain and opens the checkvalve. Oil then flows to the selector piston. Moving the selector pistonblocks the drain passage and the load piston springs are compressed.

• Lockup solenoidenergized startsclutch modulation

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Compressing the load piston springs moves the modulation reductionvalve spool down against the force of the inner spring. This initialmovement opens the supply passage (from the transmission charge pump)and permits pressure oil to flow to the clutch. As the clutch fills, pressureoil opens the ball check valve and fills the slug chamber at the top of thereduction valve spool. At the same time, oil flows through the load pistonorifice and fills the chamber between the end of the load piston and theselector piston. The load piston orifice provides a pressure drop and timedelay in the flow of oil to the load piston chamber. The load piston orificehelps control the rate of modulation. Filling the load piston chamber ismade possible when the selector piston covers the drain passage at thedecay orifice.

The load piston has now moved completely down against the stop. Themodulation cycle is completed and the clutch pressure is at its maximumsetting. Because this is a modulation reduction valve, the maximumpressure setting of the clutch is lower than the transmission chargepressure. At the end of the modulation cycle, the pressure in the slugchamber moves the reduction valve a small distance up to restrict the flowof supply oil to the clutch. This is the "metering position" of thereduction valve spool. In this position, the valve maintains precisecontrol of the clutch pressure.

Primary pressure is adjusted with shims in the load piston. Final lockupclutch pressure is not adjustable. If the primary pressure is correct andfinal lockup clutch pressure is low, the load piston should be checked tomake sure that it moves freely in the selector piston. If the load pistonmoves freely, the load piston springs should be replaced.

• Lockup clutch atmaximum pressure

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1

3

4

5

2

The transmission charging pump supplies oil to the transmissionhydraulic control valve and the shift solenoids through the inlet port (1).Excess transmission charging oil either drops to the bottom of the housingto be scavenged or flows back to the torque converter housing through theoutlet hose (2).

The torque converter lockup clutch solenoid (3) is energized by the EPTCII when DIRECT DRIVE (lockup clutch ENGAGED) is required.Transmission charge pump supply oil flows through the small hose (4) tothe lockup clutch control valve. The lockup clutch control valve thenengages the lockup clutch.

The transmission charging pressure relief valve is part of the transmissionhydraulic control valve. The relief valve limits the maximum pressure inthe transmission charging circuit. Transmission charging pressure can bemeasured at the tap (5).

1. Transmissioncontrol valve supplyport

2. Transmissioncharging oil returnport

3. Torque converterlockup clutchsolenoid

4. Lockup clutch pilotoil hose

5. Transmissioncharging pressuretap

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1. Transmission clutchpressure taps

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3 2

1

Shown is the Individual Clutch Modulation (ICM) transmission hydrauliccontrol valve. Transmission clutch pressures are measured at the pressuretaps (1).

The transmission hydraulic control valve contains a priority valve. Thepriority valve controls the pressure that is directed to the selector pistonsin each of the clutch stations. The transmission priority valve pressurehas been increased from 1720 kPa (250 psi) to 2585 kPa (375 psi).Increasing the priority valve pressure also increases the charging pressureavailable to the lockup clutch valve.

The "D" Station (2) is used to control the dual stage relief valve settingfor the clutch supply pressure (shown on next slide).

The transmission lube pressure relief valve (3) limits the maximumpressure in the transmission lube circuit.

2. "D" Station controlsdual stage reliefvalve

• Priority valve pressureincreased

3. Transmission luberelief valve

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• ICM transmissionhydraulic controlvalve

DOWNSHIFTSOLENOID

UPSHIFTSOLENOID

LOCKUPSOLENOID

F

G

H

A

B

C

TRANSMISSIONCHARGING

FILTER

ROTARYSELECTOR

SPOOL

NEUTRALIZERVALVEPRIORITY

REDUCTIONVALVE

DOWNSHIFTPRESSURE

UPSHIFTPRESSURE

TRANSMISSION CASETORQUE CONVERTER

HOUSING

CHARGINGPUMP

LUBEPUMP SCAVENGE

PUMP

COOLERBYPASSVALVE

OILCOOLER

LUBRICATIONRELIEF VALVE

PUMPPRESSURE

TO TORQUE CONVERTERRELAY VALVE

SELECTOR VALVE GROUPRELIEF VALVE

TRANSMISSION ICMHYDRAULIC SYSTEM

LOCKUP DUALSTAGE RELIEF VALVE

LUBEPRESSURE

ON

PRESSURE CONTROLGROUP

PILOT OILPRESSURE

D

E

ROTARY ACTUATOR

N1

3

The transmission control group uses a dual stage relief valve for clutchsupply pressure. At high idle in torque converter drive, transmissioncharging pressure should be 3065 kPa (445 psi) maximum. At low idle intorque converter drive, transmission charging pressure should be 2480 kPa (360 psi) minimum.

Shown is a sectional view of the ICM transmission hydraulic controlvalve group. The rotary selector spool is in a position that engages twoclutches. Pump supply oil from the lockup solenoid flows to the selectorpiston in station "D." Station "D" reduces the pump supply pressure, andthe reduced pressure flows to the lower end of the relief valve. Providingoil pressure to the lower end of the relief valve reduces the clutch supplypressure.

During torque converter lockup (DIRECT DRIVE), clutch supplypressure is reduced to extend the life of the transmission clutch seals. At high idle in direct drive, clutch supply pressure should be 1620 + 240 - 100 kPa (235 + 35 - 15 psi). The correspondingtransmission charge pressure is reduced to 2205 ± 70 kPa (320 ± 10 psi).

• Dual stage relief valve

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1. Transmission lubesupply hose

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3

1

2

Oil flows from the transmission lube section of the torque converter andtransmission pump to the transfer gears through a hose (1). Transmissionlube oil flows through the transfer gears and the transmission to cool andlubricate the internal components.

The transmission lube oil temperature sensor (2) provides an input signalto the VIMS which informs the operator of the temperature of thetransmission lube oil.

The transmission lube pressure relief valve is in the transmission casenear the transmission hydraulic control valve. The relief valve limits themaximum pressure in the transmission lube circuit. Transmission lube oilpressure can be measured at the tap (3).

At HIGH IDLE, the transmission lube pressure should be 110 to 207 kPa (16 to 30 psi). At LOW IDLE, the transmission lube pressure should be 5 to 65 kPa (.5 to 10 psi).

2. Transmission lubeoil temperaturesensor

3. Transmission lubeoil pressure tap

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• Torque converter/transmissionhydraulic system

• Four section pump:

1. Transmissionscavenge

2. Torque convertercharging

3. Transmissioncharging

4. Transmission lube

SUCTIONSCREEN

RETURNSCREEN

TC LOCKUPVALVE

TC INLETRELIEFVALVE

TC OUTLETRELIEFVALVE

TC/TRANSPUMPS

TC OUTLETSCREEN

TORQUE CONVERTER/TRANSMISSION COOLER

TC LOCKUPVALVE

TC CHARGINGFILTER

TRANSMISSIONCHARGING

FILTER

TRANSMISSIONMAGNETICSCREENS

RETURNSCREEN

TORQUE CONVERTER ANDTRANSMISSION HYDRAULIC SYSTEM

Shown is the torque converter and transmission hydraulic system. A foursection torque converter and transmission pump is located at the rear ofthe torque converter. The four sections (from front to rear) are:

1. Transmission scavenge2. Torque converter charging3. Transmission charging4. Transmission lube

The transmission scavenge pump pulls oil through the magnetic screenslocated at the bottom of the transmission. The scavenged oil from thetransmission is transferred into the torque converter housing through thetransmission oil return screen.

The three rear sections of the torque converter and transmission pump pulloil from the torque converter housing sump. Most of the required oilsupply is pulled directly from the torque converter and transmission oilcooler return oil. The remaining required oil supply is drawn through asuction screen located in the bottom of the torque converter housing.

- 119 -

• Torque convertercharging section

STMG 6823/97

Oil from the torque converter charging section of the torque converter andtransmission pump flows through the torque converter charging filter tothe inlet relief valve mounted on the torque converter. The inlet reliefvalve limits the maximum pressure of the supply oil to the torqueconverter.

Torque converter charging oil either drops to the bottom of the housing orflows through the torque converter outlet relief valve. The outlet reliefvalve limits the pressure inside the torque converter.

Most of the oil from the torque converter outlet relief valve flows throughthe torque converter outlet screen to the torque converter and transmissionoil cooler located on the right side of the engine. Oil from the torqueconverter and transmission oil cooler returns to the torque converterhousing.

Oil from the transmission charging section of the torque converter andtransmission pump flows through the transmission charging filter. Fromthe filter, transmission charging oil flows in two directions:

- Transmission charging oil flows to the torque converter lockup clutchvalve located on top of the torque converter.

- Transmission charging oil also flows to the transmission controlvalves located on top of the transmission.

Excess transmission charging oil to the transmission control valves eitherdrops to the bottom of the housing to be scavenged or flows back to thetorque converter housing.

When the torque converter lockup clutch solenoid is energized, pumpsupply oil flows to the lockup clutch control valve. The lockup clutchcontrol valve then engages the lockup clutch.

Oil flows from the transmission lube section of the torque converter andtransmission pump to the transfer gears. Transmission lube oil flowsthrough the transfer gears and the transmission to cool and lubricate theinternal components.

• Transmissioncharging section

• Transmission lubesection

106

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• EPTC II shifts thetransmissionelectronically

STARTER SOLENOID

A11

A12A13

A14A15

A16

B10

B9

A17A18

A19

A36

A37

A10 A21 A22 A23 A24 A25

TOS

TRANSMISSION

A26 A3 A4 A5

EPTC II

A - 37 PIN CONNECTORB - 10 PIN SURE-SEAL CONNECTOR

A9

B1

B6

B7

A29

A30

A27

B5

A6

A20

A28

892 - BR

893 - GN

306 - GNE750 - PU

706 - BR

720 - PU

707 - PU

711 - BR

712 - WH

713 - OR714 - YL

715 - GN

716 - BU

307 - OR

D996 - PU

D997 - YL

218- BK

217 - BK

219 - BK

227 - BK

280 - BK

709

- O

R

710

- G

N

721 -

BR

722

- W

H

723

- O

R

724

- Y

L

725

- G

N

726

- B

U

703

- B

U

704

- G

Y

705

- P

K

DATA LINK

UP, DOWN,LOCKUPSOLENOIDS

450 - YL

452- PU

2

10

12

3 4 5

11

6 7 8 9

13

D1 D2 D3

BACK-UP ALARMRELAYB2 321- BR

MACHINEID CODE

RAISE SOLENOIDA31 G714 - PU

FLOAT SOLENOIDA32 G711 - BR

LOWER SOLENOIDG712- GNA33

SHIFTLEVERSWITCH

SERVICE "SET" AND"CLEAR" SWITCHES

CONVERTERSPEED

ENGINESPEED

KEY STARTSWITCH

BODY UP

SECONDARYBRAKE

HOISTLEVER

SERVICE/RETARDER

BRAKE

TRANSMISSIONGEAR SWITCH

Electronically Programmable Transmission Control (EPTC II)

The purpose of the EPTC II is to determine the desired transmission gearand energize solenoids to shift the transmission up or down as requiredbased on information from both the operator and machine.

The EPTC II receives information from various input components such asthe shift lever switch, Transmission Output Speed (TOS) sensor,transmission gear switch and the hoist lever switch.

Based on the input information, the EPTC II determines whether thetransmission should upshift, downshift, engage the lockup clutch or limitthe transmission gear. These actions are accomplished by sending signalsto various output components.

• Shifts controlled byelectrical signals

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Output components include the upshift, downshift and lockup solenoids,the back-up alarm and others.

The EPTC II also provides the service technician with enhanceddiagnostic capabilities through the use of onboard memory, which storespossible diagnostic codes for retrieval at the time of service.

With the use of a set of service switches, the service technician can accessthe different modes to gather the stored diagnostic codes or set theadjustable transmission gear limit functions.

Input and output components on the block diagram are accompanied witha letter and number. The letter A corresponds with the 37 pin connectorand the letter B corresponds with the 10 pin Sure-Seal connector that areattached to the transmission control. The numbers next to the letterscorrespond to the pin numbers in the connector. For example, the shiftlever switch is connected to the transmission control through six wires inthe 37 pin connector at pin locations 11 through 16.

The Advanced Diesel Engine Management (ADEM II) engine control, theAutomatic Retarder Control (ARC), the Vital Information ManagementSystem (VIMS) and the EPTC II all communicate with each otherthrough the CAT Data Link. Communication between the electroniccontrols allows the sensors of each system to be shared. Many additionalbenefits are provided, such as Controlled Throttle Shifting (CTS). CTSoccurs when the EPTC II tells the engine ECM to reduce engine fuelduring a shift to lower stress to the power train.

The EPTC II is also used to control the hoist system on the 793C. Severalchanges have been made to the input and output signals through the EPTCII 37 pin CE connector. The changes are:

1. The bed raise switch has been eliminated and an input signal is nolonger transmitted through Pin 7.

2. A Pulse Width Modulation (PWM) type position sensor providesthe hoist lever input signal to Pin 28.

3. A raise solenoid output signal has been added to Pin 31. Theoutput is a ground signal to a relay which sends +24 Volts to theraise solenoid.

4. A float solenoid output signal has been added to Pin 32. Theoutput is a ground signal to a relay which sends +24 Volts to thefloat solenoid.

5. A power down solenoid output signal has been added to Pin 33.The output is a ground signal to a relay which sends +24 Volts tothe power down solenoid.

• Benefits of electroniccommunication

• EPTC II used tocontrol hoist system

• EPTC II connectorsand pin numbers

• EPTC II outputs

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• Transmissionelectronic control

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STMG 6823/97

Shown is the Electronic Programmable Transmission Control (EPTC II).The EPTC II is located to the right of the operator’s seat in the centerconsole. The control contains a diagnostic window with 12 LightEmitting Diodes (LED’s) and a three digit numeric display.

The service switches (arrow) are used to interrogate the EPTC II forstored diagnostic information, event information and to program thetransmission top gear limit functions. The switches are labeled with an"S" for "SET" and a "C" for "CLEAR."

The DIAGNOSTIC MODE of the Electronic Control is changed byDEPRESSING and HOLDING both service switches (SET and CLEAR).When the desired mode is shown on the display, the switches can bereleased. By following the instructions in the Service Manual, theserviceman can determine if the transmission electronic control system isoperating correctly.

• Service switches(arrow)

• Diagnostic modeschanged with serviceswitches

108

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EPTC II DIAGNOSTIC WINDOW

D1 D2 D3

2

10

12

3 4 5

11

6 7 8 9

13

The onboard diagnostic window houses 12 status LED's along with athree digit numeric display.

The functions of the three digit display and the status LED's are:

1. Three digits (D1, D2, D3) display numbers and letters or indicatecircuit conditions.

2. DIAG PRESENT--A RED LED which indicates that the ElectronicControl has detected a fault for which a diagnostic code has beenstored in memory. The LED is ON if the fault is still present.

3. BODY UP--An AMBER LED which is ON when the body up switchis in use as sensed by a ground from the body up switch.

• EPTC II diagnosticwindow:

- 12 status LED's

- Three digit display

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4. RETARDER--An AMBER LED which is ON when the service brakeor retarder is in use as sensed by a ground from the service/retarderbrake pressure switch.

5. BRAKE--An AMBER LED which is ON when the secondary orparking brake is in use as sensed by an open from thesecondary/parking brake pressure switch.

