precision electronic solutions - actron | automotive ... electronic solutions cp9001 instrucciónes...

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Car Computer Code ReaderDomestic GM & Saturn

Lector de Códigosde Computadorasde AutomóvilGM y Saturnnacionales de EE.UU. conSystemas MCU y EEC-IV (para EUA)

CODE SCANNER¨

precis ion electronic solut ions

CP9001

Instrucciónes en español - página 99

Lecteur de coded'ordinateurautomobileGM y Saturndomestiques Étas-Unisavec Systèmes MCU ou EEC-IV

Instructions en français - page 199

FAVOR DE LEER INSTRUCTCTIVO ANTES DE USAR EL ARTICULO

Para Nombre, Domicilio y Telefono del Importador: Ver Empaque

Tensión: 14VHecho en: China

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Congratulations on purchasing yourActron Code Scanner for accessingengine trouble codes required forrepairing vehicles equipped withcomputers. Your Actron CodeScanner is made by Actron, thelargest and most trusted name inautomotive diagnostic equipment forthe home mechanic. You can haveconfidence this product maintains thehighest quality in manufacturing, andwill provide you years of reliableservice.

This instruction manual is divided intoseveral key sections. You will finddetailed steps on using the CodeScanner and important informationabout trouble code meanings, how acomputer controls engine operation,and more!

Identifying the problem is the firststep in solving that problem. YourActron Code Scanner can help youdetermine by accessing the enginecomputer trouble codes. Armed withthat knowledge, you can either referto an appropriate service manual ordiscuss your problem with a knowl-edgeable service technician. In eitherevent you can save yourself a lot ofvaluable time and money in autorepair. And feel confident that yourvehicle’s problem has been fixed!

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1 About Codes ................................. 3

2 When to Read Codes ................... 5

3 Reading Codes............................. 6

4 Using Codes in a BasicTroubleshooting Procedure ........ 10

5 Code Meanings .......................... 14

6 Additional Code ScannerDiagnostic Features ................... 22

7 Computer Basics ........................ 25

8 Glossary ..................................... 31

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9 ABS Basics ................................. 38

10 ABS Safety ................................. 44

11 ABS Tips ..................................... 45

12 Reading ABS Codes .................. 47

System 1: Bosch 2S ......................... 51

System 2: Bosch 2U (Version A) ...... 56

System 3: Bosch 2U (Version B) ...... 62

System 4: Bosch 2U (Version C) ...... 68

System 5: Teves Mark II (Version A) 74

System 6: Teves Mark II (Version B) 79

System 7: Kelsey-Hayes RWAL ....... 84

System 8: Kelsey-Hayes 4WAL ........ 88

Applications ................................. 94

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• Always wear approved eye protection.

• Always operate the vehicle in a well ventilated area.Do not inhale exhaust gases – they are very poisonous!

• Always keep yourself, tools and test equipment away from allmoving or hot engine parts.

• Always make sure the vehicle is in park (Automatic transmis-sion) or neutral (manual transmission) and that the parkingbrake is firmly set. Block the drive wheels.

• Never leave vehicle unattended while running tests.

• Never lay tools on vehicle battery. You may short the terminalstogether causing harm to yourself, the tools or the battery.

• Never smoke or have open flames near vehicle.Vapors from gasoline and charging battery are highly flam-mable and explosive.

• Always keep a fire extinguisher suitable for gasoline/electrical/chemical fires handy.

• Always turn ignition key OFF when connecting or disconnect-ing electrical components, unless otherwise instructed.

• Always follow vehicle manufacturer’s warnings, cautions andservice procedures.

��Some vehicles are equipped with safety air bags.

You must follow vehicle service manual cautions when workingaround the air bag components or wiring. If the cautions are notfollowed, the air bag may open up unexpectedly, resulting inpersonal injury. Note that the air bag can still open up severalminutes after the ignition key is off (or even if the vehicle batteryis disconnected) because of a special energy reserve module.

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�� The computer system in today’svehicles does more than control engineoperation – it can help you findproblems, too! Special testing abilitiesare permanently programmed into thecomputer by factory engineers. Thesetests check the components connectedto the computer which are used for(typically): fuel delivery, idle speedcontrol, spark timing and emissionsystems. Mechanics have used thesetests for years. Now you can do thesame thing by using the Actron CodeScanner tool!

�� The engine computer runs the specialtests. The type of testing varies withmanufacturer, engine, model year etc.There is no “universal” test that is thesame for all vehicles. The tests examineINPUTS (electrical signals going IN tothe computer) and OUTPUTS(electrical signals coming OUT of thecomputer.) Input signals which have“wrong” values or output circuits whichdon’t behave correctly are noted by thetest program and the results are storedin the computer’s memory. These testsare important. The computer can notcontrol the engine properly if it has badinputs or outputs!

�� The test results are stored by usingcode numbers, usually called “troublecodes” or “diagnostic codes.” Forexample, a code 22 might mean“throttle position sensor signal voltageis too low.” Code meanings are listedin Section 4. Specific code definitionsvary with manufacturer, engine andmodel year, so you may want to refer toa vehicle service manual for additionalinformation. These manuals are

available from the manufacturer, otherpublishers or your local public library.(See Vehicle Service Info on page 4.)

!�� "��# �#� ��You obtain trouble codes from theengine computer memory by using theActron Code Scanner tool. Refer tosection 2 for details. After you get thetrouble codes, you can either:

• Have your vehicle professionallyserviced. Trouble codes indicateproblems found by the computer.

or,

• Repair the vehicle yourself usingtrouble codes to help pinpoint theproblem.

�� $��#�� %�� & �#� ��To find the problem cause yourself, youneed perform special test procedurescalled “diagnostics”. These proceduresare in the vehicle service manual.There are many possible causes forany problem. For example, supposeyou turned on a wall switch in yourhome and the ceiling light did not turnon. Is it a bad bulb or light socket? Isthe bulb installed correctly? Are thereproblems with the wiring or wall switch?Maybe there is no power coming intothe house! As you can see, there aremany possible causes. The diagnosticswritten for servicing a particular troublecode take into account all the possibili-ties. If you follow these procedures, youshould be able to find the problemcausing the code and fix it if you wantto “do-it-yourself.”

�� % �� &�� #��Using the Actron Code Scanner toobtain trouble codes is fast and easy.

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Trouble codes give you valuableknowledge – whether you go forprofessional vehicle servicing or “do-it-yourself. ” Now that you know what

trouble codes are and where they comefrom, you are well on your way to fixingtoday’s computer controlled vehicles!

'�� (��The following is a list of publishers who have manuals containing trouble coderepair procedures and related information. Some manuals may be available atauto parts stores or your local public library. For others, you need to write foravailability and prices, specifying the make, style and model year of your vehicle.

'�#�� (��)Chilton Book Co.Chilton WayRadnor, PA 19089

Haynes Publications861 Lawrence DriveNewbury Park, CA 91320

Cordura PublicationsMitchell Manuals, Inc.P. O. Box 26260San Diego, CA 92126

Motor’s Auto Repair ManualHearst Company250 W. 55th StreetNew York, NY 10019

Suitable manuals have titles such as:

“Electronic Engine Controls”

“Fuel Injection and FeedbackCarburetors”

“Fuel Injection and Electronic EngineControls”

“Emissions Control Manual”

...or similar titles

�� (�� *��(�� BuickTuar CompanyPost Office Box 354Flint, MI 48501

OldsmobileLansing LithographersPost Office Box 23188Lansing, MI 48909

Cadillac, Chevrolet, PontiacHelm IncorporatedPost Office Box 07130Detroit, MI 48207

Electronic engine control informationfor all GM manuals is located in“Additional Code Scanner DiagnosticFeatures”, page 22.

�� (�� Adistra Corporationc/o Saturn PublicationsPost Office Box 1000Plymouth, MI 48170

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+�#�� , ��#�) -��.Light ON and stays ON (when theengine is RUNNING)

– The computer sees a problem thatdoes not go away. (A “hard” failure.)

– The light will stay on as long as theproblem is present.

– A trouble code is stored in computermemory. (A “hard” code.)

– Use the Code Scanner at the earliestconvenient time to obtain codes.Refer to section 3, “Reading Codes”.

+�#�� , ��#�) /��-��.Light ON and then goes OFF (whenthe engine is RUNNING)

– The computer saw a problem, butthe problem went away. (An“intermittent” failure.)

– A trouble code is stored in computermemory. (An “intermittent” code.)

– The light went out because theproblem went away, but the codestays in memory.

– Use the Code Scanner at theearliest convenient time to obtaincodes. Refer to Section 3, “ReadingCodes”.

NOTE: The computer will automati-cally erase codes after several restarts(typically 50) if the problem does notreturn.

� -��% !�� 0�+�#�� , ��#��Most likely this condition is not due tocomputer system failures - but readingcodes can still be useful as part of abasic troubleshooting procedure.Review Section 4, “Using Codes”before proceeding to Section 3,“Reading Codes”.

• The “Check Engine” light comes ONwhen the engine is RUNNING

or,

• Vehicle engine is running poorly and“Check Engine” light is OFF.

�#� +�#�� , ��#�The engine computer turns the “CheckEngine” light on and off as needed.

This dashboard message light is eitheramber or red and labeled:

– “Check Engine” or,

– “Service Engine Soon” or,

– “Service Engine Now” or,

– marked with a small engine picture.

+�#�� , ��#�) 0��1��The “Check Engine” light is normallyOFF when the engine is RUNNING.

NOTE: The light will come on whenignition key is in ON position, but theengine is OFF. (For example, beforeyou start the engine.) This is a normaltest of all the dashboard messagelights.

If the “Check Engine” light does notcome on, you have an electricalproblem which needs repair. Refer tothe “Diagnostic Circuit Check” steps inthe “Basic Diagnostic Procedures”section of your vehicle service manual.(Manual sources listed on page 4.)

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1) Safety First!

• Set the parking brake.

• Put shift lever in PARK (automatictransmission) or NEUTRAL(manual transmission).

• Block the drive wheels.

• Make sure ignition key is in OFFposition.

2) Test the “Check Engine” Light

(Also called “Service Engine Soon”,“Service Engine Now” or labeledwith a small engine picture.)

• Turn ignition key from OFF to ONposition, but do not start theengine.

• Verify that the light turns on.

• The connecter is located under thedashboard on the driver’s side.

Exceptions:

–LeMans: Located behind passen-ger side kick panel. Remove snap-on cover for access.

–Fiero: Located in the centerconsole behind cover panel.

–Corvette: Sometimes located incenterconsole behind ashtray.Consult service manual for exactlocation.

• The connector may be in full view, orit may be recessed behind a panel.An opening in the panel allowsaccess to recessed connectors.

OFF

ON

• If the light does not turn on, youhave a problem with this circuitwhich must be repaired beforeproceeding. Refer to the “Diagnos-tic Circuit Check” procedure inyour vehicle service manual. (Seemanual listings on page 4.)

• Turn ignition key OFF.

3) Have a Penciland PaperReady

This is forwriting downall the codes.

4) Find the Computer Test Connector

• Service manuals call this connec-tor the Assembly Line DiagnosticLink (ALDL) connector. It may alsobe called the Assembly LineCommunication Link (ALCL) orsimply test connector.

• The connector may have a slip-oncover labeled “DiagnosticConnector.” Remove cover fortesting. Replace cover aftertesting. Some vehicles require thiscover in place for proper operation.

5) Verify Ignition Key is OFF

OFF ON

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TM

GM 1982 & higher - CP 9001

Car Computer Code Reader

6) Plug the Code Scanner into theTest Connector. Put TEST switchon ENGINE.

sequence is repeated. This continuesuntil the ignition key is turned OFF (soyou can double check your code list).

Example of code 12 only:

❊ PAUSE ❊❊FLASH (pause) FLASH FLASH

(longer pause)


(longer pause)


(even longer pause, then start over again)

Example of code series 12 and 24:


(longer pause)


(longer pause)


even longer pause, then go to next code)

❊❊ PAUSE ❊❊❊❊FLASH FLASH (pause)

FLASH FLASH FLASH FLASH

(longer pause)



(longer pause)



(even longer pause, then start all overfrom the very beginning)

• The Code Scanner only fits ONEWAY into the test connector.

• The Code Scanner will not harmthe vehicle engine computer.

NOTE: The Code Scanner does notuse all of the test connectorcontacts. Also, one Code Scannerpin may plug into an empty testconnector position. This is normal.

7) Turn Ignition Key to ON Positionbut DO NOT START THE ENGINE

You may hear some clicking sounds comingfrom under the hood. This is normal.

WARNING: Stay away from theradiator cooling fan! It may turn on.

8) Get Codes from the Flashing“Check Engine” Light

NOTE: If the light does not flash, youhave a problem which must berepaired before proceeding. Refer to“Diagnostic Circuit Check” chart invehicle service manual.

Count flashes to get trouble codes.

Code 12 looks like:


(FLASH = 1, FLASH FLASH = 2.

Put 1 and 2 together = code 12.)

Code 23 looks like:

❊❊ PAUSE ❊❊❊FLASH FLASH (pause)FLASH FLASH FLASH

• Each code is flashed three (3) timesbefore the next trouble code is sent.

• After all codes are sent, the whole

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• A code 12 is always sent evenwhen the computer sees noproblem. This tells you thecomputer diagnostic checks areworking properly. If you do not geta code 12, or if the “Check Engine”light does not flash you have aproblem which must be repaired.Refer to “Diagnostic Circuit Check”procedure in vehicle servicemanual. (See manual listing onpage 4.)

• All codes are two (2) digits long.

• Codes are sent in numeric orderfrom the lowest number to thehighest.

�� )The engine computer can send troublecodes for transmission problems - ifthe vehicle has a computer controlledtransmission.

NOTE: Some diesel powered truckshave a computer controlled transmis-sion. These vehicles will only sendtransmission related codes since thethe diesel itself is not computercontrolled.

• GM Vehicles

–The “Check Engine” light flashesboth engine codes and transmis-sion codes.

• Saturn Vehicles

–The “Check Engine” light flashesengine codes.

–The “Shift to D2” light flashestransmission codes.

Look for a code 11 flashed on the“Check Engine” light. This is a signaltelling you transmission codes will thenbe flashed on the “Shift to D2” light.Transmission codes are flashed in away similar to engine codes.

9) Turn Ignition Key OFF

10)Remove Code Scanner and Re-install Connector Cover, ifsupplied.

The computer system is now backto normal operation.

11)Refer to “Test Results” chart onpage 9

This completes the code readingprocedure.

At this point you can either:


or,


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TEST RESULTS COMMENTS

• You have a problem which needs repairbefore using the Code Scanner.

• Refer to “Diagnostic Circuit Check” chartin vehicle service manual.

No indication on“Check Engine”lightor,Did not receiveCode 12.

• Computer did NOT find a problem.

• If a driveability problem persists, perform“Visual Inspection” and “Basic MechanicalChecks” (Section 4, “Using Codes”.)

• Refer to “Diagnosis by Symptom” charts invehicle service manual. (Additionalelectrical and mechanical checks arelisted.)

Code 12 only.

• Computer found problems in vehicle.

• Follow steps in Section 4, “Using Codes”.

• Code definitions are in Section 5, “CodeMeanings”.

– GM engine and transmission codes starton page 14.

– Saturn engine codes start on page 14.

• Saturn vehicles only: Code 11 meanstransmission codes are flashed on “Shiftto D2” light.

– Saturn transmission codes start onpage 19.

Received Code 12along with othercodes.

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A driveability problem can have manypossible causes which are not relatedto the computer system. Readingcodes is one part of a good trouble-shooting procedure consisting of:

1) Visual Inspection

2) Basic Mechanical Checks

3) Reading Codes

4) Using the Vehicle Service Manual

5) Erasing Codes

2� '�� /��Doing a thorough visual and “hands-on” underhood inspection beforestarting any diagnostic procedure isessential!

You can find the cause of manydrivability problems by just looking,thereby saving yourself a lot of time.

• Are routine maintenance itemsO.K.?

– Clean air filter

– Correct fluid levels

– Recommended tire pressure

– Good ignition system components- spark plugs, wires and the like.

• Has the vehicle been servicedrecently?

– Sometimes things get recon-nected in the wrong place, or notat all.

• Don’t take shortcuts.

– Inspect hoses and wiring whichmay be difficult to see because oflocation beneath air cleanerhousings, alternators and similarcomponents.

• Inspect all vacuum hoses for:

–Correct routing. (Hoses may bemissing or misconnected.) Referto vehicle service manual, orVehicle Emission Control

Information(VECI)decallocated inthe enginecompart-ment.

–Pinches and kinks.

–Splits, cuts or breaks.

• Inspect wiring for:

–Contactwith sharpedges.(Thishappensoften.)

–Contactwith hotsurfaces, such as exhaustmanifolds.

–Pinched, burned or chafedinsulation.

–Proper routing and connections.

• Checkelectricalconnectorsfor:

–Corrosionon pins.

–Bent ordamaged pins.

–Contacts not properly seated inhousing.

–Bad wire crimps to terminals.

Problems with connectors arecommon in the engine control system.Inspect carefully. Note that someconnectors use a special grease onthe contacts to prevent corrosion. Donot wipe off! Obtain extra grease, ifneeded, from your vehicle dealer. It isa special type for this purpose.

FRONTOF CAR

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3� �� (��#�� #��Don’t overlook the basic checks listedon the next page. Mechanical problemsby themselves can always create enginetroubles. Even worse, these problemscan make a good sensor send anincorrect signal to the computer. Thenthe computer runs the engine improp-erly or sets a trouble code.

• Cylinder compression:

–Check for propercompression ineach cylinder.

–Refer to vehicleservice manual forspecifications.

• Exhaust backpressure:

–Check for anyrestrictions in theexhaust system.

• Ignition timing (If adjustable):

–Verify timing is withinspecifica-tion.

–Refer tovehicleservice manual,or VehicleEmission ControlInformation(VECI) decallocated in theengine compartment.

–Be sure to disable computer sparkadvance timing circuit, if used,when checking basic timing!

• Air induction system:

–Check forintakemanifoldvacuumleaks.

–Check forcarbon orvarnishbuild-up on throttle valve or idlespeed control device.

4� !�� Refer to Section 3, “Reading Codes”.Remember there are two types ofcodes:

• “Hard” codes – codes for problemswhich are present now.

• “Intermittent” codes – codes forproblems which happened in thepast, but are not happening now.

Remember...

–“Check Engine” light ON: You haveat least one “hard” code stored inmemory. (You may have more“hard” or “intermittent” codesstored, also.)

– “Check Engine” light OFF: Storedcodes are for “intermittent”problems. (Exception: sometimesthere are minor “hard” faults whichdo not turn on the “Check Engine”light.)

How to Tell “Hard” Codes from“Intermittent” Codes

Do the following if you are not sure:

• Write down all codes (except code12). For example: 15, 34.

• Erase codes from computermemory. (Refer to Step 5.)

• Drive vehicle for at least 10 minutesat normal temperature, cruise speedand load. (The computer may wantto verify a fault for several minutesbefore storing a code.)

• Read codes again. Codes whichreturn are “hard” faults. Codes whichdo not return are “intermittent” faults.For example, if you see code 15 (butnot 34) then you know code 15 is a“hard” code and code 34 was“intermittent”.

You troubleshoot the “hard” problemsdifferently from the “intermittent” ones.

5� 6� '�#�� (��Dealing with “Hard” Codes

• Refer tothe vehicleservicemanualdiagnosticcodecharts.These will be in Section 6E in theGM manual. Other publications havethis information in books or sections

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Made in U.S.A.

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60

90

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210

180

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270

300

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15

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called “Computerized EngineControls”, “Electronic EngineControls” or “Tune-Up Information.”

• Follow all the steps in the diagnosticprocedure for the trouble code.

• Be sure to erase the trouble codesfrom computer memory aftercompleting repair work. (See Step 5,“Erasing Codes from ComputerMemory”.)

• Drive vehicle for at least 10 minutesat normal temperature, cruise speedand load.

– Read codes again to verify troublecode is gone (problem fixed). Othercodes may have been repaired atthe same time!

Dealing with “Intermittent” Codes

These codes are for problems whichhappened in the past, but are notpresent now.

• Usually these problems are due toloose connections or bad wiring.The problem cause can often befound with a thorough visual and“hands-on” inspection. (Refer toStep 1, “Visual Inspection”.)

• Refer to the vehicle service manualdiagnostic code section. You can notuse the code chart proceduresbecause they are for “hard”problems - those which are presentnow. However, the charts havesuggestions for dealing withintermittents and can tell you wherebad connections, etc., might exist.You can also refer to the “Diagnosisby Symptom” charts.

• Be sure to erase the trouble codesfrom computer memory aftercompleting repair work. (See Step 5,“Erasing Codes from ComputerMemory”.)

• Drive vehicle for at least 10 minutesat normal temperature, cruise speedand load.

– Read codes again to verify troublecode is gone (problem fixed).Other codes may have beenrepaired at the same time!