6. BODY RAISE--An AMBER LED which is ON when the hoist leversensor is providing a signal to the electronic control.

7. HOLD--An AMBER LED which is ON when the hold pedal orswitch is in use as sensed by a ground from the hold pedal or switch.(Not used on Trucks.)

8. CONT FAILURE--A RED LED which is ON or FLASHING whenthe electronic control has FAILED and should be replaced.

9. POWER--A GREEN LED which is ON when a nominal 24 Volts isavailable between pins 1 and 2 of the electronic control 37 pinconnector.

10. TOS--An AMBER LED which is ON when the Transmission OutputSpeed (TOS) sensor is providing a signal to the electronic control.

11. TCO--An AMBER LED which is ON when the Torque ConverterOutput (TCO) speed sensor is providing a signal to the electroniccontrol.

12. EOS--An AMBER LED which is ON when the Engine Output Speed(EOS) sensor is providing a signal to the electronic control.

13. MODE 1--An AMBER LED which is ON when the electroniccontrol is NOT in Mode 0.

NOTE: The small LED at the bottom right of the three digit displayhas no diagnostic function. The small LED will always be ON.Service personnel should always view the diagnostic window with thesmall LED at the bottom right of the three digit display. When thesmall LED is at the bottom right of the three digit display, servicepersonnel know that the window is being viewed in the correctorientation.

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The Electronic Control Analyzer Programmer (ECAP) and the ElectronicTechnician (ET) Service Tools can be used in place of the EPTC IIdiagnostic window. The ECAP and ET perform the same functions as theEPTC II diagnostic window and are capable of several additionaldiagnostic functions that the EPTC II window does not display.

Additional diagnostic functions that the service tools can perform are:

- Display the EPTC II internal clock hour reading.

- Display the hour reading of the first and last occurrence for eachlogged diagnostic code.

- Display the definition for each logged diagnostic code.

- Display logged events.

- Display the lockup clutch engagement counter.

- Display the transmission gear shift counter.

• ECAP and ET servicetools

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1. Transmission gearswitch

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4

3

2

1

Shown is an example of one input component to the EPTC II and threeoutput components from the EPTC II.

The transmission gear switch (1) provides input signals to the EPTC II.The transmission gear switch inputs (also referred to as the actual gearinputs) are comprised of six wires. Five of the six wires provide a code tothe EPTC II. The code is unique for each position of the transmissiongear switch. Each transmission gear switch position will result in two ofthe five wires sending a ground signal to the EPTC II. The other threewires will remain open (ungrounded). The pair of grounded wires isunique for each gear position. The sixth wire is known as the "GroundVerify" wire, which is normally grounded. The "Ground Verify" wire isused by the EPTC II to verify that the transmission gear switch isconnected to the transmission control. The "Ground Verify" wire allowsthe EPTC II to distinguish between loss of the transmission gear switchsignals and a condition in which the transmission gear switch is betweengear detent positions.

Earlier transmission gear switches use a wiper contact assembly that doesnot require a power supply to Pin 4 of the switch. Present transmissiongear switches are Hall-Effect type switches. A power supply is requiredto power the switch. A small magnet passes over the Hall cells whichthen provide a non-contact position switching capability. The Hall-Effecttype switches use the same 10-Volt power supply as the transmissionoutput speed sensor.

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The solenoid outputs provide + Battery voltage to the upshift solenoid (2)or the downshift solenoid (3) based on the input information from theoperator and the machine. The solenoids are energized until thetransmission actual gear switch signals the EPTC II that a new gearposition has been reached. The length of time that the solenoid isenergized is usually about 0.1 seconds when a single gear upshift isdesired.

The lockup solenoid output provides + Battery voltage to the lockupclutch solenoid (4). The lockup solenoid is energized by the EPTC IIwhen in a DIRECT DRIVE gear. In FIRST gear, the solenoid will beenergized when the Transmission Output Speed (TOS) reaches apredetermined value. When the machine is in CONVERTER DRIVE, thesolenoid is de-energized by the EPTC II.

2. Upshift solenoid

3. Downshift solenoid

4. Lockup clutchsolenoid

111

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793C

STEERING

STEERING SYSTEM

This section of the presentation explains the operation of the steeringsystem. As on other Caterpillar Off-highway Trucks, the steering systemuses hydraulic force to change the direction of the front wheels. Thesystem has no mechanical connection between the steering wheel and thesteering cylinders.

- 129 -

• Steering tank

1. Upper sight gauge

2. Lower sight gauge

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STMG 6823/97

6

3

7

1

2

5

4

The steering tank is located on the right platform. Two sight gauges areon the side of the tank. When the engine is shut off and the oil is cold, theoil should be visible between the FULL and ADD OIL markings of theupper sight gauge (l). When the engine is running and the accumulatorsare fully charged, the oil level should not be below the ENGINERUNNING marking of the lower sight gauge (2). If the ENGINERUNNING level is not correct, check the nitrogen charge in eachaccumulator. A low nitrogen charge will allow excess oil to be stored inthe accumulators and will reduce the secondary steering capacity.

A combination vacuum breaker/pressure relief valve is used to limit thetank pressure. Before removing the fill cap, be sure that the engine wasshut off with the key start switch and the oil has returned to the tank fromthe accumulators. Depress the pressure release button (3) on the breatherto vent any remaining pressure from the tank.

Supply oil for the steering system is provided by a piston-type pump.Case drain oil from the pump returns to the tank through the filter (4).The remaining steering system oil returns to the tank through the mainsteering filter (5). Both filters are equipped with bypass valves to protectthe system if the filters are plugged or during cold oil start-up.

4. Case drain oil filter

5. Main steering filter

3. Combinationvacuum breaker/relief valve andpressure releasebutton

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If the steering pump fails or if the engine cannot be started, the connector (6) is used to attach an Auxiliary Power Unit (APU). The APUwill provide supply oil from the steering tank at the connector (6) tocharge the steering accumulators. Steering capability is then available totow the truck.

The steering oil temperature sensor (7) provides an input signal to theVIMS which informs the operator of the steering system oil temperature.

INSTRUCTOR NOTE: For more detailed information on servicingthe steering accumulators, refer to the Service Manual Module "793COff-highway Truck Steering System" (Form SENR1452) and theSpecial Instruction "Repair of 4T8719 Bladder Accumulator Group" (Form SEHS8757). For more information on using the APU, refer tothe Special Instructions "Using 1U5000 Auxiliary Power Unit (APU)"(Form SEHS8715) and "Using the 1U5525 Attachment Group"(Form SEHS8880).

6. APU supplementalsteering connector

7. Steering oiltemperature sensor

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2

4

3

1

The 793C is equipped with a load sensing, pressure compensated, piston-type pump (1). The steering pump is mounted to the pump drive.The pump drive is located on the inside of the right frame rail near thetorque converter.

The steering pump operates only when the engine is running and providesthe necessary flow of oil to the accumulators for steering systemoperation. The steering pump contains a load sensing controller (2) thatworks with an accumulator charging valve to monitor and control steeringpump output.

The steering pump will produce flow at high pressure until the steeringaccumulators are charged with oil and the pressure increases to 21400 ± 345 kPa (3100 ± 50 psi) at LOW IDLE. This pressure is referredto as the CUT-OUT pressure. When the CUT-OUT pressure is reached,the accumulator charging valve reduces the load sensing signal pressureto the pump load sensing controller, and the pump will destroke to theLOW PRESSURE STANDBY position. During LOW PRESSURESTANDBY, the pressure should be between 2410 and 3445 kPa (350 and 500 psi).

The pump operates at minimum swashplate angle to supply oil forlubrication, leakage and Hand Metering Unit (HMU) "thermal bleed."Because of the normal leakage in the steering system, the pressure in theaccumulators will gradually decrease to 19200 ± 315 kPa (2785 ± 45 psi).This pressure is referred to as the CUT-IN pressure.

1. Steering pump

• CUT-OUT pressure

• LOW PRESSURESTANDBY

2. Load sensingcontroller

• CUT-IN pressure

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When the pressure in the accumulators decreases to the CUT-IN pressure,the accumulator charging valve blocks the load sensing signal line to theload sensing controller from returning to the tank, and the pump willupstroke to maximum displacement (full flow).

A pressure tap (3) is located on the pump pressure switch manifold. Ifsteering pump pressure is measured at this tap during LOW PRESSURESTANDBY, a gauge acceptable for testing maximum steering systempressure must be used to avoid damaging the gauge when the steeringpump upstrokes to provide maximum oil flow.

Two pressure switches monitor the condition of the steering system on the793C. One switch (4) monitors the output of the steering pump. Thepurpose of this switch is to monitor pump supply pressure during LOWPRESSURE STANDBY. The VIMS refers to this switch as the "lowsteering pressure" switch.

The other steering pressure switch is mounted on the solenoid and reliefvalve manifold, which is located on the front frame rail below the engine.This switch monitors the steering system accumulator pressure. TheVIMS refers to this switch as the "high steering pressure" switch.

Both steering pressure switches provide input signals to the VIMS whichinforms the operator of the condition of the steering system. A steeringsystem warning is only displayed if the ground speed is above 8 km/h (5 mph).

4. Low steeringpressure switch

4. High steeringpressure switch

• Steering pressurewarnings only above8 km/h (5 mph)

3. LOW PRESSURESTANDBY pressuretap

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5

4

6

2

13

Steering pump supply oil flows through a check valve (1) to the solenoidand relief valve manifold (2). The solenoid and relief valve manifoldconnects the steering pump to the accumulator charging valve (3), theaccumulators and the steering directional valve (4). The solenoid andrelief valve manifold also provides a path to drain for the steering oil.

When checking the steering system CUT-OUT and CUT-IN pressures, agauge can be connected at the pressure tap (5).

Steering system oil samples can be taken at the steering system ScheduledOil Sampling (S•O•S) tap (6).

1. Check valve

2. Solenoid and reliefvalve manifold

3. Accumulatorcharging valve

4. Steering directionalvalve

5. Steering systempressure tap

6. Steering systemS•O•S tap

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3

2

1

Shown is a closer view of the accumulator charging valve (1). Steeringsystem CUT-OUT pressure is adjusted at the upper valve (2). Steeringsystem CUT-IN pressure is adjusted at the lower valve (3).

Steering pump supply pressure increases until the accumulator pressureacting on the accumulator charging valve shifts the cut-out and cut-inpressure valves. Together, the cut-out and cut-in pressure valves reducethe Load Sensing (LS) signal pressure (feedback pressure) to slightlyabove tank pressure. The pump is destroked to LOW PRESSURESTANDBY (CUT-OUT).

When the pressure in the accumulators decreases, the cut-in and cut-outpressure valves shift again and block the load sensing signal pressurefrom the tank. The pump load sensing signal pressure becomes equal topump pressure, and the steering pump returns to the FULL FLOWposition (CUT-IN).

1. Accumulatorcharging valve

2. CUT-OUT pressurevalve

3. CUT-IN pressurevalve

116

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• Steering pumpoperation

• Actuator pistondrained duringmaximum flow

PUMP OUTPUT

LOAD SENSINGPRESSURE

ACTUATOR PISTON

LOAD SENSINGCONTROLLER

SWASHPLATEPISTON

FLOWCOMPENSATOR

DURING CHARGING (CUT-IN)STEERING PUMP

ACCUMULATORCHARGING VALVE

TO ACCUMULATORS

FROM ACCUMULATORSCUT-OUTVALVE

CUT-INVALVE

HIGH PRESSURECUTOFF VALVE

After the engine is started, pressure increases in the steering accumulators.The pump load sensing controller is spring biased to vent the actuatorpiston pressure to drain. Venting pressure from the load sensing controllerand the actuator piston positions the spring biased swashplate tomaximum displacement (full flow).

As pressure increases in the accumulators, pump supply pressure is sensedin the accumulator charging valve and on both ends of the flowcompensator. When pressure is present on both ends of the flowcompensator, the swashplate is kept at maximum angle by the force of thespring in the pump housing and pump discharge pressure on theswashplate piston. The pistons travel in and out of the barrel andmaximum flow is provided through the outlet port. Since the pump isdriven by the engine, engine rpm also affects pump output.

NOTE: Because the signal lines are sensing pump supply pressureand not a "load" pressure, the steering system does not operate thesame as other load sensing systems with a margin pressure.

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• Accumulator chargingvalve shifts

• Signal pressuredecreases

PUMP OUTPUT

LOAD SENSINGPRESSURE

LOAD SENSINGCONTROLLER

FLOWCOMPENSATOR

SWASHPLATEPISTON

ACTUATORPISTON

LOW PRESSURE STANDBY (CUT-OUT)STEERING PUMP

TO ACCUMULATORS

FROM ACCUMULATORS

ACCUMULATORCHARGING

VALVE

CUT-OUTVALVE

CUT-INVALVE

HIGH PRESSURECUTOFF VALVE

Pump supply pressure will increase until the accumulator pressure actingon the accumulator charging valve shifts the cut-out and cut-in valves, andthe load sensing signal pressure is reduced to slightly above tank pressure.The cut-out and cut-in valves shift when the pump outlet pressure isapproximately 21400 ± 345 kPa (3100 ± 50 psi) at LOW IDLE.

Pump oil (at LOW PRESSURE STANDBY) flows past the lower end ofthe displaced flow compensator spool to the actuator piston. The actuatorpiston has a larger surface area than the swashplate piston. The oilpressure at the actuator piston overcomes the spring force of theswashplate piston and moves the swashplate to destroke the pump. Thepump is then at LOW PRESSURE STANDBY (cut-out). Pump outputpressure is equal to the setting of the flow compensator. The LOWPRESSURE STANDBY setting is between 2410 and 3445 kPa (350 and 500 psi).

• Pump at LOWPRESSURE STANDBY

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In the NEUTRAL or NO STEER position, demand for oil from theaccumulators is low. The pump operates at minimum swashplate angle tosupply oil for lubrication, leakage and HMU "thermal bleed." Because ofthe normal leakage in the steering system, the pressure in theaccumulators will gradually decrease to 19200 ± 315 kPa (2785 ± 45 psi).

When the pressure in the accumulators decreases to 19200 ± 315 kPa(2785 ± 45 psi), the accumulator charging valve cut-in and cut-out valvesshift and block the load sensing signal line pressure from the tank. Pumpoil pressurizes the load sensing signal line. The load sensing signal shiftsthe flow compensator spool and drains the actuator piston. Draining theactuator piston positions the spring biased swashplate to maximumdisplacement and full flow (CUT-IN).

At LOW lDLE in the NEUTRAL or NO STEER position, the pump willcycle between the cut-out and cut-in conditions in intervals of 30 secondsor more. Connecting a pressure gauge to the pressure tap below thesteering directional valve will indicate these steering system pressures. Ifthe pump pressure cycles in less than 30 seconds, leakage exists in thesystem and must be corrected. Typical sources of leakage can be theaccumulator bleed down solenoid or the back-up relief valve located onthe solenoid and relief valve manifold. If a machine has an HMU withthe thermal bleed orifice removed, the cycle time between cut-out andcut-in will be between 6 and 7 minutes.

If the accumulator charging pressure cannot be adjusted withinspecifications, an adjustment of the high pressure cutoff valve is required.The high pressure cutoff valve is part of the load sensing controllermounted on the steering pump. The high pressure cutoff setting is 23100 ± 345 kPa (3350 ± 50 psi) at HIGH IDLE. The high pressurecutoff setting must be a minimum of 350 kPa (50 psi) higher than theaccumulator charging (cut-out) valve setting at HIGH IDLE.