Dealing with No Trouble Codes

Have a driveability problem, but onlyget code 12? Make sure you do Step1, “Visual Inspection” and Step 2,“Basic Mechanical Checks”. If you didnot find the problem, then refer to“Diagnosis by Symptom” charts invehicle service manual.

7� �� (��%Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again. Note: Thecomputer will automatically erasecodes after several restarts (typically50) if the problem does not return.

*(Proceed as follows:

• Observe all safety precautions. (Seepage 2.)

• Turn ignition key ON.

• Insert CodeScanner.Make sureTEST switchis in ENGINEposition!

• Turn ignition keyOFF.

• Remove theECM fusefrom thefuse blockfor 10seconds.

• Replacefuse.

• RemoveCode Scanner.

If ECM fuse cannot be located, then –

• Disconnect power to the computer.To do this:

–Disconnectthe positivebatteryterminal“pigtail”,

OR

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


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TESTABS ENGINE

–Open the in-line fuse holder goingto the positive battery terminal, OR

–Disconnect negative batteryterminal – but this will also eraseother items too, such as digitalclock settings and preset digitalradio tuning.

• All the trouble codes are nowerased from computer memory!

• Wait thirty (30)seconds.

• Reconnect powerto the computer.

IMPORTANT: Thecomputer has a “learning” ability totake care of minor variations in enginecontrol operation. Whenever you erasethe computer memory by disconnect-ing power, the computer has to“relearn” various things. Vehicleperformance may be noticeablydifferent until it “relearns.” Thistemporary situation is normal. The“learning” process takes place duringwarm engine driving.

��6!0Use the GM method, or proceed asfollows:

• Observe all safety precautions. (Seepage 2.)

Warning: Stay away from theengine cooling fan. It may turn onduring this procedure.

• Turn ignition key ON, but DO NOTSTART THEENGINE.

• Put TEST switchon ENGINE.

• Plug andunplug the CodeScannerinto thetestconnector3 timeswithin 5seconds.

• All the trouble codes are nowerased from computer memory!


• RemoveCodeScanner andre-installconnectorcover, ifsupplied.

NOTE:

• The engine control computer isusually called ECM (Engine ControlModule) or PCM (Powertrain ControlModule) in the vehicle servicemanuals.

• Information flags and “Intermittent”codes may not by erased using thisprocedure. The presence of thesecodes will not cause any driveabilityor future self-diagnostics problems.

TM

GM 1982 & higher - CP 9001


SECONDS30

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�� 0��)• Code meanings can

vary with vehicle,model year, enginetype and options.

• If a code number hasmore than onedefinition listed, notethat that only onedefinition applies toyour vehicle. Consultservice manual to getthe specific definitionand troubleshootingprocedure for yourvehicle.

• Follow vehicle servicemanual procedures tofind the cause of thecode.

!��)1) Visual inspections are

important!

2) Problems with wiringand connectors arecommon, especiallyfor intermittent faults.

3) Mechanical problems(vacuum leaks,binding or stickinglinkages, etc.) canmake a good sensorsend an incorrectsignal to the computer.This can cause aTrouble Code.

4) Incorrect informationfrom a sensor maycause the computer tocontrol the engine inthe wrong way. Faultyengine operationcould even make adifferent good sensor

send an incorrectsignal to the computerand generate moretrouble codes!

�� )This page: (Codes fromflashing “Check Engine”light.)

• GM engine codes

• GM electronictransmission codes

• Saturn engine codes

Page 19 (Codes fromflashing “Shift to D2”light.)

• Saturn electronictransmission codes

Refer to Section 4,“Using Codes” fortroubleshooting tips andsteps to erase codesfrom computer memory.

"-.��/�"-��(Saturn transmission code list begins on page 19)

��Transaxle codes present(Saturn).Whenever code 11 is sent,it means transmissioncodes will be flashed nexton the “Shift to D2” light.Refer to page 19 for Saturntransmission code list.

��Diagnostic test is workingproperly. (Engine computerverifies no RPM ReferencePulses are present duringengine off testing.)

��Oxygen (O2) sensor -signal stays low (“lean”)during warm engine cruiseor sensor circuit is open orleft sensor circuit is open(dual sensor models).

��Coolant temperaturesensor (CTS) - signalvoltage is low.

��Coolant temperaturesensor (CTS) - signalvoltage is high.

��Battery or alternator problem- voltage too high or low.ORDirect ignition system(DIS) fault - line open orshorted to ground.ORIgnition system fault - Lossof 2X or Low ResolutionPulse signal.ORTransmission speed error.

��RPM signal problem.ORCamshaft sensor - circuitproblems.ORElectronic Control Module(ECM) computer circuitproblem - Pull-up resistor(Saturn).

��Camshaft or Crankshaftsensor - circuit problems.ORFuel Injector circuit is notworking properly - possibleblown fuel injector fuse.

��Ignition system fault -Intermittent 7X signal orloss of 58X signal or 6Xsignal (Saturn).

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��Throttle position sensor(TPS) - signal voltage ishigh during engine idle ordeceleration.

��Throttle position sensor(TPS) - signal voltage islow during engine idle.ORFuel cutoff relay circuit -open or shorted to ground.

��Manifold air temperature(MAT) sensor - signalvoltage is low or high.ORThrottle position sensor(TPS) error.ORMixture Control (M/C)solenoid - open or shortcircuit problems.

��Vehicle speed sensor(VSS) - open or shortcircuit problems.

��Manifold air temperature(MAT) sensor - signalvoltage is low.ORVacuum switching valvecircuit - open or shorted toground.ORATS sensor - signalvoltage is high.

��Quad-Driver module orQuad-driver No. 1 error.

��2nd gear switch.ORQuad-Driver module orQuad-driver No. 2 error.

��3rd gear switch.ORQuad-Driver module orQuad-driver No. 3 error(Corvette).OR(Transmission) Fluidpressure switch assembly- open or short circuitproblems.

��4th gear switch.ORQuad-Driver module orQuad-driver No. 3 error.ORSecondary air injectionsystem - circuit problems.

��Manifold absolute pressure(MAP) sensor - signalvoltage is low.ORFuel injector.ORPark/Neutral switch -circuit problems.ORCAM sensor - circuitproblems.OREngine speed controlgovernor malfunction.(Van)ORTurbocharger wastegateoverboost.ORWastegate electrical signal- open or shorted toground.ORPurge solenoid voltagehigh (carburetor engines)

��Barometric pressure (BARO)sensor circuit failure.ORExhaust gas recirculation(EGR) valve diagnosticswitch - closed duringengine start-up or openwhen EGR flow requestedby ECM.OREGR/EVRV.

��Mass air flow (MAF)sensor - signal voltage orfrequency is high duringengine idle.ORManifold absolute pressure(MAP) sensor - signalvoltage is high duringengine idle. (Note: Enginemis-fire or unstable idlemay cause this code.)

��Mass air flow (MAF) sensorsignal voltage orfrequency is low duringengine cruise.ORManifold absolute pressure(MAP) sensor - signalvoltage is low duringignition on.ORPressure sensor circuit -signal voltage too high orlow (carburetor engines).

��Idle air control (IAC)system problem - can notset desired RPM.

��Mass air flow (MAF) sensor- burn-off circuit problem.ORTransmission shift problem(electronically controlledtransmissions only).ORDirect ignition system (DIS)fault - loss of 24X signal orextra or missing pulses inelectronic spark timing(EST) signal.ORIgnition system fault - lossof High Resolution Pulsesignal.

��Brake switch stuck “on”.

��Brake switch circuit fault.ORKnock sensor (KS) - opencircuit problem.

��Torque converter clutch(TCC) circuit fault.ORClutch switch circuitproblems.ORKnock sensor (KS) - shortcircuit problem.

18

��Cam sensor (CAM) failure.ORCylinder select error.ORTach input error - noreference pulses duringengine run.ORElectronic spark timing(EST) circuit - open orshorted to ground duringengine run.ORDirect ignition system(DIS) fault - bypass circuitopen or shorted to groundduring engine run.ORIgnition system fault - lossof 1X Reference Pulsesignal.

��Electronic spark timing(EST) circuit - open orshorted to ground duringengine run.ORDirect ignition system(DIS) fault - bypass circuitopen or shorted to groundduring engine run.ORFuel cutoff relay circuit -open or shorted to ground.

��Electronic spark timing(EST) circuit - low voltagedetected.ORElectronic spark control(ESC) - circuit problems.

��Lean exhaust indication -oxygen (O2) sensor voltagestays low after one or twominutes of engine run.(Left sensor on dualsensor engines.)

��Rich exhaust indication -oxygen (O

2) sensor voltage

stays high after one minuteof engine run. (Left sensoron dual sensor engines.)

��Vehicle anti-theft system(VATS) failure.OR

Power steering pressureswitch failure.

��Electronic control module(ECM) computer circuitproblems - universalasynchronous receiver/transmitter (UART) link ordata loss.ORKnock sensor modulelocated in the computer isnot working properly.

��Misfire symptom.ORMass air flow (MAF)sensor - open or shortcircuit MAF sensor signal.

��High idle RPM or vacuumleak (Saturn).

��Electronic control module(ECM) computer circuitproblems - faultyprogrammable read-onlymemory (PROM), MEM-CAL, ECM or checksumerrors.

��Electronic control module(ECM) computer circuitproblems - faulty or missingCALPAC or MEM-CAL,analog to digital converter(A/D) error or Quad-Drivermodule (QDM) fault.OROil temperature sensor -signal voltage is low(Corvette).ORSystem voltage high for along period of time.(Electronic transmissionnote: this fault may causeother codes to be set.)

��Over voltage condition.(Electronic transmissionnote: this fault may causeother codes to be set.)ORExhaust gas recirculation(EGR) - system problemsor EGR Solenoid No.1problem.OR

Voltage reference error.ORVehicle anti-theft system(VATS) problems.

��Low fuel pump voltage.ORFuel pump relay.OREGR Solenoid No. 2failure.ORQuad-Driver module(QDM) output failure.ORMixture Control (M/C)solenoid - circuit voltagetoo high.

��Electronic control module(ECM) computer circuitproblems - ECM failure,serial bus error, SAD erroror fuel lean malfunction.OREGR Solenoid No. 3failure.

��Corrosivity/add coolant.ORPort throttle systemvacuum sensor problems.ORQuad-Driver “B” fault.

��Boost Control problem.

��Vehicle anti-theft system(VATS) problem.ORTransmission TemperatureSensor (TTS) - short circuitproblem in sensor orwiring.ORTransmission fluidtemperature high.

��Transmission TemperatureSensor (TTS) - open circuitproblem in sensor,connector or wiring.ORTransmission fluidtemperature low.

19

��Oxygen (O2) sensordegraded.ORPort throttle system error.ORCruise control problems -vent solenoid circuit.ORAir Conditioner (A/C)system performanceproblems.

��Gear switch circuitproblems.OROil temperature sensor -signal voltage is high(Corvette).ORCruise control problems -vacuum solenoid circuit.

��Manifold absolute pressure(MAP) sensor - signalvoltage is high.ORSmall EGR failure.ORRight oxygen (O

2) sensor

failure (dual sensorengines).ORCruise control systemproblem.

��Manifold absolute pressure(MAP) sensor - signalvoltage is low.ORMedium EGR failure.ORRight oxygen(O2) sensor -lean condition indicated(dual sensor engines).

��Large EGR failure.ORFuel injector current low.ORRight oxygen (O2) sensor -rich condition indicated(dual sensor engines).ORCruise control positionsensor problem.

��Air Conditioner (A/C)pressure sensor - circuitproblems or low A/C charge.ORElectronic Control Module(ECM) computer circuitproblem - internal resetoccurred.OR(Transmission) 3-2 shiftcontrol solenoid - circuitproblems.

��Cruise control - switchcircuit problems.ORAir Conditioner (A/C)pressure sensor - circuitproblems.ORTorque Converter Clutch(TCC) solenoid - circuitproblems.ORCruise control switches -circuit problems.

��Cruise control - systemcircuit problems.ORAir Conditioner (A/C)clutch relay - short circuit.OR(Transmission) Overdriveratio error - engine RPMgreater than input speed.

��Air Conditioner (A/C)system - pressure switchor A/C clutch relay circuitproblems.ORTorque converter clutchstuck “on”.

��Air Conditioner (A/C)pressure sensor - signalvoltage too high.

��Air Conditioner (A/C)evaporator temperaturesensor - signal voltage toolow.

��Gear Select switch - circuitproblems.OR

Vehicle Speed Sensor(VSS) - loss of signal.

��Air Conditioner (A/C)evaporator temperaturesensor - signal voltage toohigh.OR(Transmission) Pressurecontrol solenoid - circuitproblems.

��Traction control circuitvoltage low.

��Exhaust gas recirculation(EGR) system - SolenoidNo.1 problem.ORSystem voltage low -charging system problems.ORTransmission voltage low -low system voltagepossibly caused bygenerator voltage supplycircuit or power traincontrol module (PCM).

��Exhaust gas recirculation(EGR) system - SolenoidNo.2 problem.

��Exhaust gas recirculation(EGR) system - SolenoidNo.3 problem.ORPrimary cooling fan relaydriver circuit - circuitproblems.

��Secondary cooling fanrelay driver circuit - circuitproblems.

��Vehicle Speed Sensor(VSS) - signal voltage toohigh.ORTransmission TemperatureSensor (TTS) - hightemperature indicated.

��Vehicle Speed Sensor(VSS) - signal voltage toolow.

20

��Brake switch circuitproblems.ORAnti-Lock Brake System(ABS) message fault(Saturn).OR(Transmission) Solenoid“B” (3-2 shift solenoid) -open or short circuitproblems.

��Ignition system fault - 3Xsignal problem.ORElectronic control module(ECM) computer circuitproblem - internalcommunications failure(Saturn).OR(Transmission) Solenoid“A” (1-2 shift solenoid) -open or short circuitproblems.

��Torque Converter Clutch(TCC) solenoid - circuitproblems.ORReverse Inhibit - open orshort circuit in reverseinhibit solenoid coil.

��3-2 Control solenoid - openor short circuit problems.ORSkip shift solenoid - openor short circuit problems.

��Electronic control module(ECM) computer circuitproblems - faultyprogrammable read-onlymemory (PROM).OR(Transmission) Input oroutput speed sensor -circuit problems. (Speedsensor signals do notagree with with selectedgear range.)ORTorque converter clutch(TCC) - TCC ismechanically stuck on.

��Electronic control module(ECM) computer circuitproblems - faulty analog-to-digital (A/D) converter.OR(Transmission) Low gearerror - transmission in 3rdor 4th gear when computercommanding 1st or 2ndgear.

��Electronic control module(ECM) computer circuitproblems - faultyelectrically erasableprogrammable read-onlymemory (EEPROM).OR(Transmission) High gearerror - transmission in 1stor 2nd gear whencomputer commanding 3rdor 4th gear.

��Electronic control module(ECM) computer circuitproblem - internal resetoccurred.

��Skip shift light - open orshort circuit problems inskip shift light circuit.

��Pressure control solenoid -transmission line pressurenot at desired level.

��Change oil light - wrongvoltage is present in lightcircuit for more than 26seconds.

��Transmission voltage low -low system voltagepossibly caused bygenerator voltage supplycircuit or power traincontrol module (PCM).ORLow oil light - wrongvoltage is present in lightcircuit for more than 26seconds.

��Vehicle speed sensor(VSS) - output circuitproblems.

��Tachometer output circuit

problems.

21

� ��(GM/Saturn engine codes and GM transmission code list begins on page 14.)

Note: Code numberslabeled “InformationFlag” may be sent alongwith the regular(unlabeled) troublecodes. The computersends Information Flagsto help you find thecause of a trouble code.Note that conditionswhich only cause anInformation Flag will notturn on the “CheckEngine” light. Refer tovehicle service manualtroubleshooting charts.

�� Line pressure high.

�� Line pressure low.

�� Hot light.

��No 1st gear.OR

�� Electrical variable orifice(EVO) fault.

�� No gears available.

��2nd gear stuck “on”.

��No 2nd gear.

��No 3rd gear.

��No 4th gear.

��No torque converter clutch.

��Torque converter clutchstuck”on”.

�� Quick quad-driver outputfault - open or short circuiton any of the qaud-drivermodule circuits (QDM) thatlasts 5 seconds or longer.

��Transaxle temperaturecircuit open.

��Transaxle temperaturecircuit grounded.

�� Powertrain Control Module(PCM) computer circuitproblem - communicationsfailure.

��No turbine speed signal.

��Turbine speed signalnoise.

��Vehicle Speed Sensor(VSS) circuit - no signal.

��Vehicle Speed Sensor(VSS) circuit - signal noise.

�� Master relay - open orgrounded.

�� Master relay - shorted.

�� Gear selector switch circuitproblem - no signal.

�� Gear selector switch circuitproblem - invalid signal.

�� Powertrain Control Module(PCM) computer circuitproblem - communicationinterrupt failure.

��Hold mode voltage is toolow.OR

�� Reference inputintermittent or noisy -missing or extra ignitionreference pulses aredetected by powertraincontrol module (PCM).

�� Gear selector error signal.

�� Powertrain Control Module(PCM) computer circuitproblem - serial link datainvalid.

��Hold mode stuck “on”.OR

�� Battery voltage out ofrange - battery voltage hasdropped below 11 volts orhas increased above 17volts.

��Hold mode stuck “off”.OR

�� ESC (Knock present) -powertrain control module(PCM) can not reduceengine knock by retardingtiming.

��Powertrain Control Module(PCM) computer circuitproblem - analog to digital(A/D) converter error.OR

�� 5-volt reference ground -flag will set if manifold

22

absolute pressure (MAP)sensor signal, handwheelsensor signal, throttleposition sensor (TPS)signal are zero volts.

��Transaxle temperaturesensor failure.

�� Generic Field-EffectTransistor (FET) driverfailure.

�� Powertrain Control Module(PCM) computer circuitproblem - non volatileRandom Access Memory(RAM) failure.

�� Battery voltage unstable -battery voltage changesmore than 3 voltsinstantaneously.

�� Powertrain Control Module(PCM) computer circuitproblem - ProgrammableRead-Only Memory(PROM) failure.OR

�� 6X Signal fault - 6X pulsesdo not occur betweeneach reference pulse or a6X pulse does notimmediately follow areference pulse. Possibleopen or intermittent in DISmodule harness.

�� Powertrain Control Module(PCM) computer circuitproblem - interrupt failure.

�� Powertrain Control Module(PCM) computer circuitproblem - Random AccessMemory (RAM) failure.OR

�� Option check sum error -flag will be set if tire sizeand options do notcompare with those storedin the powertrain controlmodule (PCM).

�� Powertrain Control Module(PCM) computer circuitproblem - ElectricallyErasable ProgrammableRead-Only Memory (EEPROM) failure.

�� Ignition voltage problem -too high or low.

�� Clamp shorted.

�� Clamp open.OR

�� Handwheel sensor circuitfault - handwheel sensoroutput voltage is out oftolerance.

�� Line circuit grounded oropen.

�� Line circuit shorted.

�� 2nd line circuit - groundedor open.OR

�� Cooling system hightemperature - enginecoolant temperature isgreater than 239°F(118°C).

�� 2nd line circuit - shorted.OR

�� Cooling system lowtemperature - enginecoolant temperature is lessthan 32°F (0°C).

�� 3rd line circuit - groundedor open.OR

�� Coolant sensor signalunstable - coolanttemperature sensor (CTS)indicates a change of morethan 59°F (15°C)instantaneously.

�� 3rd line circuit - shorted.OR

�� Coolant/Transmissiontemperature sensor ratioerror - indicates a degradingcoolant temperature sensor(CTS) if transmissiontemperature sensor (TTS) isworking properly.

�� 3rd gear stuck “on”.OR

�� Air temperature sensorsignal unstable - airtemperature sensor (ATS)indicates a change of morethan 59°F (15°C)instantaneously.

23

�� 4th line circuit - groundedor open.OR

�� Throttle position sensor(TPS) to manifold absolutepressure (MAP) sensorvoltage out of range - flagis set if TPS and MAPvoltage readings dontagree with internalrelational tables stored inthe powertrain controlmodule (PCM).

�� 4th line circuit - shorted.

�� 4th gear stuck “on”.

�� Torque Converter Clutch(TCC) circuit - grounded oropen.

�� Torque Converter Clutch(TCC) circuit - shorted.

�� Transaxle temperatureunstable.

�� Transaxle temperaturelow.OR

�� Low coolant - coolantswitch opens for 20seconds with enginerunning.

�� Brake switch stuck open.

�� Brake switch stuck closed.

�� Engine speed invalid.

�� Torque Converter Clutch(TCC) hold circuit -grounded or open.

�� Torque Converter Clutch(TCC) hold circuit -shorted.

�� Master relay stuck “on”.

�� Assembly Line DiagnosticLink (ALDL) - serialcommunication linkinterrupt.

�� Clamp circuit - intermittentfault.

�� Torque Converter Clutch(TCC) hold circuit -intermittent fault.

�� Master enable relay circuit- intermittent fault.

�� Line circuit - intermittentfault.