To adjust the high pressure cutoff valve on the load sensing controller,turn the cut-out valve adjustment screw completely in and count thenumber of turns so it can be returned to its original position later. Withthe engine at HIGH IDLE, adjust the high pressure cutoff valve to 23100 ± 345 kPa (3350 ± 50 psi). Return the cut-out valve adjustmentscrew to its original position and re-test the cut-out and cut-in valvepressures.

NOTE: When testing or adjusting any steering system pressuresettings, always allow the accumulator charge cycle to occur at leastten times before measuring the pressure. Failure to allow thecharging cycle to occur ten times will result in inaccurate readings.

• Accumulator pressuredecreases

• Cut-in and cut-outvalves shift

• Pump returns to fullflow

• High pressure cutoffvalve adjustment

• Cycle time betweenCUT-OUT and CUT-IN:

- With thermal bleedorifice, 30 secondsor more

- Without thermalbleed orifice,between 6 and 7minutes

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• Accumulator chargingvalve

FROM PUMP TO PUMP CONTROLSIGNAL PORT

CUT-OUT VALVE

CUT-IN VALVE

FROMACCUMULATOR

TO TANK

FEEDBACK ORIFICE

ACCUMULATOR CHARGE VALVEDURING CHARGING (CUT-IN)

Shown is a sectional view of the accumulator charging valve duringCHARGING (CUT-IN).

During CHARGING, the cut-out spool is held to the right by the spring.The cut-out spool blocks the pump and load sensing signal passages fromthe feedback orifice. Signal pressure is equal to pump pressure and thehigh signal pressure causes the pump to upstroke to maximumdisplacement (full flow).

As accumulator pressure increases, the cut-out spool will move to the leftagainst the spring force. When accumulator pressure reaches the cut-outsetting, the cut-out spool will open the pump and load sensing signalpassages to the feedback orifice. The feedback orifice reduces the loadsensing signal pressure to slightly more than tank pressure.

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• Accumulator chargingvalve

FROM PUMP TO PUMP CONTROLSIGNAL PORT

CUT-OUT VALVE

CUT-IN VALVE

FROMACCUMULATOR

TO TANK

FEEDBACK ORIFICE

ACCUMULATOR CHARGE VALVELOW PRESSURE STANDBY (CUT-OUT)

Shown is a sectional view of the accumulator charging valve in the LOWPRESSURE STANDBY (CUT-OUT) position.

In the CUT-OUT position, accumulator pressure has increased to the cut-out setting and both the cut-in and cut-out stems are fully shifted againstthe springs. The pump and load sensing signal passages are open to thefeedback orifice. The feedback orifice reduces the signal pressure toslightly more than tank pressure.

The feedback orifice is only required to initiate and maintain CUT-OUT.As the accumulator pressure decreases, the feedback pressure holds thecut-out spool to the left until the cut-in valve opens and vents the feedbackpressure to the tank. The feedback pressure during CUT-OUT assistsshifting against the spring. At the beginning of CUT-IN, the feedbackpressure assists the spring force.

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• Accumulator chargingvalve

FROM PUMP TO PUMP CONTROLSIGNAL PORT

CUT-OUT VALVE

CUT-IN VALVE

FROMACCUMULATOR

TO TANK

FEEDBACK ORIFICE

ACCUMULATOR CHARGE VALVEBEGINNING STAGE OF CUT-IN

Shown is a sectional view of the accumulator charging valve in thebeginning stage of CUT-IN.

When accumulator pressure decreases to the cut-in pressure, the cut-inspool will move to the right and allow feedback pressure into the cut-invalve and cut-out valve spring chambers. The feedback pressure assiststhe cut-out and cut-in valve springs with shifting the cut-out and cut-inspools to the right.

The cut-in spool continues to move to the right and blocks the centerpassage to the cut-out spool. When the center passage to the cut-out spoolis blocked, signal pressure becomes equal to pump pressure.

CUT-IN will occur when the cut-out spool shifts to a position in which thepump load sensing signal is no longer vented to feedback pressure. Signalpressure becomes equal to pump pressure, the pump upstrokes and thecharging cycle begins.

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1. Check valve

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STMG 6823/97

1

6

2

1

3

45

Steering pump supply oil flows through a check valve (1) to the solenoidand relief valve manifold. The solenoid and relief valve manifoldconnects the steering pump to the accumulator charging valve, theaccumulators and the steering directional valve. The solenoid and reliefvalve manifold also provides a path to drain for the steering oil.

A steering accumulator pressure switch (2), an accumulator bleed downsolenoid (3), a back-up relief valve (4), a steering system Scheduled OilSampling (S•O•S) tap (5) and a supplemental steering connector (6) arelocated on the solenoid and relief valve manifold.

The check valve (1) prevents accumulator oil from flowing back to thesteering pump when the pump destrokes to LOW PRESSURESTANDBY.

The steering accumulator pressure switch (2) monitors the steeringaccumulator pressure. The VIMS refers to this switch as the "HighSteering Pressure" switch.

The steering accumulator pressure switch provides an input signal to theVIMS which informs the operator of the steering system condition. Asteering system warning is displayed only if the ground speed is above 8 km/h (5 mph).

2. Steeringaccumulatorpressure switch

3. Accumulator bleeddown solenoid

4. Back-up relief valve

5. Steering systemS•O•S tap

6. Supplementalsteering connector

• Steering pressurewarnings only above8 km/h (5 mph)

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The accumulator bleed down solenoid (3) is used to drain pressure oilfrom the accumulators when the truck is not in operation.

The back-up relief valve (4) is used to drain pressure oil if the steeringpump high pressure cutoff valve does not open.

Steering system oil samples can be taken at the steering system ScheduledOil Sampling (S•O•S) tap (5)

To operate the steering circuit on a disabled truck, an Auxiliary PowerUnit (APU) connects to the supplemental steering connector (6) on thesolenoid and relief valve manifold and to a suction port on the hydraulictank (see Slide No. 112). The APU will provide supply oil to charge theaccumulators. Steering capability is then available to tow the truck.

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• Solenoid and reliefvalve manifold

BACK-UP RELIEF VALVE

BLEED DOWNSOLENOID

TO TANK

SUPPLY FROM PUMPAND ACCUMULATORS

SOLENOID AND RELIEF VALVE MANIFOLD

Shown is a sectional view of the solenoid and relief valve manifold. Theaccumulator bleed down solenoid is activated by the bleed down solenoidshutdown control when the key start switch is moved to the OFF position.The bleed down solenoid shutdown control holds the solenoid open for 70 seconds.

Pressure oil from the accumulators is sensed by the bleed down solenoid.When the solenoid is energized, the plunger moves and connects thepressure oil to the drain passage. Pressure oil flows through an orifice,past the plunger, to the tank. The orifice limits the return oil flow fromthe accumulators to a rate which is LOWER than the flow limit(restriction) of the steering oil filter in the hydraulic tank. When thesolenoid is de-energized, spring force moves the plunger and pressure oilcannot go to drain.

• Bleed down solenoiddrains accumulators

- 144 -

• Back-up relief valveprotects system ifpump does notdestroke

STMG 6823/97

The back-up relief valve protects the steering system if the steering pumpmalfunctions (fails to destroke). Pressure oil from the steering pumpworks against the end of the back-up relief valve and the spring. Therelief valve unseats (opens) if oil pressure reaches approximately 26000 ± 400 kPa (3775 ± 60 psi) at a flow of 8 ± 2 L/min. (2 ± .5 gpm).Oil then flows past the relief valve and drains to the tank.

The back-up relief valve must only be adjusted on a test bench. Thepressure setting of the back-up relief valve can be changed by adjustingthe spring force that keeps the relief valve seated (closed). To change therelief valve setting, remove the protective cap and turn the adjustmentscrew clockwise to increase or counterclockwise to decrease the pressuresetting. One revolution of the setscrew will change the pressure setting3800 kPa (550 psi).

A functional test of the back-up relief valve can be performed on themachine by installing a manual hydraulic pump at the location of theAuxiliary Power Unit (APU) connector and installing blocker plates toprevent oil from flowing to the accumulators. See the service manual formore detailed information.

NOTE: Using the functional test procedure to adjust the back-uprelief valve will provide only an approximate setting. Accuratesetting of the back-up relief valve can only be performed on ahydraulic test bench.

• Adjust back-up reliefvalve on test benchonly

• Functional test ofback-up relief valve(on machine)

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1. Steering directionalvalve

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STMG 6823/97

2

1

The steering directional valve (1) is pilot operated from the HMU in theoperator’s station. Five pilot lines connect these two components. Thepilot lines send pilot oil from the HMU to shift the spools in the steeringdirectional valve. The spools control the amount and direction of pressureoil sent to the steering cylinders. Four pilot lines are used for pumpsupply, tank return, left turn and right turn. The fifth pilot line is for theload sensing signal.

When checking the steering system cut-out and cut-in pressures, a gaugecan be connected at the pressure tap (2).

2. Steering systempressure tap

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• Steering directionalvalve components:

- Priority spool

- Amplifier spool withcombiner/checkspool

- Directional spool

- Relief/makeup valves

- Back pressure valve

RELIEF/MAKEUPVALVE

LEFT TURNCYLINDER BACK PRESSURE

VALVE

LEFT TURN PILOT OIL

AMPLIFIER SPOOL

RIGHT TURN PILOT OIL

COMBINER/CHECKSPOOL

LOAD SENSING PORTFROM

ACCUMULATOR

PRIORITY SPOOL

STEERING DIRECTIONAL VALVE

RIGHT TURNCYLINDER

RELIEF/MAKEUPVALVE

HAND METERINGUNIT SUPPLY ANDTHERMAL BLEED

NO TURN

TO TANK

Shown is a sectional view of the steering directional valve. The maincomponents of the steering directional valve are: the priority spool, theamplifier spool with internal combiner/check spool, the directional spool,the relief/makeup valves and the back pressure valve.

Pressure oil from the accumulators flows past the spring biased priorityspool and is blocked by the amplifier spool. The same pressure oil flowsthrough an orifice to the right end of the priority spool. The orificestabilizes the flow to the priority spool and must be present to open andclose the priority spool as the flow demand changes. The same pressureoil flows to the HMU. After all the passages fill with pressure oil, thepriority spool shifts to the left, but remains partially open. In thisposition, the priority spool allows a small amount of oil flow (thermalbleed) to the HMU and decreases the pressure to the HMU supply port.The lower pressure prevents the HMU from sticking.

- 147 -STMG 6823/97

With the truck in the NEUTRAL or NO TURN position, all four workingports (supply, tank, right turn and left turn) are vented to the tank throughthe HMU. The directional spool is held in the center position by thecentering springs.

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• Steering directionalvalve during a RIGHTTURN

AMPLIFIER SPOOL

BACK PRESSURE VALVE

LEFT TURN PILOT OILRIGHT TURN PILOT OIL

COMBINER/CHECK SPOOL

LOAD SENSING PORT

FROMACCUMULATOR

HAND METERINGUNIT SUPPLY ANDTHERMAL BLEED

PRIORITY SPOOL

STEERING DIRECTIONAL VALVERIGHT TURN

RELIEF/MAKEUP VALVE

LEFT TURNCYLINDER

RIGHT TURNCYLINDER

RELIEF/MAKEUP VALVE

TO TANK

When the steering wheel is turned to the RIGHT, the "thermal bleed" andventing of the four work ports to the tank is stopped. The increasedsupply pressure flows to the HMU and the load sensing pilot line. Theload sensing pilot line directs cylinder pressure to the priority spool in thedirectional valve. Cylinder pressure is present in the HMU because pilotoil combines with accumulator oil in the combiner/check valve spool inthe directional valve. The increased pressure in the load sensing linecauses the priority spool to move to the right and allows more oil to flowto the HMU through the supply line. The load sensing pump supplypressure varies with the steering load. The priority spool movesproportionally, allowing sufficient oil flow to meet the steeringrequirements.

Pilot oil flows through a stabilizing orifice to the right turn pilot port ofthe directional valve and moves the directional spool. Movement of thedirectional spool allows pilot oil to flow to the amplifier andcombiner/check spools.

• Pilot oil movesdirectional spool

• Load sensing pilotpressure movespriority spool

- 149 -

• Pilot oil movesamplifier spool

STMG 6823/97

The pilot oil divides at the amplifier spool. Pilot oil flows through anarrow groove around the combiner/check spool. The pilot oil ismomentarily blocked until the amplifier spool moves far enough to theright to allow partial oil flow through one of eight orifices.

Pilot oil also flows through a connecting pin hole and a stabilizing orificeto the left end of the amplifier spool and causes the amplifier spool tomove to the right. Accumulator oil at the spring end (right end) of theamplifier spool flows through a mid-connecting pin to the left end of theamplifier spool and also causes the amplifier spool to move to the right.

When the amplifier spool moves to the right, accumulator oil flows to theinner chamber, forcing the combiner/check spool to the left. Accumulatoroil then flows through seven of the eight orifices. Pilot and accumulatoroil combine. Oil flows across the directional spool (which has alreadyshifted) for a RIGHT TURN.

The faster the steering wheel is turned, the farther the directional spooland the amplifier spool are shifted. A higher flow rate is available, whichcauses the truck to turn faster. The ratio of pilot and pump supply oil thatcombine is always the same because one orifice is dedicated to pilot flowand seven orifices are dedicated to accumulator supply flow.

Return oil from the cylinders flows across the directional spool, aroundthe relief/makeup valve, forces the back pressure valve open and returnsto the tank.

During a turn, if a front wheel strikes a large obstruction that cannotmove, oil pressure in that steering cylinder and oil line increases. Oilflow to the cylinder is reversed. This pressure spike is felt in theamplifier spool. The combiner/check spool moves to the right and blocksthe seven pump supply oil orifices to the steering cylinders. Theamplifier spool moves to the left and blocks the pilot oil orifice. Pilot oilflow to the steering cylinders stops. The pressure spike is not felt at theHMU. If the pressure spike is large enough, the relief/makeup valvedrains the pressure oil to the tank as previously described.

• Turning steeringwheel faster providesmore flow to cylinders

• Pressure spike movescombiner/check spooland blocks flow toHMU

• Pilot and accumulatoroil combine incombiner/check spool

- 150 -

• HMU (arrow)

126

STMG 6823/97

The Hand Metering Unit (HMU) (arrow) is located at the base of thesteering column behind a cover at the front of the cab. The HMU isconnected to the steering wheel and controlled by the operator.

The HMU meters the amount of oil sent to the steering directional valveby the speed at which the steering wheel is turned. The faster the HMU isturned, the higher the flow sent to the steering cylinders, and the faster thewheels will change direction.

• Meters oil todirectional valve

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• HMU in NEUTRAL

CONTROLSECTION

SLEEVE

SPOOL

DRIVEPIN

L R PT

THERMAL BLEEDPASSAGES

CENTERINGSPRINGS

METERINGSECTION

ROTOR

STATOR

HAND METERING UNITNEUTRAL POSITION

Shown is a sectional view of the HMU in the NEUTRAL (NO TURN)position. The metering section is a small hydraulic pump which producesa specific (metered) amount of oil flow. This metered oil is then directedby the control section to the left or right turn port. As the steering wheelis turned faster, the flow of oil increases. More oil is sent to the steeringcylinders, which allows the cylinders to move faster.

When the steering wheel is in the NEUTRAL position (steering wheelstationary), the holes in the sleeve and the passages in the spool are notaligned. However, a small amount of pump oil from the inlet is allowedto flow through the center of the HMU. This small amount of oil flow(thermal bleed) keeps the HMU full and ready for a quick response tosteering demands. The thermal bleed oil also helps keep the HMU warmduring cold weather operation.