�� Torque Converter Clutch(TCC) circuit - intermittentfault.

�� 2nd gear circuit -intermittent fault.

�� 3rd gear circuit -intermittent fault.

�� 4th gear circuit -intermittent fault.

24

�� 0�� 1��

This section contains...

Relay and Solenoid Circuit Test: Youcan switch on most of the computercontrolled relay and solenoid circuits -for checking relay operation or makingwiring voltage checks.

Field Service Test (Fuel InjectedEngines Only): A quick check of thefuel control system to verify properoperation.

Become familiar with Code Scanneruse (Section 3) before using thefollowing procedures.

+��

Computer controlled relay and solenoidcoils are commonly wired as follows:

• One side of the coil is connected toa source of vehicle battery power.

• The other side of the coil is wired tothe computer.

Inside the computer housing is atransistor switch (often called a

“driver”). The computer energizes thecoil by using the transistor switch.

Transistor ON:

– Transistor electrically connects endof coil to circuit ground.

– Coil is ON because circuit iscomplete. (Coil connected to batterypower and ground.)

Transistor OFF:

– Transistor disconnects end of coilfrom circuit ground.

– Coil is OFF because circuit is open.(Coil not connected to circuitground.)

You can switch on most of thecomputer controlled relay and solenoidcircuits except the fuel pump relay andfuel injectors. This is helpful forchecking relay operation or makingwiring voltage checks. Do the following:

1) Safety First!





TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


RELAY COIL

12 voltsor more

Less than1.5 volts

COMPUTER

Transistor ON*

!��8 10

RELAY COIL

12 voltsor more

COMPUTER

Transistor OFF*

!��8 199

12 voltsor more

*Transistor action isshown using a “switch”representation forclarity.

2) Plug the Code Scannerinto the Test Connector.Put TEST switch onENGINE.

25

3) Turn Ignition Key to ON Positionbut DO NOT START THE ENGINE.

• WARNING: Stay away from theradiator cooling fan! It may turn on.

• Ignore the flashing Check Enginelight.

4) Computer Controlled Relays andSolenoids are Turned ON

Exception: Fuel pump and fuelinjectors are OFF. (Refer to vehicleservice manual for any otherexceptions.)

• Make any relay orsolenoid circuitchecks at thistime. Note thefollowing specialcircuit actions...

– Fuel InjectedEngines Only:The Idle AirControl (IAC)motor fully extends (most vehicles)or moves back and forth.

– Carbureted Engines Only: TheIdle Speed Control (ISC) motor, ifused on vehicle, moves back andforth. Also, the Exhaust GasRecirculation (EGR) solenoid isenergized for 25 seconds.

5) Turn Ignition Key OFF.

• Remove Code Scanner and re-install connector cover, if supplied.

• The computer system is now backto normal.

• This completes the Relay andSolenoid Circuit Test.

Model CP 7676Automotive Volt/Ohm

Dwell/Tach

10 Megohm V

V

Ohms

RPMX10

Dwell

4CYL

5CYL

CYL6

CYL8

4CYL

5CYL

CYL6

CYL8

OFF

2002K

200K

2M

20K

2

20

750

200

20M

Digital Engine Analyzer

1�� 2�1��(�3��

This is a quick check of the fuel controlsystem to verify proper operation -especially after repair work. Servicemanuals call this the “Field ServiceMode”. Do the following:

1) Safety First!





2) Test the “Check Engine” Light

(Also called“Service EngineSoon”, “ServiceEngine Now” orlabeled with a small engine picture.)

• Turn ignition key from OFF to ONposition, but do not start theengine.

• Verify that the light turns on.

• If the light does not turn on, youhave a problem with this circuitwhich must be repaired beforeproceeding. Refer to the “Diagnos-tic Circuit Check” procedure in yourvehicle service manual. (Seemanual listings on page 4.)

3) Start the Engine

WARNING: Always operate vehiclein well ventilated area. Exhaustgases are very poisonous!

4) Plug theCodeScanner intothe TestConnector.Put TESTswitch onENGINE.

The enginecomputer is nowin the “Field

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


26

Diagnostic Mode.” The flashing“Check Engine” light shows how thefuel control system is operating.See below.

Read Section 7, “Computer Basics”or Section 8, “Glossary” for anexplanation of Open Loop andClosed Loop operation.

IMPORTANT: The oxygen sensorneeds to be hot so the computercan check the signal for proper fueldelivery. Warm the engine by idlingfor 2 minutes at 2000 RPM. Then,gently rev the engine from idle topart throttle several times. (Thiscreates a changing sensor signalfor the computer.) Finally, keep thethrottle steady, or at idle, for the restof the test.

4�� 5� ��

The computer is inOpen Loopoperation. Thecomputer will run“open loop” if it doesnot see an oxygen sensor signalbecause...

– The oxygen sensor is not hotenough to operate (normalcondition if engine too cold orsensor cooled down during idle)

or,

– Open circuit problems exist (badsensor or wiring). Note that thiscondition will generate a troublecode.

4��

The computer is inClosed Loopoperation. Theoxygen sensor issending a signal.

• Light equally ON and OFF whileflashing: system is runningcorrectly (proper air/fuel mixture).

• Light mostly ON while flashing:system is running “rich”.

• Light mostly OFF while flashing:system is running “lean”.

Various mechanical, electronic orwiring problems can cause thecomputer to sense a “rich” or “lean”running engine. Usually theseconditions will generate a trouble code,such as 44 (lean exhaust) or 45 (richexhaust). Follow vehicle servicemanual troubleshooting charts to findthe cause. The Field Service Test letsyou check to see if the problem wasfixed. (Light flashing equally ON andOFF once a second.)

Note: While in the “Field ServiceMode”...

– New trouble codes are not storedin computer memory.

– On some engines, the computerwill send a signal for a fixed sparkadvance.


• RemoveCodeScannerand re-installconnectorcover, ifsupplied.

• Thecomputersystem isnow backto normal.

• This completes the Field ServiceTest.

TM

GM 1982 & higher - CP 9001


27

��%�� &

This section further explains the enginecomputer control system, the types ofsensors and how the computer controlsfuel delivery, idle speed and timing.

The following is an introduction tocomputer controlled engine systems.Additional information may be found inbooks dealing with this subjectavailable at your local library or autoparts store. The more you know aboutthe computer system, the better andfaster you can troubleshoot and fixproblems.

:#% ��;Computer controls were installed invehicles to meet Federal Governmentregulations for lower emissions andbetter fuel economy. This all began inthe early 1980’s when purely mechani-cal control systems just were not goodenough anymore. A computer could beprogrammed to precisely control theengine under various operatingconditions and eliminate somemechanical parts making the enginemore reliable.

Note that vehicle service manuals referto the computer as either the ECM(Engine Control Module) or PCM(Powertrain Control Module).

:#� �#� �� The main control areas of the computerare:

• Fuel delivery

• Idle speed

• Spark advance timing

• Emission devices (EGR valve,carbon cannister, etc.)

The changes made to the basic engineto allow a computer to control thesetasks are the only differences betweenan older engine and a computerizedone. A little later we will discuss justhow the computer handles these tasks.

:#� # 01� �#��;A computer controlled engine isbasically the same as earlier types. It isstill an internal combustion engine withpistons, spark plugs, valves and cams.The ignition, charging, starting, andexhaust systems are almost the same,as well. You test and repair thesesystems the same way as before,using familiar tools. The instructionmanuals for these tools show you howto perform the tests. Your compressiongauge, vacuum pump, dwell-tachmeter, engine analyzer, timing light,etc., are still valuable!

�#� �� %��The computer module is the “heart” ofthe system. It is sealed in a metal boxand linked to the rest of the system bya wiring harness. The computermodule is located in the passengercompartment, usually behind thedashboard or front kick panels. Thisprotects the electronics from moisture,extreme temperatures and excessvibration, which are common in theengine compartment.

The computer is permanentlyprogrammed by factory engineers. Theprogram is a complex list of instruc-tions telling the computer how tocontrol the engine under variousdriving conditions. To do its job, thecomputer needs to know what ishappening and then it needs devices tocontrol things.

�� #� ��The computer can only work withelectrical signals The job of the sensoris to take something the computerneeds to know, such as enginetemperature, and convert it to anelectrical signal which the computer

28

can understand.You can think of sensors as “high tech”senders the devices found in oldervehicles for gauges and dashboardmessage lights (oil pressure, fuel level,etc.) Signals running into the computerare referred to as “inputs.”

Sensors monitor such things as:

• Engine temperature

• Intake manifold vacuum

• Throttle position

• RPM

• Incoming air (temperature, amount)

• Exhaust gas oxygen content

Most engine computer systems will usethe sensor types listed above.Additional sensors may be useddepending upon the engine, vehicletype or other tasks the computer mustdo. Note that information from onesensor may be used by the computerfor many different tasks. For example,engine temperature is something thecomputer needs to know whencontrolling fuel delivery, spark timing,idle speed and emission systems. Thesensor information may be veryimportant for one engine controlfunction, but only used to “fine tune” asecond one.

�#�� %�� • Thermistor – This is a resistor

whose resistance changes withtemperature. It is used to measuretemperatures of coolant or incomingair. It has two wires connected to it.

• Potentiometer – This signals aposition, such as throttle position orEGR valve position. It connects tothree wires: one for power, one forground and one to carry the positionsignal back to the computer.

• Switches – These are either ON

OUTP

UT

ACTU

ATORS

SENSORS INPUT

BRAINS OF THE COMPUTER

(voltage signal to the computer) orOFF (no voltage signal to thecomputer). Switches connect to twowires and tell the computer simplethings, such as whether or not theair conditioner is running.

• Signal Generator – These createtheir own signal to tell the computerof some condition, such as exhaustgas oxygen content, camshaftposition, or intake manifold vacuum.They may have one, two or threewires connected to them.

�#� �� #�� "��#��The computer can only send outelectrical signals (referred to as“outputs”). Devices called actuatorsare powered by the computer tocontrol things. Actuator types include:

• Solenoids – These are used tocontrol a vacuum signal, bleed air,control fuel flow, etc.

• Relays – These switch highamperage power devices on and off,such as electric fuel pumps orelectric cooling fans.

• Motors – Small motors are oftenused to control idle speed.

1�#�� Not all of the computer outgoingsignals go to actuators. Sometimesinformation is sent to electronicmodules, such as ignition or tripcomputer.

<�" �#� �� %Good performance and low emissionsdepend upon precise fuel control.Early computer controlled vehiclesused electronically adjustablecarburetors, but fuel injectors weresoon introduced.

The job of the computer is to providethe optimum mixture of air and fuel(air/fuel ratio) to the engine for bestperformance under all operatingconditions.