- 152 -STMG 6823/97

When the steering wheel is turned, the spool, pin and drive start to turn.The sleeve does not turn at the same time because the diameter of theholes for the pin in the sleeve is slightly larger than the diameter of thepin. The slight delay in sleeve movement allows the spool to turn farenough inside the sleeve to align the holes in the sleeve with the groovesin the spool. The oil path for thermal bleed is then blocked by the rotationof the spool and the sleeve.

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• HMU during RIGHTTURN

METERINGSECTION

CONTROLSECTION

SLEEVE

SPOOL

DRIVEPIN

ROTOR

L R PT

STATOR

CENTERINGSPRINGS

HAND METERING UNITRIGHT TURN

When the steering wheel is turned to the RIGHT and the holes in thesleeve are aligned with the grooves in the spool, pump oil (P) at the inletflows through the holes in the sleeve and the grooves in the spool. The oilin the grooves goes through other holes in the sleeve and into the lowerpassage. Oil flows through the lower passage to the metering section andis then directed into a space between the stator and the rotor.

The rotor is splined to the drive. As the drive turns, the rotor turns anddirects oil through the upper passage. The metered oil flows through theholes in the sleeve, the grooves in the spool and out of the sleeve throughthe right turn port (R). Metered oil from the port goes to the steeringdirectional valve.

Return oil from the steering cylinders flows through the tank port (T) inthe HMU to the steering tank.

- 154 -STMG 6823/97

When the steering wheel rotation is stopped, the spool, pin, drive androtor stop turning. The centering springs that were compressed when thespool was moving now bring the spool and sleeve back to a NEUTRALposition. The holes in the sleeve no longer align with the grooves in thespool. Oil flow from the pump stops. Pilot oil to the steering directionalvalve stops, causing the wheels to remain stationary.

- 155 -

• Steering accumulators(arrow)

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STMG 6823/97

Two steering accumulators (arrow) provide the supply oil during normaloperation and temporary secondary steering if a loss of pump oil flowoccurs.

Inside the accumulators is a rubber bladder that is charged with nitrogen.The nitrogen charge provides energy for normal steering and secondarysteering capability if steering pump flow stops.

To check the secondary steering system, the engine must be shut off withthe manual shutdown switch while leaving the key start switch in the ONposition. When the manual shutdown switch is used, the bleed downsolenoid is not energized and the accumulators do not bleed down. Thetruck can then be steered with the engine stopped.

INSTRUCTOR NOTE: More detailed information on servicing thesteering accumulators is in the Special Instruction "Repair of 4T8719Bladder Accumulator Group" (Form SEHS8757).

High pressure oil remains in the accumulators if the manualshutdown switch is used. To release the oil pressure in theaccumulators, turn the key start switch to the OFF position and turnthe steering wheel left and right until the oil is drained from theaccumulators (steering wheel can no longer be turned).

WARNING

- 156 -

• Shutdown control(arrow)

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STMG 6823/97

Shown is the shutdown control (arrow) for the steering accumulator bleeddown solenoid. The control is located in the cab below the centerconsole.

The steering accumulator bleed down solenoid is activated by the controlwhen the key start switch is moved to the OFF position. The bleed downsolenoid shutdown control holds the solenoid open for 70 seconds.

131

• Hoist systemcontrolled by EPTC II

HOIST SYSTEM

The hoist system on the 793C Off-highway Truck is electronicallycontrolled by the Electronic Programmable Transmission Control (EPTC II).

The hoist control system operates similarly to the previous trucks. Thefour operating positions are: RAISE, HOLD, FLOAT and LOWER.

- 157 -STMG 6823/97

793C

HOIST SYSTEM

132

The operator controls the hoist lever (arrow). The four positions of thehoist lever are RAISE, HOLD, FLOAT and LOWER.

The truck should normally be operated with the hoist lever in the FLOATposition. Operating with the hoist lever in the FLOAT position allows thehoist valve to provide some downward hydraulic pressure on the hoistcylinders and prevents an empty body from bouncing on rough haulroads.

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• Hoist lever (arrow)

• Hoist lever normally inFLOAT position

• Hoist control positionsensor (arrow)

133

The hoist lever controls a Pulse Width Modulated (PWM) position sensor.The PWM sensor sends duty cycle input signals to the EPTC II.Depending on the position of the sensor and the corresponding duty cycle,one or two relays located behind the cab are energized. The relays thensend +24 Volts to one or two of the three solenoids located on the hoistvalve. The hoist valve is mounted on the frame near the right hoistcylinder.

The hoist lever sensor also replaces the body raise switch (transmissionneutralizer switch) that was located behind the operator’s seat. The hoistlever sensor performs three functions:

- Raises and lowers the body.- Neutralizes the transmission in REVERSE.- Starts a cycle for the Truck Production Management System(TPMS).

INSTRUCTOR NOTE: To see the hoist system input and outputconnections to the EPTC II, refer to Slide No. 106.

- 159 -STMG 6823/97

• Sensor energizesthree solenoids onhoist valve

• Sensor performs threefunctions:

- Raises and lowersbody

- Neutralizestransmission inREVERSE

- Starts cycle forTPMS HOIST LEVER POSITION DUTY CYCLE RANGE

RAISE 15% TO 29%

HOLD 33% TO 51%

FLOAT 55% TO 70%

LOWER 76% TO 90%

• Hoist and brake oilhydraulic tank

• Oil level sight gauges(arrows)

134

Shown is the hoist and brake oil hydraulic tank and the oil level sightgauges (arrows). The oil level is normally checked with the upper sightgauge. The oil level should first be checked with cold oil and the enginestopped. The level should again be checked with warm oil and the enginerunning.

The lower sight gauge is used when filling the hydraulic tank with thehoist cylinders in the RAISED position. When the hoist cylinders arelowered, the hydraulic oil level will increase. After the hoist cylinders arelowered, check the hydraulic tank oil level with the upper sight gauge asstated above.

- 160 -STMG 6823/97

• Lower sight gaugeused for filling tankwith hoist cylindersRAISED

• Rear of hoist andbrake oil tank:

135

Shown is the rear of the hoist and brake oil hydraulic tank. The hoistsystem pumps pull oil from the hydraulic tank through the suction screens (1) located in the rear of the tank.

Two rear brake oil cooler relief valves are located in the hydraulic tank atthe left center connection (2). The setting of the oil cooler relief valves is790 kPa (115 psi).

- 161 -STMG 6823/97

2

1

1. Suction screens

2. Rear brake oil coolerrelief valve location

1. Two section hoistpump

136

The hoist system oil is supplied by a two section pump (1) located at thetop rear of the pump drive. Oil flows from the hoist pump to the hoistvalve through two screens located above the hoist valve.

The hoist system relief pressures are different in the RAISE and LOWERpositions.

The hoist system relief pressure during RAISE is 20370 + 700 - 0 kPa(2955 + 100 - 0 psi).

The hoist system relief pressure during LOWER is 3450 + 350 - 0 kPa(500 + 50 - 0 psi).

The hoist system pressure can be measured at the two pressure taps (2).

- 162 -STMG 6823/97

21

2. Hoist systempressure taps

• Relief pressuresdifferent for RAISEand LOWER

1. Hoist screens

2. Hoist screen bypassswitches

137

Oil flows from the hoist pump through the hoist screens (1) to the hoistcontrol valve. Two hoist screen bypass switches (2) provide input signalsto the VIMS which informs the operator if the hoist screens are restricted.

The hoist valve uses parking brake release pressure as the pilot oil to shiftthe directional spool inside the hoist valve. The parking brake release oilpressure is 4700 ± 200 kPa (680 ± 30 psi). Three solenoid valves are usedto direct the pilot oil to the ends of the directional spool. The solenoidvalve (3) is energized by the hoist lever sensor in the cab when the sensoris in the RAISE position.

A counterbalance valve (4) is mounted on the left side of the hoist valve.The counterbalance valve prevents cavitation of the cylinders when thebody raises faster than the pumps can supply oil to the cylinders (causedby a sudden shift of the load).

When the hoist valve is in the HOLD or FLOAT position, all the hoistpump oil flows through the large hose (5) to the front brake oil coolerfilters located outside the left frame. Excess oil from the parking brakerelease valve joins the hoist pump oil at the fitting connected to the largehose (5).

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1

3

2

5

4

3. RAISE positionsolenoid valve

4. Counterbalancevalve

5. Hose to front brakeoil cooler filters

138

An oil cooler relief valve is located in the hoist valve behind the largeplug (1). The relief valve limits the front brake oil cooling pressure whenthe hoist valve is in the HOLD or FLOAT position. The setting of the oilcooler relief valve is 790 kPa (115 psi).

The hoist valve uses parking brake release pressure as the pilot oil to shiftthe directional spool inside the hoist valve. The parking brake release oilis supplied to the solenoid valves through the small tubes (2). Threesolenoid valves are used to direct the pilot oil to the ends of thedirectional spool. The solenoid valves (3) on the front of the hoist controlvalve are energized by the hoist lever sensor in the cab when the sensor isin the LOWER or FLOAT position. No solenoids are energized when thehoist lever sensor is in the HOLD position.

Supply oil flows to the raise port of the hoist cylinders from the upperright port (4) when the hoist lever sensor is in the RAISE position.

Supply oil flows to the lower port of the hoist cylinders from the lowerleft port (5) when the hoist lever sensor is in the LOWER or FLOATposition.

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2

3

5 1

4

2. Pilot oil supply tubes

3. LOWER and FLOATsolenoid valves

4. Hoist cylinder RAISEport

5. Hoist cylinderLOWER and FLOATport

1. Front brake oilcooler relief valveplug

139

• Hoist valve in HOLD Shown is a sectional view of the hoist valve in the HOLD position. Thepilot oil at both ends of the directional spool is vented to the tank. Thespool is held in the centered position by two centering springs. Passagesin the directional spool vent the dual stage relief valve signal stem to thetank. All the hoist pump oil flows through the front brake oil filters to thefront brake oil cooler.

The position of the directional spool blocks the oil in the head end of thehoist cylinders. Oil in the rod end of the hoist cylinders is connected tothe front brake cooling oil by a small vent slot cut in the directional spool.

A gauge connected to the hoist system pressure taps while the hoist valveis in the HOLD position would show the restriction pressure of the frontbrake oil cooling circuit. The maximum pressure in the circuit shouldcorrespond to the setting of the front brake oil cooler relief valve. Thesetting of the oil cooler relief valve is 790 kPa (115 psi).

- 165 -STMG 6823/97

TO HOISTCYLINDERROD END

TO HOISTCYLINDERHEAD END

TO FRONT BRAKEOIL COOLERS

FRONT BRAKEOIL COOLER

RELIEF VALVE

PARKING BRAKERELEASE PRESSURE

RAISESOLENOID

LOWERSOLENOID

FLOATSOLENOID

PARKING BRAKERELEASE PRESSURE

PUMP SUPPLY PORT

TO TANK

LOW PRESSURERELIEF VALVE

HIGH PRESSURERELIEF VALVE

COUNTERBALANCEVALVE

793CHOIST CONTROL VALVE

HOLD

DUAL STAGERELIEF VALVESIGNAL STEM

ROD END VENT SLOT

LOAD CHECKVALVE

140

• Hoist valve in RAISE Shown is a sectional view of the hoist valve in the RAISE position. TheLOWER and FLOAT solenoids are de-energized and pilot oil is vented tothe tank. The RAISE solenoid is energized and directs pilot oil pressureto the upper end of the directional spool. Pump oil flows past thedirectional spool to the head end of the hoist cylinders.

When the directional spool is initially shifted, the two load check valves(one shown) remain closed until the pump supply pressure is higher thanthe pressure in the hoist cylinders. The load check valves prevent thebody from dropping before the RAISE pressure increases.

The directional spool also sends hoist cylinder raise pressure to the dualstage relief valve signal stem and the counterbalance valve. The dualstage relief valve signal stem moves down and blocks the supply pressurefrom opening the low pressure relief valve. The counterbalance valve isheld open by the hoist cylinder raise pressure. Oil flowing from the rodend of the hoist cylinders flows freely to the front brake oil cooler.

- 166 -STMG 6823/97

ON

FROM HOISTCYLINDERROD END

TO HOISTCYLINDERHEAD END

TO FRONT BRAKEOIL COOLERS

FRONT BRAKEOIL COOLER

RELIEF VALVE

PARKING BRAKERELEASE PRESSURE

RAISESOLENOID

LOWERSOLENOID

FLOATSOLENOID

PARKING BRAKERELEASE PRESSURE

PUMP SUPPLY PORT

TO TANK

LOW PRESSURERELIEF VALVE

HIGH PRESSURERELIEF VALVE

COUNTERBALANCEVALVE

793CHOIST CONTROL VALVE

RAISE

DUAL STAGERELIEF VALVESIGNAL STEM

ROD END VENT SLOT

LOAD CHECKVALVE

If the pressure in the head end of the hoist cylinders exceeds 20370 + 700 - 0 kPa (2955 + 100 - 0 psi), the high pressure relief valvewill open. When the high pressure relief valve opens, the dump spoolmoves to the left and pump oil flows to the front brake oil cooler.

The high pressure hoist relief valve setting is checked at the two pressuretaps located on the hoist pump. Check the relief pressures with the hoistlever in the RAISE position and the engine at HIGH IDLE.

- 167 -STMG 6823/97

• High pressure reliefsetting checkedduring RAISE at HIGHIDLE

141

• Counterbalance valve During RAISE, the counterbalance valve prevents the dump body fromrunning ahead of the hoist pumps if the load shifts rapidly to the rear andattempts to pull the hoist cylinders. Signal pressure from the head end ofthe hoist cylinders holds the counterbalance valve open. Oil from the rodend of the hoist cylinders flows unrestricted through the counterbalancevalve to the front brake oil cooler. If the head end pressure decreasesbelow 6900 ± 690 kPa (1000 ± 100 psi), the counterbalance valve movesdown and restricts the flow of oil from the rod end of the cylinders to thefront brake oil cooler.

During LOWER and FLOAT, the counterbalance valve allowsunrestricted flow from the pump to the rod end of the hoist cylinders.

- 168 -STMG 6823/97

RAISE POSITION

HEAD ENDSIGNAL PRESSURE

FROM HOISTCYLINDERROD END

TO FRONTBRAKE OIL

COOLER

LOWER AND FLOAT POSITIONS

TO HOISTCYLINDERROD END

FROMPUMP

HOISTCOUNTERBALANCE VALVE

142

• Hoist valve in LOWER(power down)

Shown is a sectional view of the hoist valve in the LOWER (power down)position. The RAISE solenoid is de-energized and pilot oil is vented tothe tank. The LOWER and FLOAT solenoids are both energized anddirect pilot oil pressure to the lower end of the directional spool.

Supply oil from the pump flows past the directional spool, through thecounterbalance valve, to the rod end of the hoist cylinders. Oil in the headend of the hoist cylinders flows to the tank.

The directional spool also vents the passage to the dual stage relief valvesignal stem. The dual stage relief valve signal stem allows supplypressure to be limited by the low pressure relief valve.

- 169 -STMG 6823/97

ON

ON

TO HOISTCYLINDERROD END

FROM HOISTCYLINDERHEAD END

TO FRONT BRAKEOIL COOLERS

FRONT BRAKEOIL COOLER

RELIEF VALVE

PARKING BRAKERELEASE PRESSURE

RAISESOLENOID

LOWERSOLENOID

FLOATSOLENOID

PARKING BRAKERELEASE PRESSURE

PUMP SUPPLY PORT

TO TANK

LOW PRESSURERELIEF VALVE

HIGH PRESSURERELIEF VALVE

COUNTERBALANCEVALVE

793CHOIST CONTROL VALVE

LOWER (POWER DOWN)

DUAL STAGERELIEF VALVESIGNAL STEM

ROD END VENT SLOT

LOAD CHECKVALVE

If the pressure in the rod end of the hoist cylinders exceeds 3450 + 350 - 0 kPa (500 + 50 - 0 psi), the low pressure relief valve willopen. When the low pressure relief valve opens, the dump spool movesto the left and pump oil flows to the front brake oil cooler.

The low pressure hoist relief valve setting is checked at the two pressuretaps located on the hoist pump. Check the relief pressures with the hoistlever in the LOWER position and the engine at HIGH IDLE.

- 170 -STMG 6823/97

• Low pressure reliefsetting checkedduring LOWER atHIGH IDLE

143

• Hoist valve in FLOAT Shown is a sectional view of the hoist valve in the FLOAT position. TheRAISE and LOWER solenoids are de-energized and pilot oil is vented tothe tank. The FLOAT solenoid is energized and directs pilot oil pressureto the lower end of the small diameter spool located below the directionalspool. The small diameter spool pushes against the directional spool andmoves the directional spool up. Because the pilot pressure is acting on asmaller surface area, the directional spool does not move up as far asduring LOWER.

Pump supply oil flows past the directional spool, through thecounterbalance valve, to the rod end of the hoist cylinders. Oil in the headend of the hoist cylinders flows to the tank. The directional valve is in aposition that permits the pressure of the oil flowing to the front brake oilcooler to be felt at the rod end of the hoist cylinders.

The rod end pressure helps to hold the body against the frame whentraveling. The hoist lever should always be in the FLOAT position whiletraveling.

- 171 -STMG 6823/97

ON

TO HOISTCYLINDERROD END

FROM HOISTCYLINDERHEAD END

TO FRONT BRAKEOIL COOLERS

FRONT BRAKEOIL COOLER

RELIEF VALVE

PARKING BRAKERELEASE PRESSURE

RAISESOLENOID

LOWERSOLENOID

FLOATSOLENOID

PARKING BRAKERELEASE PRESSURE

PUMP SUPPLY PORT

TO TANK

LOW PRESSURERELIEF VALVE

HIGH PRESSURERELIEF VALVE

COUNTERBALANCEVALVE

793CHOIST CONTROL VALVE

FLOAT

DUAL STAGERELIEF VALVESIGNAL STEM

ROD END VENT SLOT

LOAD CHECKVALVE

• Operate truck withhoist lever in FLOAT

• Front brake oil coolerfilters (arrow)

144

When the hoist valve is in the HOLD or FLOAT position, all the hoistpump oil flows through the front brake oil cooler filters (arrow) locatedoutside the left frame. Excess oil from the parking brake release valvealso flows through these filters.

Oil flows from the front brake oil cooler filters to the front brake oilcooler located above the torque converter.

- 172 -STMG 6823/97

1. Front brake oilcooler diverter valve

2. Front brake oilcooler

145

The hoist and parking brake release pump oil flows from the front brakeoil cooler filters, through the front brake oil cooler diverter valve (1), tothe front brake oil cooler (2).

When the service or retarder brakes are applied, air pressure is sent to thefront brake oil cooler diverter valve. Normally, front brake cooling oil isdiverted around the cooler and goes directly to the front brakes. When airis sent to the diverter valve piston, front brake cooling oil is allowed toflow through the front brake oil cooler. Since the coolers use the coolantfrom the aftercooler circuit, diverting oil around the coolers providescooler aftercooler air during high power demands (when climbing a gradewith the brakes RELEASED, for example).

- 173 -STMG 6823/97

12

• Two-stage hoistcylinders

146

Shown are the twin two-stage hoist cylinders used to raise the body.

- 174 -STMG 6823/97

147

• Hoist system circuit The hoist system pumps pull oil from the hydraulic tank through suctionscreens.

Oil flows from the hoist pump through the hoist screens to the hoistcontrol valve.

The hoist valve uses parking brake release pressure as the pilot oil to shiftthe directional spool inside the hoist valve. Three solenoid valves areused to direct the pilot oil to the ends of the directional spool. Thesolenoid valve on the right is energized in the RAISE position. The twosolenoid valves on the left are energized in the LOWER or FLOATposition.

When the hoist valve is in the HOLD or FLOAT position, all the hoistpump oil flows through the front brake oil cooler filters. Excess oil fromthe parking brake release valve joins with the hoist pump oil and alsoflows through the front brake oil cooler filters.

- 175 -STMG 6823/97

FROM PARKINGBRAKE RELEASE

VALVE

TO HOIST CYLINDERHEAD END

TO HOIST CYLINDERROD END

FROM PARKINGBRAKE RELEASE

VALVE

FRONT BRAKEOIL COOLER

FILTERS

FRONT BRAKEOIL COOLER

DIVERTERVALVE

FRONT BRAKES

HOISTPUMP

HOISTSCREENS

SUCTIONSCREENS

HOISTSYSTEM

HOLDPOSITION

An oil cooler relief valve is located in the hoist valve. The relief valvelimits the front brake oil cooling pressure when the hoist valve is in theHOLD or FLOAT position.

Hoist and parking brake release pump oil flows from the front brake oilcooler filters, through the front brake oil cooler diverter valve, to the frontbrake oil cooler.

Service or retarder brake air pressure is sent to the front brake oil coolerdiverter valve. Normally, front brake cooling oil is diverted around thecooler and goes directly to the front brakes. When air is sent to thediverter valve pistons, front brake cooling oil is allowed to flow throughthe front brake oil cooler. Since the coolers use the coolant from theaftercooler circuit, diverting oil around the coolers provides cooleraftercooler air during high power demands.

Two hydraulic cylinders are used to raise the body away from the frameof the truck. When the hoist lever is held in the RAISE position, supplyoil flows to the head end of the hoist cylinders and moves the two stagecylinders to their extended lengths. The oil from the rod end of thecylinders flows through the hoist valve into the front brake oil coolingcircuit.

When the hoist lever is moved to the LOWER or FLOAT position and thecylinders are extended, supply oil enters the rod end of the hoist cylindersand lowers the second stage of the cylinders. The oil from the head endof the cylinders flows through the hoist valve to the hydraulic tank.

- 176 -STMG 6823/97

148

AIR SYSTEM AND BRAKES

Two separate brake systems are used on the 793C Off-highway Truck.The two brake systems are: the parking/secondary brake system and theservice/retarder brake system.

The parking/secondary brakes are spring engaged and hydraulicallyreleased. The service/retarder brakes are engaged hydraulically by an air-over-oil brake system.

The 793C Truck is also equipped with an air system. An engine driven aircompressor supplies the air and fills two reservoirs. Air from thereservoirs provides energy to perform several functions:

1. Engine start-up2. Service and retarder brake control3. Secondary and parking brake control4. Windshield washer and wiper5. Automatic lubrication injection6. Horn7. Exhaust bypass (wastegate) control

- 177 -STMG 6823/97

793C

AIR SYSTEM AND BRAKES

• Two brake systems:

- Parking/secondarybrake system

- Service/retarderbrake system

• Air system functions

• Oil cooled brakeassembly

• Duo-Cone sealsprevent oil fromleaking or transferring

149

Shown is a cutaway illustration of an oil cooled brake assembly. Thebrakes are environmentally sealed and adjustment free. Oil continuallyflows through the brake discs for cooling. Duo-Cone seals prevent thecooling oil from leaking to the ground or transferring into the axlehousing. The wheel bearing adjustment must be maintained to keep theDuo-Cone seals from leaking.

The smaller piston (yellow) is used to ENGAGE the secondary andparking brakes. The parking brakes are spring ENGAGED andhydraulically RELEASED.

The larger piston (purple) is used to ENGAGE the retarder/service brakes.The retarder/service brakes are engaged hydraulically by an air-over-oilbrake system.

- 178 -STMG 6823/97

• Small pistonENGAGES secondaryand parking brakes

• Large pistonENGAGESretarder/servicebrakes

150

Operator Controls

Several brake system control components are located on the centerconsole at the right of the operator's seat.

The parking brake air switch (1) controls the flow of air to the parkingbrake release valve.

The windshield wiper/washer switch (2) controls the flow of air to thepump in the windshield washer reservoir and to the wiper motor in frontof the cab.

The brake retraction switch (3) is an electrical switch used to activate theelectric pump that supplies oil for towing.

The Traction Control System (TCS) switch (4) is used to test the TCS(formerly referred to as the "Automatic Electronic Traction Aid").

- 179 -STMG 6823/97

1 2 3

4

3. Brake retractionswitch

4. TCS switch

2. Windshieldwiper/washer switch

1. Parking brake airswitch

• Secondary brake lever(red)

• Retarder lever (black)

151

Operator controls on the steering column are the secondary brake lever (red) and the retarder lever (black).

The secondary brake lever allows the operator to modulate theengagement of the parking brakes. The secondary brake lever engagesthe same brake system as the parking brake air switch.

The retarder brake lever allows the operator to modulate the engagementof the service brakes. The retarder brake lever engages the same brakesystem as the service brake pedal (shown in the next slide).

- 180 -STMG 6823/97

• Service brake pedal(arrow)

• Horn control button(not shown)

152

On the floor is the service brake pedal (arrow).

Located on the floor to the left of the steering column is the horn controlbutton (not shown).

- 181 -STMG 6823/97

• Air compressor

153

Air Charging System

The air system is charged by an air compressor mounted on the left frontof the engine.

System pressure is controlled by the governor (arrow). The governormaintains the system pressure between 660 and 830 kPa (95 and 120 psi).

- 182 -STMG 6823/97

• Air compressorgovernor (arrow)

1. Air dryer

154

Air flows from the air compressor to the air dryer (1) located in front ofthe left front tire. The air dryer removes contaminants and moisture fromthe air system. The condition of the desiccant in the air dryer should bechecked every 250 hours and changed periodically (determined by thehumidity of the local climate).

When the air compressor governor senses that system air pressure is at thecut-out pressure of 830 kPa (120 psi), the governor sends an air pressuresignal to the purge valve through the hose (2). The purge valve opens andair pressure that is trapped in the air dryer is exhausted through thedesiccant, an oil filter and the purge valve.

An air system relief valve (3) is located on the air dryer. A heatingelement (4) prevents moisture in the dryer from freezing in cold weather.

- 183 -STMG 6823/97

42

1

3

3. Air system reliefvalve

4. Heating element

2. Purge valve signalhose

1. Service/retarderbrake reservoir

155

Air flows through the air dryer and fills two reservoirs. Theservice/retarder brake reservoir (1) is located on the right platform. Thisreservoir also supplies air for the air start system.

The second reservoir is located behind the cab and supplies air for theparking/secondary brake system.

A relief valve (2) is installed in the service/retarder brake reservoir. Thisrelief valve serves as a back-up for the relief valve on the air dryer.

Condensation should be drained from the tank daily through the drainvalve (3).

- 184 -STMG 6823/97

2

1

3

3. Condensation drainvalve

2. Relief valve

• Parking/secondarybrake reservoir

156

Located behind the operator’s station is the parking/secondary brake airreservoir. A drain valve is located on the right side of the cab. Moistureshould be drained from the reservoir daily through the drain valve (see Slide No. 28).

A check valve (arrow) prevents a loss of air if an air line breaks upstreamof the air reservoir.

- 185 -STMG 6823/97

• Check valve (arrow)

1. Pressure protectionvalve

157

Located behind the operator’s station is a pressure protection valve (1). Ifone of the accessory circuits fails, the pressure protection valve maintainsa minimum of 482 kPa (70 psi) in the service brake circuit.

Also located behind the operator’s station is the air system pressure sensor (2). The air system pressure sensor provides an input signal to theVIMS which informs the operator if a problem exists in the air system.

The solenoid air valve (3) provides controlled supply air for the automaticlubrication system. The solenoid air valve is controlled by the VIMS.The VIMS ENERGIZES the solenoid ten minutes after the machine isstarted. The VIMS keeps the solenoid ENERGIZED for 75 seconds andthen DE-ENERGIZES it. Every 60 minutes thereafter, the VIMSENERGIZES the solenoid for 75 seconds until the machine is stopped(turned off). These settings are adjustable through the VIMS keypad inthe cab.

- 186 -STMG 6823/97

2

1

3

2. Air system pressuresensor

3. Automaticlubrication solenoidair valve

158

• Air charging systemschematic

This schematic shows the flow of air through the air charging system. Airflows from the air compressor, through the air dryer, to theservice/retarder brake reservoir.

Air from the service/retarder brake reservoir enters the pressure protectionvalve. When the pressure in the service/retarder reservoir reaches 550 kPa (80 psi), the pressure protection valve allows air to flow to theparking/secondary brake reservoir, the air start system, the automaticlubrication system and the accessory circuits (wiper and horn).

All reservoirs have a check valve at the air supply port to prevent a loss ofair if a leak upstream of the reservoirs occurs.

- 187 -STMG 6823/97

AIR COMPRESSORAND GOVERNOR

AIRDRYER

REMOTE AIRSUPPLY CONNECTOR

PRESSUREPROTECTION

VALVE

SERVICE/RETARDERBRAKE RESERVOIR

AIR CHARGING SYSTEM

PARKING/SECONDARYBRAKE RESERVOIR

LOW AIRSWITCH

• Secondary brake valve(arrow)

• Modulates parkingbrake engagement

159

Parking and Secondary Brake System

The secondary brake valve (arrow) is controlled by the secondary brakelever in the cab. Normally, air flows through the secondary brake valve tothe parking brake release valve. When the secondary brake lever is pulleddown, the valve blocks the flow of air to the parking brake release valve.

Blocking the air from the parking brake release valve positions the spoolin the valve to drain the oil from the parking brakes, which allows thesprings in the parking brake to ENGAGE the brakes. The secondarybrake valve can be used to modulate parking brake engagement bymetering the amount of air flow to the parking brake release valve.

The parking brake air switch on the center console in the cab also controlsthe flow of air to the parking brake release valve.

- 188 -STMG 6823/97

1. Parking brakerelease valve

160

Supply air from the parking brake air switch in the cab or the secondarybrake valve flows to an air chamber in the parking brake release valve (1).The parking brake release valve contains an air piston that moves a spool.The spool either directs oil to RELEASE the parking brakes or drains oilto ENGAGE the parking brakes. A relief valve inside the parking brakerelease valve limits the system pressure for releasing the brakes.

The same oil that supplies the parking brake release valve also suppliesoil to the Traction Control System (TCS) valve (2). The TCS valveautomatically ENGAGES and RELEASES the rear parking brake of awheel that is spinning at least 60% faster than the other rear wheel. TheTCS test switch in the cab can be used to test the TCS.

The left and right rear parking/secondary brake pressures can be measuredat the pressure taps (3).

If the parking brakes need to be released for service work or towing, theelectric motor (4) can be energized by the brake retraction switch locatedin the cab. The motor drives a pump which sends oil through the parkingbrake release valve to RELEASE the parking brakes. Towing pumppressure is controlled by a relief valve in the towing pump.

Air pressure is also needed to release the brakes for towing. The pistonchamber in the parking brake release valve must be pressurized to movethe spool in the valve. The oil from the electrically driven brake releasepump can then flow to the rear brakes.

- 189 -STMG 6823/97

4

2

31

• Air pressure neededto release brakes fortowing

4. Towing motor

• Pump provides flow torelease parkingbrakes for towing

2. TCS valve

3. Rearparking/secondarybrake taps

1. Parking brakerelease pump

2. Rear brake oilcooling pumps

161

Shown is the parking brake release pump (1) and the three rear brake oilcooling pumps (2). Parking brake release supply oil flows from theparking brake release pump, through the parking brake release oil filter, tothe parking brake release valve.