29

The computer needs to know:

• ...what the engine operatingcondition is.

Sensors used: coolant temperature,throttle position, manifold absolutepressure, mass air flow, RPM.

• ...how much air is coming into theengine.

Sensors used: mass air flow or acombination of manifold absolutepressure, manifold air temperature,RPM.

• ...how much fuel is being delivered.

The computer knows this by howlong it turns on the fuel injectors.(The computer uses a “feedbackcontrol” or “duty cycle” solenoid onelectronic controlled carburetors.)

• ...that everything is working the wayit should.

Sensor used: exhaust gas oxygensensor.

Note: Not all engines use every sensorlisted above.

�� "�� An example of “Open Loop” opera-tion...

The coolant temperature sensor tellsthe computer how warm the engine is.Factory engineers know what the bestair/fuel mixture is for the engine atvarious operating temperatures. (Morefuel is needed for a cold engine.) Thisinformation is permanently pro-grammed into the computer. After thecomputer knows the engine tempera-ture, it determines the amount of aircoming in, then it will look at itsprogramming to find out how much fuelto deliver and operate the fuel injectorsaccordingly. (Engines with electroniccarburetors don’t do any of this. Theyhave a thermostatically controlledchoke just like non-computer engines.)

This process is an example of “OpenLoop” operation by the computer. Thecontrol system performs an action(expecting a certain result), but has noway of verifying if the desired resultswere achieved. In this case, thecomputer operates a fuel injector

expecting a certain amount of fuel tobe delivered. (The computer assumeseverything in the fuel system isoperating as expected.) In open loopoperation, the computer has no way ofchecking the actual amount of fueldelivered. Thus, a faulty fuel injector orincorrect fuel pressure can change theamount of fuel delivered and thecomputer would not know it.

The computer system is forced tooperate “open loop” because nosensor type is available which canmeasure air/fuel ratios when theengine is cold.

<�� An example of “Closed Loop”operation...

The computer watches the coolanttemperature and throttle positionsensors to tell when the engine is allwarmed up and cruising. As before,the computer determines the amountof air coming into the engine, thendelivers the amount of fuel that shouldprovide the optimum air/fuel mixture.The big difference is that this time thecomputer uses the oxygen sensor tocheck how well its doing and re-adjustthings, if needed, to make sure the fueldelivery is correct. For example: If theoxygen sensor indicates a “rich”condition, the computer will compen-sate by reducing fuel delivery until theoxygen sensor signals an optimum air/fuel ratio. Likewise, the computer willcompensate for a “lean” condition byadding fuel until the oxygen sensoronce again signals an optimum air/fuelmixture.

This is an example of “Closed Loop”operation. The control systemperforms an action (expecting acertain result), then checks the resultsand corrects its actions (if necessary)until the desired results are achieved.

The oxygen sensor only works when itis very hot. During cold engine warm-up, and sometimes at idle, the sensorwill be too cool to operate (no signalsent). The computer must operate"open loop" during this time because itcannot use the sensor to check the air/fuel ratio.

30

��= �� As long as the engine and oxygensensor are hot, the computer canoperate “closed loop” for best economyand least emissions. During the driveconditions listed on the left, thecomputer may have to ignore the sensorand run “open loop,” relying on internalprogramming for fuel delivery instruc-tions. During idle, for example, theoxygen sensor may cool down and stopsending a signal. A different situationcan occur during wide-open-throttleacceleration. The computer sometimesadds additional fuel (on purpose) fortemporary acceleration power. Thecomputer knows it is running “rich” so itignores the sensor signal until the wide-open-throttle condition is over.

<�" �#� �� Throttle position and RPM sensors tellthe computer when the vehicle isidling. (Sometimes an idle positionswitch on the throttle is used.) Thecomputer simply watches RPM andadjusts an idle speed control deviceon the vehicle to maintain the desiredidle condition. Note that this is anotherexample of “closed loop” operation.The computer performs an action(activating an idle control device),then watches the results of its action(engine RPM) and readjusts asnecessary until the desired idle speedis achieved.

There are two types of idle speedcontrol devices. The first is anadjustable throttle stop that ispositioned by a computer controlledmotor. The second method lets thethrottle close completely. An airpassage bypassing the throttle allowsthe engine to idle. A computercontrolled motor adjusts air flowthrough the bypass to set idle speed.

Smaller engines can stumble or stallat idle when the air conditionercompressor turns on or the powersteering is used. To prevent this,switches tell the computer when thesedemands are coming so it canincrease the idle accordingly.

<�" �#� �� You set spark timing in a non-computerengine by using a timing light andadjusting the distributor at idle RPM.During vehicle operation, timing ischanged by either engine vacuum(vacuum advance function) or by engineRPM (centrifugal advance function.)These spark timing changes are donemechanically inside the distributor.

Computer controlled vehicles using adistributor still have you set spark timingby using a timing light and adjusting thedistributor at idle RPM. The timingchanges which occur during vehicleoperation, however, are controlledelectronically. The computer looks atsensors to determine vehicle speed,engine load and temperature. (RPM,throttle position, coolant temperatureand manifold pressure or mass air flowsensors are used.) Then, the computeradjusts timing according to factoryprogrammed instructions. Somevehicles have a “knock” sensor. Thecomputer can “fine tune” the sparktiming if this sensor signals an engineknock condition. A timing signal (“EST” -Electronic Spark Timing) is sent by thecomputer to an ignition module whicheventually creates the spark. Thecomputer uses a crankshaft positionsensor to determine piston position, soit can send the spark timing (EST)signal at the proper moment.

Newer ignition systems use nodistributor. There are several versions,such as Computer Controlled CoilIgnition (C3I), Direct Ignition System(DIS), Integrated Direct Ignition (IDI)and Opti-Spark. These systems usemultiple ignition coils. (2 spark plugsare wired to each coil.) Sensors forcrankshaft position or camshaftposition (or both) are used by both theignition module and computer to firethe correct coil at the proper time. Thecomputer provides spark advancetiming as before - by looking at vehiclespeed, engine load and temperature.(RPM, throttle position, coolanttemperature and manifold pressure ormass air flow sensors are used.) Referto vehicle service manual for detailedignition system descriptions.

31

��

> �*! ��&#�� * !��'� �The EGR valve lets exhaust gases re-enter the intake manifold and mix withthe incoming air/fuel. The presence ofexhaust gases reduces combustiontemperatures in the cylinders and thisreduces poisonous NOx emissions.The computer controls the flow ofgases through the EGR valve. TheEGR system is only used during warmengine cruise conditions. A partiallyopen EGR valve at other times cancause stalling. Various types of EGRsystems are used on different vehicles.The EGR valve may be operated byengine vacuum or with a computercontrolled electrical (or vacuum)signal. See “EGR” in Glossary (Section8) for more details.

> �� /�?�� %��This system reduces harmful carbonmonoxide (CO) and hydrocarbon (HC)emissions. The computer takes outsideair from an air pump and directs it tothe exhaust manifold during enginewarm-up. (The extra air helps partiallyburned exhaust gases to completelyburn and reduce pollution.) Afterwarm-up, depending upon vehicle, theair pump may send air down to thecatalytic convertor or “dump” it back tothe atmosphere. Various types of AirInjection systems are used on differentvehicles. Refer to “Air InjectionSystem” in Glossary (Section 8) formore explanation.

> 9�� !�� %�%��A special canister collects vaporsevaporating from the fuel tank,preventing them from escaping into theatmosphere and causing pollution.During warm engine cruise conditions,the computer opens a connectionbetween the cannister and the engine(by energizing the Purge solenoid.)Then, engine vacuum draws the trappedfuel vapors into the engine for burning.

��

The computer often controls variousother functions around the vehicle.Detailed explanations may be found inyour vehicle service manual. Sometypical examples are...

> �� #The computer can turn off the airconditioner to reduce engine loading.This is desirable during heavyacceleration, engine cranking or lowspeed steering maneuvering. Thecomputer may also disable the airconditioner when refrigerant pressureis too low (or high) to prevent A/Cdamage. The computer stops the airconditioner by using a relay todisconnect voltage from the A/Cclutch.

> !�� 9�The computer controls engine coolingfan (electric type) operation on mostvehicles. Usually the fan is turned onwhen engine temperature exceeds acertain level or when the air condition-ing system is used. The computer usesa relay to power the fan. Somevehicles have a second fan for extracooling.

> '�� -�"�� Saturn Vehicles: This system deliverspower steering assist based on vehiclespeed - little assist during high speedstraight steering, increased assist forlow speed turns. The computercontrols fluid flow in the power steeringpump using an Electronic VariableOrifice (EVO) actuator (more fluid flowprovides more steering assist). Referto “EVO” in Glossary (Section 8) formore details.

> ��@�� #The computer can control the lock-upclutch in an automatic transmissiontorque convertor. The clutch is lockedduring steady, warm engine cruise

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conditions. This improves fueleconomy by eliminating power loss inthe torque convertor. The computerenergizes a solenoid to achieve lock-up. Signals from the engine coolanttemperature, throttle position andvehicle speed sensors are used.

> �� Some transmissions have computercontrolled shifting. Two solenoidsmounted in the transmission areenergized singly, or in combination, toselect a gear ratio. The solenoidsdirect the flow of fluid within thetransmission to cause the shift. Thecomputer uses throttle position,

vehicle speed, engine loading andother sensor information to determinethe optimum shift performance. Thecomputer can also set the shift quality- anywhere from harsh to smooth. Thisis done using a “force motor” actuatorin the transmission to adjust theinternal fluid line pressure.

-�� The Glossary (Section 8) describesvarious sensors and actuators used incomputer controlled engine systems.You can learn more by reading thesedefinitions.

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�� Air conditioner.

�� !�� "The ECM uses this relay toenergize the A/C clutch - toturn the A/C system on oroff.

�� #��$�� %�&� �'An input signal to the ECMindicating that either the A/C compressor is running orthat A/C operation is beingrequested (depends uponvehicle). Then ECMadjusts idle speed toprevent engine stallingwhen the A/C system isengaged. The ECM mayalso turn on the enginecooling fan.

�� (��This sensor is connectedto the A/C refrigerant line.It measures refrigerantpressure and sends avoltage signal to the ECM.The ECM will turn off theA/C system (by de-energizing the A/C Clutchrelay) to preventcompressor damage if thepressure is too high or low.

�� (��)��!This is a mechanical switchconnected to the A/Crefrigerant line. The switch isactivated (which sends asignal to the ECM) when theA/C refrigerant pressurebecomes too low. The ECMwill turn off the A/C system(by de-energizing the A/CClutch relay) to preventcompressor damage. Somevehicles have a secondswitch activated when therefrigerant pressure is toohigh.