- 190 -STMG 6823/97

12

1. Parking brakerelease filter

2. Parking brakerelease pressure tap

162

Oil flows from the parking brake release pump, through the parking brakerelease filter (1), to the parking brake release valve. Parking brake releasepressure can be measured at the filter by removing a plug and installing apressure tap (2).

- 191 -STMG 6823/97

2

1

163

Normally, supply oil flows from the parking brake release pump, throughthe parking brake release filter, to the parking brake release valve. If airpressure is present from the parking brake air switch or secondary brakevalve, supply oil flows past the relief valve, the check valve and the spoolto RELEASE the parking brakes. The relief valve controls systempressure for releasing the brakes and for the pilot oil that is supplied to thethree solenoids on the hoist valve. The setting of the relief valve in theparking brake valve is 4700 ± 200 kPa (680 ± 30 psi).

This schematic shows the flow of oil through the parking brake releasesystem when the towing system is energized.

Oil flow from the parking brake release pump has stopped. The towingmotor is energized, and air pressure is present above the parking brakerelease valve piston. The air pressure moves the spool in the parkingbrake release valve down to block the drain port.

- 192 -STMG 6823/97

PARKING BRAKERELEASE VALVE

TOWING SYSTEM

TOWING PUMP

PARKINGBRAKE

RELEASEPUMP

TOWING PUMPRELIEF VALVE

CHECKVALVE

TO HOISTPILOT SYSTEM

RELIEFVALVE

• Parking brake releasesystem during towing

• Parking brake reliefvalve limits hoist pilotpressure

• Normal parking andsecondary brakeoperation

Oil flows from the towing pump to the parking brake release valve andthe parking brakes. The check valve in the parking brake release valveblocks the oil from the towing pump from flowing to the parking brakerelease pump.

During towing, the parking brake release pressure is controlled by a reliefvalve in the towing pump. When the relief valve opens, oil transfers fromthe pressure side to the suction side of the towing pump. The setting ofthe relief valve is approximately 3790 kPa (550 psi).

A check valve in the outlet port of the towing pump prevents oil fromflowing to the towing pump during normal operation.

To check the brake release system used for towing, install a gauge on theparking brake release pressure tap on the rear axle. Use a long gaugehose so the gauge can be held in the cab. With the parking brake airswitch in the RELEASE position and the key start switch in the ONposition, energize the parking brake release switch on the dash used fortowing. The parking brake release pressure should increase to 3790 kPa(550 psi). Turn off the switch when the pressure stops increasing.

The parking brake release pressure must increase to a minimum of 3790 kPa (550 psi). During towing, the brake retraction switch on thedash must be energized whenever the parking brake release pressuredecreases below this pressure or the brakes will drag.

NOTE: At least 550 kPa (80 psi) air pressure must be available at theparking brake release valve to ensure full release of the brakes fortowing.

NOTICE

Energize the brake retraction switch only when additional pressure isrequired to release the brakes. Leaving the brake retraction (towing)motor energized continuously will cause damage to the motor.

- 193 -STMG 6823/97

• Procedure to checkparking brake releasesystem for towing

• Towing pump checkvalve

• Relief valve in towingpump controls brakerelease pressure

164

• Parking/secondarybrake system

Shown is the parking/secondary brake hydraulic and air system with thesecondary brakes RELEASED and the parking brakes ENGAGED.

Supply air from the parking/secondary brake air reservoir flows throughthe secondary brake valve and is blocked by the parking brake air switch.

No air pressure is present to move the spool in the parking brake releasevalve. Supply oil from the parking brake release pump is blocked by thespool. Oil from the parking brake is open to drain through the parkingbrake release valve, which allows the springs in the parking brake toENGAGE the brakes.

A parking/secondary brake switch that provides an input signal to theEPTC II is located in the air line between the parking brake switch and theparking brake release valve. When the parking or secondary brakes areENGAGED, the switch signals the EPTC II to allow rapid downshifts.

- 194 -STMG 6823/97

PARKINGBRAKESWITCH

SECONDARYBRAKEVALVE

PARKINGBRAKERELEASEVALVE

PARKING/SECONDARY BRAKES

PARKING BRAKES ENGAGED

TO HOISTPILOT SYSTEM

PARKINGBRAKE

RELEASEPUMP

SECONDARY BRAKES RELEASED

PARKING/SECONDARYBRAKE RESERVOIR

EPTC IISWITCH

• Parking/secondarybrake switch forEPTC II input

1. Service brake valve

2. Manual retardervalve

3. Automatic RetarderControl (ARC) valve

165

Service and Retarder Brake System

The service brake valve (1) is controlled by the brake pedal in the cab.Supply air for the service brake valve, the manual retarder valve (2) andthe Automatic Retarder Control (ARC) valve (3) is supplied from thebottom port of the service brake valve.

When the manual retarder is engaged, air flows from the manual retardervalve through the top of the left double check valve (4) and the rightdouble check valve (5) to a relay valve near the brake master cylindersand a diverter valve on the front brake oil cooler.

When the service brakes are engaged, air flows from the service brakevalve through the bottom of the left double check valve (4). If the manualretarder and the service brakes are engaged at the same time, air from thesystem with the highest pressure will flow through the left double checkvalve (4) and the right double check valve (5) to a relay valve near thebrake master cylinders and a diverter valve on the front brake oil cooler.

- 195 -STMG 6823/97

1

4

6 32 5

4. Left double checkvalve

5. Right double checkvalve

6. Brake ON switch

• Service brakes andmanual retarderengage same relayvalve

The Automatic Retarder Control (ARC) system function is to modulatetruck braking (retarding) when descending a long grade to maintain aconstant engine speed. Previously, the ARC was installed in parallel withthe manual retarder and the service brakes. On current machines, theARC system is separate from the manual retarder and the service brakes.

When the ARC is engaged, air flows from the ARC valve (3) to a separaterelay valve located near the brake master cylinders. Air also flows fromthe ARC valve through the right double check valve (5) to the divertervalve on the front brake oil cooler.

The switch (6) turns on the amber BRAKE ON light on the dash in theoperator’s station when any of the brakes are engaged (manual retarder,service brake or automatic retarder).

- 196 -STMG 6823/97

• ARC brake systemengages separaterelay valve

1. Service brake andmanual retarderrelay valve

2. ARC relay valve

166

The front brake relay valve (1) receives metered air from only the servicebrake valve or the manual retarder valve. The rear brake relay valve (2)receives metered air from only the Automatic Retarder Control (ARC)valve.

When the service brakes or manual retarder brakes are ENGAGED, thefront relay valve opens and metered air flows from the service brakereservoir, through the double check valves (3), to the brake cylinders (4and 5). The brake relay valve reduces the time required to engage andrelease the brakes.

When the ARC brake system is ENGAGED, the rear relay valve opensand metered air flows from the service brake reservoir, through the doublecheck valves (3), to the brake cylinders (4 and 5). The brake relay valvereduces the time required to engage and release the brakes.

The double check valves (3) are used to separate the service brakes andmanual retarder brakes from the ARC brake system.

The brake cylinders operate by air-over-oil. When the metered air entersthe brake cylinders, a piston moves down and pressurizes the oil in thebottom of the cylinders. Two cylinders (4) supply oil to the front brakes,and two cylinders (5) supply oil to the rear brakes. The pressure oil fromthe brake cylinders flows to a slack adjuster.

- 197 -STMG 6823/97

4

1

5

2

3

• Double check valvesseparate brakesystems

• Brake relay valvesreduce time to engageand release brakes

3. Double check valves

4. Front brakecylinders

5. Rear brakecylinders

167

As the brake discs in the brake assemblies wear, more oil is needed fromthe brake cylinders to compensate for the wear. The makeup oil tank (1)supplies makeup oil for the brake cylinders. Oil from the brake coolingcircuit provides a continuous supply of oil to the makeup oil tank. Lowbrake cooling flow can cause the makeup oil reserve to decrease andcause the brake cylinders to overstroke.

To check for makeup oil flow, remove the cover from the makeup oiltank. With the engine at HIGH IDLE, a stream of oil filling the tankshould be visible. If a stream of oil is not visible, the hydraulic systemmay have a restriction, pump flow may be low, or the brake oil coolingrelief valve may be stuck open or set too low.

If air is in the system or a loss of oil downstream from the cylindersoccurs, the piston in the cylinder will overstroke and cause an indicatorrod to extend and open the brake overstroke switch (2). The switchprovides an input signal to the VIMS which informs the operator of thecondition of the service/retarder brake oil circuit. If an overstrokecondition occurs, the problem must be repaired and the indicator rodpushed in to end the warning.

Air can be removed from the brake cylinders through the bleed screws (3).

Shown is one of the four brake oil temperature sensors (4) located in thebrake cooling oil return tube.

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1

2

4

3

1. Brake oil makeuptank

• Check brake makeupoil flow

2. Brake overstrokeswitch

3. Brake cylinder bleedscrew

4. Brake oiltemperature sensor

• Front brake oil coolerdiverter valve (arrow)

168

The air that flows to the two relay valves also flows to the front brake oilcooler diverter valve (arrow). Normally, front brake cooling oil isdiverted around the cooler and goes directly to the front brakes. When airis sent to the diverter valve piston, brake oil is allowed to flow throughthe front brake oil cooler. Since the cooler uses the coolant from theengine aftercooler circuit, diverting oil around the cooler provides lowertemperature aftercooler air during high power demands (when climbing agrade with the brakes RELEASED, for example).

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169

• Brake cylinderENGAGED

This slide shows a sectional view of the brake cylinder when the brakesare ENGAGED.

Air pressure from the brake relay valve enters at the air inlet. The airpressure moves the air piston and the attached rod closes the valve in theoil piston. When the valve in the oil piston is closed, the oil pistonpressurizes the oil in the cylinder. The pressure oil flows to a slackadjuster.

If air is in the system or a loss of oil downstream from the cylindersoccurs, the piston in the cylinder will overstroke and cause the indicatorrod to extend and open the brake overstroke switch. The switch providesan input signal to the VIMS which informs the operator of the conditionof the service/retarder brake oil circuit. If an overstroke condition occurs,the problem must be repaired and the indicator rod pushed in to end thewarning.

When the air pressure is removed from behind the air piston, the springmoves the air piston and the attached rod opens the valve in the oil piston.Any makeup oil that is needed flows into the passage at the top of the oilchamber, through the valve, and into the oil chamber at the right of the oilpiston.

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AIRPISTON

RODSPRING

INDICATOR ROD

AIRINLET

BRAKE CYLINDERBRAKES ENGAGED

VALVE

OILPISTON

FROMMAKEUP

TANK

TOSLACK

ADJUSTER

• Brake overstrokeswitch indicates lossof brake oil

1. Slack adjuster

170

The truck is equipped with two slack adjusters--one for the front brakesand one for the rear brakes. The slack adjuster (1) shown is for the rearbrakes. The slack adjusters compensate for brake disc wear by allowing asmall volume of oil to flow through the slack adjuster and remain betweenthe slack adjuster and the brake piston under low pressure. The slackadjusters maintain a slight pressure on the brake piston at all times.

Brake cooling oil pressure maintains a small clearance between the brakediscs.

The service brake oil pressure can be measured at the two taps (2) locatedon top of the slack adjusters.

Air can be removed from the service brakes through the two remote bleedvalves (3).

The parking brake release pressure can be measured at the two taps (4) onthe axle housing.

NOTE: Air can be removed from the front service brakes throughbleed valves located on each wheel.

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1

33

4 4

2

• Cooling oil pressuremaintains clearancebetween discs

2. Service brakepressure taps

3. Service brake bleedvalves

4. Parking brakerelease pressuretaps

171

• Slack adjusterRELEASED andENGAGED

• Large piston moves toENGAGE brakes

• Small piston allowsmakeup oil to brakes

This slide shows sectional views of the slack adjuster when the brakes areRELEASED and ENGAGED.

When the brakes are ENGAGED, oil from the brake cylinders enters theslack adjusters and the two large pistons move outward. Each large pistonsupplies oil to one wheel brake. The large pistons pressurize the oil to theservice brake pistons and ENGAGE the brakes.

Normally, the service brakes are FULLY ENGAGED before the largepistons in the slack adjusters reach the end of their stroke. As the brakediscs wear, the service brake piston will travel farther to FULLYENGAGE the brakes. When the service brake piston travels farther, thelarge piston in the slack adjuster moves farther out and contacts the endcover. The pressure in the slack adjuster increases until the small pistonmoves and allows makeup oil from the brake cylinders to flow to theservice brake piston.

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OIL FLOWTO BRAKECYLINDER

OIL FLOWFROM BRAKE

CYLINDER

FROMWHEELBRAKES

TOWHEELBRAKES

TOWHEELBRAKES

BRAKES ENGAGEDBRAKES RELEASED

LARGE PISTON

SMALL PISTON

FROMWHEELBRAKES

BRAKE SLACK ADJUSTER

When the brakes are RELEASED, the springs in the service brakes pushthe service brake pistons away from the brake discs. The oil from theservice brake pistons pushes the large pistons in the slack adjuster to thecenter of the slack adjuster. Makeup oil that was used to ENGAGE thebrakes is replenished at the brake cylinders from the makeup tank.

The spring behind the large piston causes some oil pressure to be felt onthe service brake piston when the brakes are RELEASED. Keeping somepressure on the brake piston provides rapid brake engagement with aminimum amount of brake cylinder piston travel.

The slack adjusters can be checked for correct operation by opening theservice brake bleed screw with the brakes RELEASED. A small amountof oil should flow from the bleed screw when the screw is opened. Thesmall flow of oil verifies that the spring behind the large piston in theslack adjuster is maintaining some pressure on the service brake piston.

Another check to verify correct slack adjuster operation is to connect agauge to the pressure tap on top of the slack adjuster and another gauge atthe service brake bleed screw location on the brake anchor plate casting.With system air pressure at maximum and the service brake pedaldepressed, the pressure reading on both gauges should be approximatelythe same.

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• Large piston springkeeps pressure onservice brake piston

• Check slack adjusterfor correct operation

• Service brake springsmove large pistons tocenter of slackadjuster

172

• Service/retarder brakeair system

This schematic shows the flow of air through the service/retarder brake airsystem when the retarder (manual and automatic) is RELEASED and theservice brakes are ENGAGED. Supply air pressure flows from the largeservice brake air reservoir to the relay valves and the service brake valve.Supply air pressure flows from the service brake valve to the manualretarder valve and the ARC valve.

The manual retarder valve blocks the flow of air to a double check valve.The ARC solenoids also block the flow of air to a double check valve andthe ARC relay valve. The service brake valve allows air to flow to adouble check valve that blocks the passage to the manual retarder valve.Air pressure from the service brake valve flows through the double checkvalve to the service brake relay valve and a second double check valve tothe front brake oil cooler diverter valve.

The service brake relay valve opens and metered air flows from the largeservice brake air reservoir to the brake cylinders. A pair of double checkvalves above the brake cylinders prevent the flow of service brake air tothe ARC relay valve.

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SERVICERELAYVALVE

FRONT BRAKEOIL COOLER

DIVERTERVALVE

SERVICEBRAKEVALVE

RETARDERVALVE

BRAKE CYLINDERS

ARCVALVE

SERVICE/RETARDER BRAKE AIR SYSTEMSERVICE BRAKE ENGAGED

ARCRELAYVALVE

BRAKEENGAGEDSWITCH

173

• Rear brake oil coolingcircuit

Shown is the rear brake oil cooling circuit. The rear brake cooling pumps(see Slide No. 161) pull oil from the hydraulic tank through suctionscreens. Rear brake cooling oil pressure is controlled by two oil coolerrelief valves located inside the hydraulic tank (see Slide No. 135).

Oil flows from the rear brake cooling pumps through two rear brake oilcoolers located behind the right front tire (see Slide No. 8). Oil flowsfrom the rear brake oil coolers, through the rear brakes, and returns to thehydraulic tank.

Most of the oil that flows into the parking brake release valve flowsthrough the valve and joins with the hoist system oil. The parking brakerelease and hoist system oil is used to cool the front brakes (see Slide No. 147).

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REAR BRAKESBRAKE COOLINGPUMPS

SUCTIONSCREENS

REAR BRAKECOOLING CIRCUIT

REAR BRAKEOIL COOLERS

OIL COOLERRELIEF VALVES

PARKING BRAKERELEASE FILTER

HOIST VALVE

PARKING BRAKERELEASE PUMP

PUMPDRIVE

PARKING BRAKERELEASE VALVE

TRACTIONCONTROLSYSTEM

(TCS)VALVE

TO REARBRAKES

TO FRONTBRAKES

• Parking brake releaseoil used to cool frontbrakes

174

AUTOMATIC RETARDER CONTROL (ARC)

The Automatic Retarder Control (ARC) system function is to modulatetruck braking (retarding) when descending a long grade to maintain aconstant engine speed. The ARC system uses the service/retarder brakesystem to engage the brakes. If the ON/OFF switch is moved to the ONposition, the ARC will be activated if the throttle pedal is not depressedand the parking/secondary brakes are RELEASED. The ARC system isdisabled when the throttle is depressed or when the parking/secondarybrakes are ENGAGED.

The ARC is not connected to the service brakes and the manual retarder.When the ARC is engaged, air flows from the ARC valve to a separaterelay valve located near the brake master cylinders. Air also flows fromthe ARC valve, through a double check valve, to the diverter valve on thefront brake oil cooler.

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AUTOMATICRETARDER

VALVE

SUPPLYSOLENOID

CONTROLSOLENOID

AUTO RETARDERPRESSURE SWITCH

VENT

SUPPLY OUTPUT

CONTROL OUTPUT

AUTO RETARDER PRESSURE INPUT

BRAKE ENGAGED PRESSURE INPUT

BRAKE ENGAGED LAMP OUTPUT

BRAKEENGAGED

LAMP

DATA LINK

HARNESSCODE PLUG

Diagnostic Display

(ARC)CONTROL

RETARDERAUTOMATIC

HARNESSCODE INPUTSENGINE SPEED

SERVICE INPUTS

ARC ON/OFFSWITCH

ON INPUT

OFF INPUT

CLEAR

SET

AIR FROMSERVICE BRAKE

RESERVOIR

BRAKEPEDAL

AND VALVE

RETARDERLEVER

AND VALVE

BRAKE ENGAGEDSWITCH

TO SERVICEBRAKE

RELAY VALVE

VENT

ENGINE SPEEDSENSOR

A11

A26

A17-1936,37

A3

A4

A10

A5

A16

A21

B9B10B6

B7

TO FRONT BRAKEOIL COOLER

DIVERTER VALVETO ARC

RELAY VALVEA - 37 PIN CONNECTORB - 10 PIN SURE-SEAL CONNECTOR

• Automatic RetarderControl (ARC)

The ARC is set at the factory to maintain a constant engine speed of 1900 ± 50 rpm (engine speed setting is programmable). When the ARCinitially takes control of retarding, the engine speed may oscillate out ofthe ± 50 rpm target, but the engine speed should stabilize within a fewseconds.

For proper operation of the ARC, the operator needs only to activate thecontrol with the ARC ON/OFF switch and select the correct gear for thegrade, load, and ground conditions. The ARC is designed to allow thetransmission to upshift to the gear selected by the shift lever. After thetransmission shifts to the gear selected by the operator and the enginespeed exceeds 1900 rpm, the ARC will apply the retarder as needed tomaintain a constant engine speed.

The ARC system also provides Engine Overspeed protection. If anunsafe engine speed is reached, the ARC will engage the brakes, even ifthe ARC ON/OFF switch is in the OFF position and the throttle isdepressed.

Trucks approaching an overspeed condition will sound a horn and activatea light at 2100 rpm. If the operator ignores the light and horn, the ARCwill engage the retarder at 2180 rpm. If the engine speed continues toincrease, the EPTC II transmission control will either upshift (one gearonly above shift lever position) or unlock the torque converter (if the shiftlever is in the top gear position) at 2300 rpm.

The ARC also provides service personnel with enhanced diagnosticcapabilities through the use of onboard memory, which stores possiblefaults, solenoid cycle counts and other service information for retrieval atthe time of service.

By using of a set of service switches, service personnel can accessdifferent modes to gather the stored diagnostic information or set theadjustable engine speed control setting.

The Auto Retarder Control receives signals from several switches andsensors. The control analyzes the various input signals and sends signalsto the output components. The output components are two solenoids anda lamp.

INSTRUCTOR NOTE: For more detailed information about theAutomatic Retarder Control (ARC) system, refer to the ServiceManual Module "Automatic Retarder Control System" (FormSENR5683) and the Technical Instruction Module "AutomaticRetarder Control System" (Form SEGV2593).

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• ARC providesprogramming anddiagnostic capability

• ARC provides engineoverspeed protection

• ARC set to maintain1900 engine rpm

1. Automatic RetarderControl (ARC)

2. ARC diagnosticwindow

175

Shown is the Automatic Retarder Control (1). The ARC is located in thecompartment behind the cab. The control contains a diagnostic window(2) with 12 Light Emitting Diodes (LED’s) and a three digit numericdisplay.

The service switches (3) are used to interrogate the ARC for storeddiagnostic information, event information and to program the enginecontrol speed. The switches are labeled with an "S" for "SET" and a "C" for "CLEAR."

The DIAGNOSTIC MODE of the electronic control is changed byDEPRESSING and HOLDING both service switches (SET and CLEAR).When the desired mode is shown on the display, the switches can bereleased. By following the instructions in the Service Manual, theserviceman can determine if the ARC system is operating correctly.

The Electronic Control Analyzer Programmer (ECAP) and the ElectronicTechnician (ET) Service Tools can be used in place of the ARC diagnosticwindow. The ECAP and ET perform the same functions as the ARCdiagnostic window and are capable of a few additional diagnostics thatthe ARC window does not display (see Slide No. 109).

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1

2

3

• ECAP and ET servicetools

3. Service switches

176

• ARC diagnosticwindow:

- 12 status LED's

- Three digit display

The onboard diagnostic window houses 12 status LED's along with athree digit numeric display.

The function of the three digit display and the status LED's are:

1. POWER--A GREEN LED which is ON when a nominal 24 Volts isavailable between pins 1 and 2 of the electronic control 37 pinconnector.

2. CONTROL EVENT--A RED LED which is ON or FLASHING whenthe electronic control has FAILED and should be replaced.

3. ARC PRESSURE--An AMBER LED which is ON when the ARCpressure switch is OPEN, which indicates the presence of brake airpressure at the ARC valve.

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ARC DIAGNOSTIC WINDOW

1 2 3 5 6 74

8

D1 D2 D3

10

12

11

13

4. RETARDER PRESSURE--An AMBER LED which is ON when theservice brakes, manual retarder or auto retarder is in use as sensed bythe CLOSED brake engaged pressure switch.

5. SECONDARY BRAKE--An AMBER LED which is ON when thesecondary or parking brake is in use as sensed by the OPENsecondary brake pressure switch.

6. THROTTLE PEDAL--An AMBER LED which is ON when thethrottle pedal is depressed as sensed by a signal from the throttlesensor.

7. ENG SPEED--An AMBER LED which is ON when the engine speedsensor is providing a signal to the control.

8. DIAGNOSTIC PRESENT--A RED LED which indicates that theelectronic control has detected a fault for which a diagnostic code hasbeen stored in memory. The LED is ON if the fault is still present.

9. Three digits (D1, D2, D3) display numbers and letters or indicatecircuit conditions.

10. SERVICE MODE--An AMBER LED which is ON when theelectronic control is NOT in Mode 0.

11. Not used at this time.

12. Not used at this time.

13. Not used at this time.

NOTE: The small LED at the bottom right of the three digit displayhas no diagnostic function. The small LED will always be ON.Service personnel should always view the diagnostic window with thesmall LED at the bottom right of the three digit display. When thesmall LED is at the bottom right of the three digit display, servicepersonnel know that the window is being viewed in the correctorientation.

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177

TRACTION CONTROL SYSTEM (TCS)

The Traction Control System (TCS) uses the rear parking/secondarybrakes (spring engaged and hydraulically released) to decrease therevolutions of a spinning wheel. The TCS allows the tire with betterunderfoot conditions to receive an increased amount of torque. Thecontrols for the system are contained in the TCS electronic control. TheTCS electronic control monitors the drive wheels through three inputsignals: one at each drive axle, and one at the transmission output shaft.When a spinning drive wheel is detected, the electronic control sends asignal to the selector and proportional valves which in turn engage thebrake of the affected wheel. When the condition has improved and theratio between the right and left axles returns to 1:1, the electronic controlsends a signal to release the brake.

The TCS was formerly referred to as the "Automatic Electronic TractionAid." The operation of the system has not changed. The main differenceis the appearance of the electronic control. The TCS electronic controlnow looks like the EPTC II and ARC electronic controls. The LightEmitting Diodes (LED's) function the same as on the previous AETAelectronic control, but they are arranged in a rectangular pattern instead ofa straight line. The three digit numeric displays do not provide anyfunction at this time. Also, the TCS is not on the CAT Data Link at thistime and cannot communicate with the other electronic controls or theECAP and ET service tools.

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• TCS formerly calledAETA

• TCS uses parking/secondary brakesystem

1. TCS electroniccontrol

178

The TCS electronic control (1) is located in the compartment behind thecab. The electronic control contains a diagnostic window with 12 LightEmitting Diodes (LED’s) and a three digit numeric display. Thediagnostic window is used to interrogate the TCS for diagnosticinformation. No Service Switches or Diagnostic Modes other than Mode 0 are available at this time.

A service/retarder brake switch (2) provides an input signal to the TCSand performs two functions:

1. When the service brakes or retarder are ENGAGED, the TCS functionis stopped.

2. The service/retarder brake switch provides the input signal needed toperform a diagnostic test. When the TCS test switch and the retarderlever are ENGAGED simultaneously, the TCS will engage each rearbrake independently. Install two pressure gauges on the TCS valveand observe the pressure readings during the test cycle. The left brakepressure will decrease and increase. After a short pause, the rightbrake pressure will decrease and increase. The test will repeat as longas the TCS test switch and the retarder lever are ENGAGED.

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2

1

2. Service/retarderbrake switch

- Stops TCS function

- Performs diagnostictest

179

The TCS electronic control receives information from various inputcomponents such as the left and right wheel speed sensors, thetransmission speed sensor and the service/retarder brake switch.

Based on the input information, the TCS electronic control determineswhether the left or right rear brake should be ENGAGED. These actionsare accomplished by sending signals to various output components.

Output components include the selector solenoid and the proportionalsolenoid.

Input and output components on the block diagram are accompanied witha letter and number. The letter "A" corresponds with the 37 pin CE(Caterpillar Environmental) connector and the letter "B" corresponds withthe 10 pin Sure-Seal connector that are attached to the TCS electroniccontrol. The numbers next to the letters correspond to the pin numbers inthe connector. For example, the TCS test switch is connected to the TCSelectronic control through a wire in the 10 pin Sure-Seal connector at pinlocation 2.

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SELECTORSOLENOID - LEFT

TRANSMISSIONSPEED

B10

B9

A17

A18

A19

A36

A37

TCS

A - 37 PIN CE CONNECTORB - 10 PIN SURE-SEAL CONNECTOR

A3

A10

MACHINEHARNESS

CODES

892 - BR

893 - GN

774 - YL

213- BK

214 - BK

212 - BK

215 - BK

216 - BK

DATA LINK(NOT USED)

768 - OR

+V TO BRAKE SWITCHAND WHEEL SENSORSA9 767 - WH

BATTERY +BATTERY -

A1132- PK

A2255 - BK

SELECTORSOLENOID - RIGHT

A4 775 - BR

PROPORTIONALSOLENOIDA5 773 - GY

LEFT WHEELSPEED

A29769 - BU

RIGHT WHEELSPEED

A30770 - GN

SERVICE/RETARDERBRAKE SWITCH

B1772 - BR

TESTSWITCH

B2700 - PK

D1 D2 D3

12

2

4

5 6 7

3

8 9 10 11

13

• TCS controls brakingwith electrical inputsand outputs

180

The TCS onboard diagnostic window houses 12 status LED's along with athree digit numeric display.

The functions of the three digit display and the status LED's are:

1. Three digits (D1, D2, D3) display numbers and letters or indicatecircuit conditions (not used at this time).

2. LEFT AXLE PICKUP--An AMBER LED which is ON or FLASHINGwhen the machine is moving.

3. RIGHT AXLE PICKUP--An AMBER LED which is ON orFLASHING when the machine is moving.

4. TRANSMISSION PICKUP--An AMBER LED which is ON orFLASHING when the machine is moving.

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TCS DIAGNOSTIC WINDOW

D1 D2 D3

12

2

4

5 6 7

3

8 9 10 11

13

• TCS diagnosticwindow:

- 12 status LED's

- Three digit display

5. SPEED PICKUP--An AMBER LED which is ON or FLASHING toindicate a fault (see NOTE below).

6. VEHICLE IDENTIFICATION--An AMBER LED which is ON whenthe harness code is incorrect.

7. LEFT BRAKE SOLENOID OR HARNESS--An AMBER LEDwhich is ON when an OPEN or SHORT is present in the left brakesolenoid or harness circuit.

8. PROPORTIONAL VALVE OR HARNESS--An AMBER LEDwhich is ON when an OPEN or SHORT is present in the proportionalsolenoid valve or harness circuit.

9. RIGHT BRAKE SOLENOID OR HARNESS--An AMBER LEDwhich is ON when an OPEN or SHORT is present in the right brakesolenoid or harness circuit.

10. CONTROL BOX--A RED LED which is ON or FLASHING whenthe electronic control has FAILED and should be replaced.

11. POWER--A GREEN LED which is ON when a nominal 24 Volts isavailable between pins 1 and 2 of the electronic control 37 pinconnector.

12. SERVICE BRAKE OR RETARDER--An AMBER LED which is ONwhen the service brake or retarder is in use as sensed by a GROUNDfrom the service/retarder brake pressure switch.

13. TEST MODE--A RED LED which indicates that the test switch is inthe ON position.

NOTE: To determine which pickup is at fault, actuate the test switchwhile driving in a straight line without wheel slippage. Count thenumber of flashes in each five second series of flashes: one flashindicates the left axle pickup, two flashes indicate the right axlepickup, three flashes indicate the transmission pickup, four flashesindicate to check the LED's.

The small LED at the bottom right of the three digit display has nodiagnostic function. The small LED will always be ON. Servicepersonnel should always view the diagnostic window with the smallLED at the bottom right of the three digit display.

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• TCS valve

181

The Traction Control System (TCS) valve is mounted inside the rear ofthe right frame rail. Two solenoids are mounted on the valve.

Electrical signals from the TCS electronic control cause the selectorsolenoid valve (1) to shift and select either the left or right parking brake.If the selector valve shifts to the left parking brake hydraulic circuit, thecontrol oil is drained. The left reducing spool of the control valve can thenshift and engage the parking brake.