�� Devices which arepowered by the ECM tocontrol things. Actuatortypes include relays,

solenoids and motors.Actuators allow the ECM tocontrol engine operation.

��*Air/fuel.

��+��, ��$��,'�"��-This is an emission controlsystem operated by theECM. During cold enginewarm-up, an air pumpinjects outside air into theexhaust manifold to helpburn hot exhaust gases.This reduces pollution andspeeds warm-up of oxygensensor and catalyticconvertor. After the engineis warm, the air will eitherbe “dumped” back to theatmosphere (or into the aircleaner) or sent into thecatalytic convertor. Thereare various versions of theAIR system - dependsupon vehicle.

The air pump is usually beltdriven by the vehicleengine. The ECM controlsair flow from the pump byoperating two electricallypowered solenoid valves.An Electric Air Divert Valveeither “dumps” the air toatmosphere (valveunpowered) or sends theair further into the system(valve energized). Asecond control valve, theAir Switching Valve, routsthe air to the catalyticconvertor (valveunpowered) or to theexhaust manifold (valveenergized). These twovalves may be individualcomponents or combinedinto one assembly.

The ECM normally sendsair to the catalyticconvertor during warmengine operation. The ECMwill divert air away from theconvertor to preventoverheating damage undercertain operatingconditions, such as decel,high RPM or “rich” air/fuelconditions.

Some vehicles have anelectrically powered airpump controlled by theECM. This system injectsair into the exhaustmanifold only when thepump is energized andrunning. No air is injectedwhen the pump is off. (Theair pump does not send airto the catalytic convertor inthis version.)

�./.Assembly Line DiagnosticLink. This is the connectorthat the Code Scannerplugs into for testingpurposes. The connector iswired to the ECM, and isusually located under thedashboard on the driver’sside.

0�,#Barometric PressureSensor. (See MAP sensorfor explanation.)

0��1Used on certainsupercharger equippedengines. This solenoid isnormally energized by theECM. (This allows thesupercharger system tooperate normally.) TheECM de-energizes thesolenoid during higherengine speed and loadconditions to reduce boostpressure.

0� 2��)��!��&� �An input signal to the ECMindicating that the brakepedal is being depressed.Vehicles with CruiseControl systems monitorthe brake switch todetermine when to engage(or disengage) the cruisecontrol function. The brakeswitch may also have acircuit supplying power tothe Torque ConvertorClutch (TCC) solenoid.This connection ensuresthe TCC solenoid willdisengage when the brakepedal is depressed (seeTCC definition).

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�Computer CommandControl. The name for theGM electronic enginecontrol system used onmost vehicles built since1982.

�.(�3A “spare tire” for the ECM.It is circuitry which canoperate the vehicle fuelinjectors in a limitedfashion should the ECMmalfunction. The CALPAKis hidden behind anaccess door on the ECM.The replaceable CALPAKmodule is only used oncertain ECM’s.

�4Camshaft Position Sensor.This sensor sends afrequency signal to theECM. Vehicles withsequential fuel injection(SFI) use this signal tosynchronize the injectorfiring order. Some DIStype ignition systems usethis signal to synchronizespark plug firing.

��1�.��$ �.'This is when a controlsystem performs an action(expecting a certainresult), then checks theresults and corrects itsactions (if necessary) untilthe desired results areachieved. Example: Fueldelivery. The ECMoperates a fuel injector ina way which should deliveran optimum air/fuelmixture - if everything inthe fuel system isoperating as expected! Inclosed loop operation, theECM uses the oxygensensor to check theresults. (Fuel delivery maybe different than expectedbecause of variations infuel pressure or injectoroperation.) If the oxygensensor indicates a “rich”condition, the ECM willcompensate by reducingfuel delivery until theoxygen sensor signals anoptimum air/fuel ratio.Likewise, the ECM willcompensate for a “lean”condition by adding fueluntil the oxygen sensoronce again signals anoptimum air/fuel mixture.

Thus, closed loop operationmeans the ECM can “finetune” control of a system toget an exact result providingthe ECM has a sensor (orother means) to checkresults.

��"An unbroken, continuouscircuit through which anelectric current can flow.

(%Crankshaft PositionSensor. This sensor sendsa frequency signal to theECM. It is used toreference fuel injectoroperation and synchronizespark plug firing ondistributorless ignitionsystems (DIS).

5%Coolant TemperatureSensor. This sensor is athermistor - a resistorwhose resistancedecreases with increasesin temperature. The sensoris threaded into the engineblock and contacts theengine coolant. The ECMuses this signal for controlof fuel delivery, sparkadvance and EGR flow.

/�&�� %�&� �An electronic signal whichhas only two (2) voltagevalues: a “low” value (closeto zero) and a “high” value(usually 5 volts or greater).Sometimes the low voltagecondition is called “Off” andthe high voltage conditionis called “On”. Signalswhich can have anyvoltage value are called“analog” signals.

/�%Distributorless IgnitionSystem or Direct IgnitionSystem. A system whichproduces the ignition sparkwithout the use of adistributor.

/��6�A transistor “switch” insidethe ECM used to applypower to an externaldevice. This allows theECM to control relays,solenoids and smallmotors.

/��"� "��A term applied tofrequency signals - thosewhich are constantlyswitching between a smallvoltage value (close tozero) and a larger value(usually 5 volts or greater).Duty cycle is thepercentage of time thesignal has a large voltagevalue. For example, if thesignal is “high” (largevoltage) half of the timethen the duty cycle is 50%.If the signal is “high” onlyone fourth of the time, thenthe duty cycle is 25%. Aduty cycle of 0% meansthe signal is always at a“low” value and notchanging. A duty cycle of100% means the signal isalways at a “high” valueand not changing. Theengine control computeruses duty cycle typesignals when it wants morethan just “on-off” control ofan actuator. This is how itworks: A 50% duty cyclesignal going to a vacuumswitching solenoid meansthe solenoid will be “on”(passing full vacuum) halfthe time and “off” (passingno vacuum) half the time.The average amount ofvacuum passing throughthe solenoid will be onehalf the full value becausethe solenoid is only “on” forone half the time. (Thesignal switches at a rapidrate, such as ten times asecond.) Thus, thecomputer can get avacuum controlled actuatorto move half way between“no vacuum” position and“full vacuum” position.Other positions can beachieved by changing theduty cycle of the controlsignal which in turnchanges the averageamount of control vacuum.

/74Digital Volt Meter. Aninstrument using a numericreadout to displaymeasured voltage valuesas opposed to a movingneedle on a gauge face.Usually the instrument hasother measuringcapabilities, such asresistance and current,and may be called a DMM

35

(Digital Multi-Meter). MostDVM’s have 10 Megohminput impedance. Thismeans the circuit undertest will not beelectronically disturbedwhen the DVM isconnected for a measure-ment.

8 4Electronic Control Module.The “brains” of the enginecontrol system. It is acomputer housed in ametal box with a number ofsensors and actuatorsconnected with a wiringharness. Its job is tocontrol fuel delivery, idlespeed, spark advancetiming and emissionsystems. The ECMreceives information fromsensors, then energizesvarious actuators to controlthe engine. The ECM issometimes called PCM(Powertrain ControlModule) in vehicles havingother computers. Theseother computers are usedfor climate control,entertainment systems,etc.

8*�Electronic Fuel Injection. Aterm applied to any systemwhere a computer controlsfuel delivery to an engineby using fuel injectors.

89,Exhaust Gas Recirculation.The EGR systemrecirculates exhaust gasesback into the intakemanifold to reduce NO

xemissions. The EGR valvecontrols the flow ofexhaust gases back intothe intake manifold. SomeEGR valves are operatedwith a vacuum signal whileothers are electricallycontrolled. The amount ofEGR valve openingdetermines the flowthrough the valve. EGRrecirculation is only usedduring warm engine cruiseconditions. EGR flow atother times can causestalling or no starts. Thereare three types of EGRsystems controlled by theECM.One system uses exhaust

backpressure to operatethe EGR valve. The ECMdoes not control the EGRvalve in this case, but itcan switch off the valvecompletely when desired.(The ECM operates asolenoid switch to cut offthe backpressure controlsignal to the valve.)The second system usesan EGR valve entirelycontrolled by the ECM.This valve contains threeindividual flow passages:small flow, medium flowand large flow. Eachpassage has an electricsolenoid. (Passage closedwhen solenoid off -passage open whensolenoid energized.) TheECM energizes one ormore solenoids incombination to set updifferent flow rates throughthe valve as required.The third system is alsodirectly controlled by theECM. This EGR valve isvacuum operated (thenormally closed valveopens as vacuum isapplied). The ECM appliescontrol vacuum to the EGRvalve using a solenoidconnected to a vacuumsource. The ECM uses aduty cycle type signal tovary the amount of vacuumpassing through thesolenoid. (See “DutyCycle” definition.)

84�ElectromagneticInterference. Undesiredsignals interfering with aneeded signal. Forexample: static on a radiobrought about by lightningflashes or closeness tohigh voltage power lines.

8% Electronic Spark Control.This is an ignition systemfunction which works onvehicles having a knocksensor mounted on theengine block. The knocksensor is wired to circuitryin a separate module(early version) or inside theECM (later version). If thesensor detects engineknock, the ESC functionalerts the ECM which willimmediately retard the

spark to eliminate theknock condition.

8%5Electronic Spark Timing.An ignition system wherethe ECM controls the sparkadvance timing. A signalcalled EST goes from theECM to the ignition modulewhich fires the spark coil.The ECM determinesoptimum spark timing fromsensor information - enginespeed and RPM, throttleposition, coolanttemperature, engine load,vehicle speed, Park/Neutralswitch position and knocksensor condition.

87#Electronic Variable Orificeactuator. This is a solenoidmounted in the powersteering pump. It is used insome variable assist powersteering systems. Thesolenoid controls theamount of fluid passed tothe steering gear.Increasing fluid flowgenerates more powersteering assist. The ECMcontrols the solenoid byusing a duty cycle typesignal (see “duty cycle”definition). The ECM usesinformation from thevehicle speed sensor(VSS) and the handwheelsensor (which sends asignal related to the rate ofsteering wheel turning).During low speed turns, theECM increases the EVOsolenoid opening to provideadditional steering assist.The ECM decreasessteering assist duringstraight line driving byreducing fluid flow throughthe EVO solenoid.

87,7Electronic VacuumRegulator Valve. Thisactuator is controlled bythe ECM and is used tovary the amount of vacuumapplied to a vacuumoperated device - usuallythe EGR valve.