The proportional solenoid valve (2) controls the volume of oil beingdrained from the selected parking brake control circuit. The rate of flowis controlled by a signal from the TCS electronic control.

The pressure taps (3) can be used to measure the left and rightparking/secondary brake pressure when performing diagnostic tests on theTCS.

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1

3

2

3

1. Selector solenoid

2. Proportionalsolenoid

3. Left and rightparking/secondarybrake pressure taps

182

• TCS operation withbrakes RELEASED

Shown is the TCS with the engine running and the brakes RELEASED.

When the machine is started:

• Oil flows from parking brake release section of the pump throughthe oil filter where the flow is divided. One line from the filterdirects oil to the parking and secondary brake valve. The otherline sends oil to the pump signal port (right end of signal piston) ofthe TCS control valve.

• Oil flow to the TCS control valve causes the ball check piston tomove to the left and unseat the drain ball check valve. Openingthe drain ball check valve opens a drain passage to the hydraulictank.

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PARKINGBRAKEVALVE

TRANSMISSIONSPEED SENSOR

SPEEDDISTRIBUTOR

TESTSWITCHBRAKE

SWITCH

SELECTORSOLENOID

PROPORTIONALSOLENOID

SCREEN

ORIFICE

LEFTDRIVE AXLE

RIGHTDRIVE AXLE

TRACTION CONTROL SYSTEM (TCS)ENGINE RUNNING/BRAKES RELEASED

BALL CHECK

OUTPUTSIGNALS

INPUTSIGNALS

When the operator releases the parking brakes:

• Air pressure is increased at the parking brake valve forcing thevalve spool down.

• Parking brake release oil can now flow through the parking andsecondary brake valve to the TCS control valve.

• In the control valve, oil closes the parking/secondary ball checkvalve and flows through the screen.

• Oil flows through the right and left brake control circuit orifices.

• Oil flows to the ends of the left and right brake reducing valvespools.

• When the control circuit pressure is high enough, the reducingspools shift toward the center of the TCS control valve andparking brake release oil flows to release the brakes.

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183

• TCS operation withleft brake ENGAGED

Shown is the TCS with the engine running and the left brake ENGAGED.When signals from the sensors indicate that the left wheel is spinning 60% faster than the right wheel, the following sequence of events occurs:

1. The TCS sends a signal to the selector solenoid valve and theproportional solenoid valve.

2. The selector solenoid valve opens a passage between the outer endof the left brake pressure reducing valve and the proportionalsolenoid valve.

3. The proportional solenoid valve opens a passage from the selectorsolenoid valve to drain. The proportional solenoid valve alsocontrols the rate at which the oil is allowed to flow to drain.

4. Control circuit oil drains through the selector valve and enters theproportional valve.

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PARKINGBRAKEVALVE

TRANSMISSIONSPEED SENSOR

SPEEDDISTRIBUTOR

TESTSWITCHBRAKE

SWITCH

SELECTORSOLENOID

PROPORTIONALSOLENOID

SCREEN

ORIFICE

LEFTDRIVE AXLE

RIGHTDRIVE AXLE

TRACTION CONTROL SYSTEM (TCS)ENGINE RUNNING/LEFT BRAKE ENGAGED

BALL CHECK

OUTPUTSIGNALS

INPUTSIGNALS

5. The reducing valve spool for the left parking brake shifts andblocks the flow of oil to the parking brake.

6. Oil in the left parking brake control circuit begins to drain.

7. The left parking brake begins to ENGAGE.

8. The left brake orifice restricts the flow of oil from the parkingbrake valve.

When the signals from the sensors indicate that the left wheel is no longerspinning, the following occurs:

• The TCS stops sending signals to the selector solenoid and theproportional solenoid.

• The selector solenoid valve and proportional solenoid valve blockthe passage to drain and allow the control circuit pressure toincrease.

• The left brake reducing valve spool shifts to the center positionand blocks the passage to drain.

• Parking brake release oil is directed to the left parking brake andthe brake is RELEASED.

INSTRUCTOR NOTE: More detailed information about theTraction Control System (TCS) can be found in the Service ManualModule "Automatic Electronic Traction Aid" (Form SENR2986) andthe Technical Instruction Module "Automatic Electronic TractionAid" (Form SEGV2585).

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184

CONCLUSION

This presentation has provided a basic introduction to the Caterpillar793C Off-highway Truck. All the major component locations wereidentified and the major systems were discussed. When used inconjunction with the service manual, the information in this packageshould permit the serviceman to analyze problems in any of the majorsystems on these trucks.

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SLIDE LIST

1. Model view (left side)2. Model view (right side)3. Model view (front)4. Model view (rear)5. Subtitle slide--Walk Around Inspection6. Front wheel7. Wheel breather and suspension

breathers8. Rear brake coolers and other filters9. Hydraulic tank

10. Final drive11. Rear axle housing12. Body up retaining cable13. Fuel tank14. Primary fuel filter and drain15. Torque converter and transmission sight

gauges16. Torque converter outlet screen17. Brake cylinder breather18. Front brake oil cooler filters19. Front suspension cylinder and air dryer20. Engine oil filters21. Engine oil change connector22. Secondary fuel filters23. Manual shutdown switch24. Air filters25. Shunt tank26. Air tank, grease tank and steering tank27. Steering tank28. Secondary brake reservoir drain29. Operator's station/shift console30. Operator's station/center console31. Operator's station/light switches32. Operator's station/instrument panel33. Operator's station/VIMS34. VIMS component diagram35. Operator's station/fuse panel36. Operator's station/ET laptop37. Operator's station/VIMS laptop38. Operator's station/hoist lever control39. Operator's station/steering column40. Operator's station/pedals41. Engine/right side

42. Electronic engine control system diagram

43. ADEM II electronic control44. Atmospheric pressure sensor45. 3516B improvements46. Fuel filter restriction switch47. Crankcase pressure sensor48. 3516B improvements49. 3516B improvements50. 3516B improvements51. Engine oil pre-lubrication52. Variable speed fan control53. Engine oil renewal system54. Engine oil level sensors55. Exhaust bypass control56. Shunt tank57. Radiator58. Engine (front)59. Coolant flow switch60. Oil coolers61. Rear brake oil coolers62. Jacket water coolant flow circuit63. Engine (left side)64. Rear aftercooler sensor65. Front brake oil cooler66. Aftercooler coolant flow circuit67. Engine oil pump68. Engine oil filters69. Engine oil system flow circuit70. Primary fuel filter71. Fuel transfer pump72. Secondary fuel filters73. Fuel pressure regulator74. Fuel system flow circuit75. Air filters76. Turbo inlet sensor77. Turbochargers78. Exhaust temperature sensors79. Exhaust system flow circuit80. Subtitle slide--Power Train81. Torque converter82. Torque converter (converter drive)83. Torque converter (direct drive)

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SLIDE LIST

84. Transmission, transfer gears and differential

85. Transmission (section)86. Differential87. Rear axle flow control valve88. Rear axle89. Rear axle flow control valve90. Rear axle cooling system schematic91. Double reduction planetary (section)92. Torque converter/transmission pumps93. Transmission scavenge screen94. Torque converter suction screen95. Torque converter charging filter96. Torque converter inlet relief valve97. Torque converter outlet relief valve98. Transmission charging filter99. Torque converter lockup valve

100. Torque converter lockup circuit101. Transmission control and solenoids102. Transmission ICM control103. Transmission ICM hydraulic circuit104. Transfer gears105. Torque converter/transmission hydraulic

circuit106. EPTC II input/output diagram107. EPTC II108. EPTC II diagnostic window109. ET laptop computer110. Shift solenoids and actual gear switch111. Subtitle slide--Steering System112. Steering tank and filter113. Steering pump114. Steering valves115. Steering accumulator charging valve116. Steering pump (during charging)117. Steering pump (low pressure standby)118. Accumulator charging valve (during

charging)119. Accumulator charging valve (low

pressure standby)120. Accumulator charging valve (beginning

of cut-in)121. Steering solenoid and relief valve

122. Steering solenoid and relief valve (section)

123. Steering directional valve124. Steering directional valve (hold)125. Steering directional valve (right turn)126. Hand metering unit127. Hand metering unit (neutral)128. Hand metering unit (right turn)129. Steering accumulators130. Steering bleed down control131. Subtitle slide--Hoist System132. Hoist lever switch133. Hoist lever sensor134. Hydraulic tank135. Hydraulic tank screens136. Hoist pumps137. Hoist control valve (rear)138. Hoist control valve (left side)139. Hoist control valve (hold)140. Hoist control valve (raise)141. Hoist counterbalance valve142. Hoist control valve (lower)143. Hoist control valve (float)144. Front brake oil cooler filters145. Front brake oil cooler146. Hoist cylinders147. Hoist system circuit148. Subtitle slide--Air System and Brakes149. Oil cooled brake (section)150. Operator's station/center console151. Operator's station/steering column152. Operator's station/pedals153. Air compressor and governor154. Air dryer155. Service air reservoir (right platform)156. Secondary air reservoir (behind cab)157. Pressure protection valve158. Air charging system159. Secondary brake valve160. Parking brake release valve161. Brake pumps162. Parking brake release filter163. Towing system

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SLIDE LIST

164. Parking and secondary brake air system

165. Front of cab/service and retarder brake valves

166. Relay valves, brake cylinders and makeup tank

167. Brake cylinders and makeup tank168. Front brake oil cooler diverter valve169. Brake cylinder (engaged)170. Slack adjuster171. Slack adjuster (section)172. Service brake and retarder air system173. Brake cooling circuit174. ARC system diagram175. ARC control box176. ARC diagnostic window177. TCS control box (old and new)178. TCS control box179. TCS system diagram180. TCS diagnostic window181. TCS valve182. TCS system (brakes released)183. TCS system (brakes engaged)184. Model view

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Serviceman's Handout No. 1

793C SERVICE TOOLS

MAINTENANCE

2P8250 Filter Strap Wrench4C5084 Filter Cutting Tool4C9301 Coolant Conditioner Test Kit4C4911 Battery Load Tester1U9921 Battery Post Cleaner5P0957 Coolant and Battery Tester (°F)5P3514 Coolant and Battery Tester (°C)9U5617 Suspension oil fill unit5P8610 Nitrogen Charging Adapter (for charging two suspension cylinders)7S5437 Nitrogen Charging Group7S9394 Tire Fill Air Hose (6 ft. long)7F8240 Tire Valve Repair Tool1P0545 Tire Gauge6V4040 Nitrogen Tire Inflation Kit1U5551 Valve Extension (for charging steering accumulators)5P1720 Seal Pick

ENGINE

9S9082 Engine Turning Tool4C8241* Valve Lash Setting Gauge1U5440 Fuel Flow Monitor Group1U5470 Engine Pressure Gauge Group9U5132 Injector Height Tool Group

*3500B engines require the 125-2744 Base instead of the 125-2742 Base used with 3500engines.

ELECTRONIC CONTROL DIAGNOSTICS

Laptop Computer for VIMS/Electronic TechnicianIBM Compatible Computer with DB-9 orDB-25 Pin RS-232 Serial Port

Vital Information Management System (VIMS)JERD2093 Caterpillar Common Services SoftwareJERD2137 VIMS Software LicenseJERD2138 VIMS Software SubscriptionJERD2139 VIMS Software Subscription (additional copies)127-9797 VIMS Computer to Truck Adapter Cable

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Serviceman's Handout No. 2

ELECTRONIC CONTROL DIAGNOSTICS

Electronic Control Analyzer Programmer (ECAP)8T8697 ECAPNEXG4521 Machine Functions Service Program Module (SPM) for ECAP7X1700* Communication AdapterNEXG4523* Service Program Module (SPM) for Communication Adapter139-4166* ECAP/ET Cable (connects Communication Adapter to machine;

can also be used for Flash programming)7X1420 ECAP Cable (earlier ECAP plastic port 1)7X1851 ECAP Cable (current ECAP metal port 1)7X1703 Holder for Communication Adapter7X1180 ECAP Internal Expansion Board7X1695* Timing Probe Cable6V2197* Timing Probe Magnetic Pickup6V3093* Timing Probe Adapter Sleeve

*Also required to run ET.

Electronic Technician (ET)JERD2124 Electronic Technician (ET) Software License (alternate to ECAP)JERD2129 ET Software Subscription (Engines and Machines)JERD2142 ET Software Subscription (Machines Only)7X1425 ET Adapter Cable (connects ET to Communication Adapter)LERQ3133 HyperACCESS/5--Flash File Download Software

ELECTRICAL

4C3406 Deutsch Connector Kit (HD10 with crimp tool)9U7246 Deutsch Connector Kit (DT no crimp tool)1U5804 Deutsch Connector Crimp Tool (part of 4C3406)6V3000 Sure Seal Repair Kit1P2305 Terminal and Connector Repair Kit8T0900 AC/DC Clamp-on Ammeter6V7070 Digital Multimeter (Beckman)9U7330 Fluke 87 Digital Multimeter

(reads Pulse Width Modulation PWM on EUI/ARC/VIMS)8T3224 Multimeter Probes (for checking CE connectors)7X1710 Signal Reading Probe Group (spade slides in connectors)4C9024 Service Tool and Soldering Iron Battery9U7560 Field Soldering Iron Group (used with 4C9024)5P4205 5/32 T-handle Allen wrench for DRC connectors

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Serviceman's Handout No. 3

POWER TRAIN

8T5200 Signal Generator (substitutes transmission/engine speed signals)6V4157 Transmission/Hydraulic System Pressure Gauge Group6V6064 Test Cover (top of ICM transmission)

TEMPERATURE MEASUREMENTS

4C6500 Digital Thermometer Group8T2844 Temperature Recorder Stickers4C6090 Multichannel Temperature Selector Group6V9130 Temperature Adapter Group (for Digital Multimeter)8T5334 Surface Temperature Probe123-6700 Infrared Thermometer with Laser Sight

MISCELLANEOUS

1U5481 Pressure Gauge Group1U5482 Pressure Adapter Group for 1U54814C4892 ORFS Fitting and Gauge Group8T5320 Hydraulic Test Group (contains blocker plates)5P1404 Adapter (7/8-14 x 9/16-18 for brake bleed port)1U5000 Auxiliary Power Unit (gas engine pump for towing)1U5525 Auxiliary Power Unit Attachment Group1U8869 Digital Dial Indicator6V6042 Dial Indicator Contact Group8T5096 Magnetic Dial Indicator Group8T1000 Digital Positioner GroupFT1975 Suspension Gauge Block

Stop Watch

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Serviceman's Handout No. 4

VIMS KEYPAD OPERATIONS

- Scroll parameters monitored by VIMS by depressing the GAUGE key.

- Payload Monitor ON/OFF PAYLOAD 7295623

- Calibrate Payload Monitor PAYCAL 729225

- Payload Resettable Totals TOT 868

- Reset Displayed Data RESET 73738

- Display Self Test TEST 8378

- Reset Service Light SVCLIT 782548

- Set Lube Cycle Times LUBSET 582738

- Manual Lube LUBMAN 582626

- Show Acknowledged Events EACK 3225

- Show Event Statistics ESTAT 37828

- Show Event List ELIST 35478

- Start Event Recorder EREC 3732

- Start/Stop Data Logger DLOG 3564

- Reset Data Logger DLRES 35737

- Odometer Set/Reset ODO 636(requires VIMS PC connection)

- Machine Status MSTAT 67828

- Change Language LA 52

- Change Units UN 86

- Change Backlight BLT 258

- Change Display Contrast CON 266(requires Updated Message Center)

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INSTRUCTOR NOTES

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INSTRUCTOR NOTES

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INSTRUCTOR NOTES

SESV1682 Printed in U.S.A.3/97