*0 Feedback Carburetor. Thisis used on early versions ofcomputer controlledengines. It is a carburetorwhich can have its fuel

36

delivery modified by anelectronic signal from theECM. The signal controls a“mixture control solenoid”(MCS) attached to thecarburetor body.

*�:��"The frequency of anelectronic signal is ameasure of how often thesignal repeats a voltagepattern in a one secondtime span. For example:suppose a signal starts atzero volts, goes to fivevolts then returns to zeroagain. If this patternrepeats itself 100 times inone second, then thesignal frequency is 100cycles per second - or 100Hertz.

*��+��An electronically controlledflow valve. Fuel injectorsare connected to apressurized fuel supply.(The pressure is created bya fuel pump.) No flowoccurs when the injector isoff (not energized). Whenthe injector is powered, itopens fully allowing the fuelto flow. The ECM controlsfuel delivery by varying theamount of time the injectorsare turned on.

*��(�-�,� "The ECM energizes thisrelay to apply power to thevehicle fuel pump. Forsafety reasons, the ECMremoves power from thefuel pump when ignitionsignals are not present.

*��(�-��&� �This is a wire between theECM and the fuel pumpmotor power terminal. TheECM uses this signal toverify when voltage is atthe fuel pump (fordiagnosing fuel pumpproblems).

9 ��)��!�These are switches (usuallytwo) located inside certainautomatic transmissions.The ECM monitors theswitches to determine whattransmission gear isengaged. The switches are

activated by hydraulicpressure and may benormally open or closed,depending upon vehicle.The ECM uses gearinformation for control of thetorque convertor clutch,some emission systems andfor transmission diagnosticpurposes.

9��1The return path for currentto flow back to its source.(Usually the negativebattery terminal.) It is alsothe reference point fromwhich voltage measure-ments are made. That is, itis the connection place forthe minus (-) test lead fromthe voltmeter.

; ��8��This sensor is a three wiretype containing electroniccircuitry. Two wires supplypower and ground. Thethird wire carries thesensor signal back to theECM. The sensor consistsof a permanent magnetand a small modulecontaining a transistorizedHall Effect switch. A smallair gap separates thesensor and the magnet.The magnetic field causesthe Hall switch to turn onand send out a low voltagesignal. If a metal strip (ironor steel) is placed in thegap, it will block themagnetic field fromreaching the Hall device.This causes the Hall switchto turn off and send a highvoltage signal out on thesignal wire.The metal strips (blades)are part of a cup or diskattached to a rotatingcomponent such as thecrankshaft or camshaft. Asthe blades pass throughthe sensor gap. the signalvoltage will switch high andlow creating a series ofpulses. The ECMdetermines the speed ofrotation by measuring howfast pulses appear. HallEffect type sensors may beused to measure speedand position of thecrankshaft or camshaft -for spark timing or fuelinjector control.

; �1)!��This is a three wire sensor(power, ground and signalwires). It is used in someVariable Assist PowerSteering systems. TheECM uses the sensorsignal to determine howfast the steering wheel isbeing turned. Then theECM can apply the correctamount of power steeringassist based on vehiclespeed. See “EVO”(Electronic VariableOrifice) actuator definitionfor more information.

;8�High Energy Ignition.Ignition system whichpulses the spark coil byusing transistor switchesinstead of mechanicalbreaker points. Theelectronics are in a modulewhich use a referencesignal coming from amagnetic pick-up coildriven by the camshaft.

;��<�$;<'A term for frequency -cycles per second.

�� Idle Air Control. This is adevice mounted on thethrottle body. It adjusts theamount of air bypassing aclosed throttle so that theECM can control idlespeed. The IAC is astepper motor whichmoves a pintle within theair bypass passage. Whenthe ECM wants to changeidle speed, it will move thepintle backwards, for moreair and faster idle, or it willmove it forward for less airand slower idle. (SeeStepper Motor definition.)

�1��)��!This is a switch built intothe tip of the ISC (IdleSpeed Control) motorspindle. (See “ISC”definition.) During idle, thethrottle rests against theISC spindle and activatesthe switch. The ECM usesthis switch signal toidentify closed throttlecondition, then operatesthe engine in an “idle” or“deceleration” mode.

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��Electrical signals runninginto the ECM. Thesesignals come fromsensors, switches or otherelectronic modules. Theygive the ECM informationabout vehicle operation.

�% Idle Speed Control. Thisrefers to a small electricmotor mounted on thethrottle body and controlledby the ECM. The ISC motormoves a spindle back andforth. When the throttle isreleased during idle, it restson this spindle. The ECMcan control idle speed byadjusting this spindleposition. The ECMdetermines the desired idlespeed by looking at batteryvoltage, coolanttemperature, engine loadand RPM.

3��2��This sensor is used todetect engine detonation(knocking). When sparkknock occurs, the sensorsends a pulsing signal.Depending upon vehicle,this signal goes either to theECM or a separate ESC(Electronic Spark Control)module. Then the sparkadvance is retarded toeliminate detonation. Thesensor contains apiezoelectric element and isthreaded into the engineblock. Vibrating the elementgenerates the signal.Special construction makesthe element only sensitive tothe engine vibrationsassociated with knocking.

4�*Mass Air Flow sensor. Thissensor measures theamount of air entering theengine and sends afrequency or voltage signal(depends upon sensor type)to the ECM. The signalvoltage or frequencyincreases when the amountof incoming air goes up.This gives the ECMinformation required forcontrol of fuel delivery andspark advance.

4�(Manifold AbsolutePressure sensor. This

sensor measures manifoldvacuum and sends afrequency or voltage signal(depends upon sensortype) to the ECM. Thisgives the ECM informationon engine load for controlof fuel delivery, sparkadvance and EGR flow.

4�5Manifold Air Temperaturesensor. This sensor is athermistor - a resistorwhose resistancedecreases withtemperature. It is threadedinto the intake manifold sothe ECM can determinethe temperature of theincoming air. This is usedfor fuel deliverycalculations.

4 %Mixture Control Solenoid.Used on computercontrolled vehicles havingcarburetors. Built into thecarburetor, it allows theECM to “fine tune” fueldelivery during warmengine cruise.

484 �.A small electronicassembly containing thethe functions of both thePROM and CALPAK. It ishidden behind an accessdoor on the ECM and isreplaceable. Only someECM’s have MEMCAL.

4*�Multi-Port Fuel Injection.(See MPFI definition.)

4�1A type of operatingcondition, such as “idlemode” or “cruise mode.”

4(*�Multi-Port Fuel Injection. Afuel injection system usingone injector for eachcylinder. The injectors aremounted in the intakemanifold. The injectors arefired in groups rather thanindividually.

#��$��'A break in the continuity ofa circuit such that nocurrent can flow.

#��.��$#�.'This is when the controlsystem performs an action(expecting a certain result),but has no way of verifyingif the desired results wereachieved. Example: theECM operates a fuelinjector expecting a certainamount of fuel to bedelivered. (The ECMassumes everything in thefuel system is performingas expected.) In open loopoperation, the ECM has noway of checking the actualamount of fuel delivered.Thus, a faulty fuel injectoror incorrect fuel pressurecan change the amount offuel delivered and the ECMwould not know it.In general, a controlsystem operates “openloop” only when there is nopractical way to monitor theresults of an action.Example: Fuel deliveryduring cold engine warm-up. The computer runs“open loop”because theoxygen sensor is not readyto send a signal. Withoutthe sensor signal, thecomputer can not checkthe actual amount of fueldelivered.

#�

Oxygen sensor. Theoxygen sensor is threadedinto the exhaust manifold,directly into the stream ofthe exhaust gases. TheECM uses the sensor to“fine tune” fuel delivery.The sensor generates avoltage of 0.6 to 1.1 voltswhen the exhaust gas isrich (low oxygen content).The voltage changes to 0.4volts or less when theexhaust gas is lean (highoxygen content). Thesensor only operates afterit reaches a temperature of349°C (660°F).

#��Electrical signals sent fromthe ECM. These signalsmay activate relays orother actuators for controlpurposes around thevehicle. The signals canalso send information fromthe ECM to other electronicmodules, such as ignitionor trip computer.

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(�=Park/Neutral switch. Thisswitch tells the ECM whenthe gear shift lever is in thePark or Neutral position.Then the ECM will operatethe engine in an “idle”mode.

(�� >��1This solenoid is locatedinside certain automatictransmissions. The ECMuses this solenoid to varythe internal line pressure,as required, based onengine load condition

(,#4Programmable Read-OnlyMemory. A small,replaceable electricalcomponent hidden behindan access door on theECM. The PROM containspermanent programminginformation the ECM needsto operate a specificvehicle model. Includedare vehicle weight, engineand transmission type,axle ratio and otherspecifics.

(%Power steering switch.This tells the ECM whenpower steering is beingused. The ECM canprevent stalling on a small,idling engine by watchingthis switch and increasingidle speed if powersteering is being used.

(��&��1This device controls theflow of fuel vapors from thecarbon canister to theintake manifold. Thecanister collects vaporsevaporating from the fueltank, preventing them fromescaping into theatmosphere and causingpollution. During warmengine cruise conditions,the ECM energizes thePurge solenoid so thetrapped vapors are drawninto the engine andburned.

?� 1�/��6�An electrical device insidethe ECM. It functions asfour separate electronic“switches” allowing the

ECM to energize relays orsolenoids.

,� "A mechanical device forswitching high currentcircuits on and off. It iselectronically controlled bya low current circuit.Relays allows a low powerECM signal to control ahigh power device such asan electric cooling fan.

,�� This sensor type consistsof a permanent magnetwith a coil of wire wrappedaround it. Nearby thesensor is a toothed“reluctor” ring made of ironor steel. The ring isattached around a rotatingcomponent such as thecrankshaft. Whenever atooth from the ring passesby the sensor, it attractsthe magnetic field linessurrounding the magnet.As the field lines move,they pass through the wirecoil which generates asmall voltage pulse(magnetic inductionprinciple). Thus, a voltagepulse is generated everytime a tooth passes by thesensor coil. The ECMdetermines the speed ofrotation by measuring howfast pulses appear.Reluctance sensors maybe used for:

Crankshaft or Camshaft –speed, position (sparktiming or fuel injectorcontrol).

Driveshaft – vehicle speed(transmission or torqueconvertor control, coolingfan use, variable assistpower steering and “cruisecontrol”).

Wheel Speed – anti-lockbrake or traction controlsystems.

%��Device which give the ECMinformation. The ECM canonly work with electricalsignals. The job of thesensor is to take somethingthe ECM needs to know,such as engine tempera-

ture, and convert it to anelectrical signal which theECM can understand. TheECM uses sensors tomeasure such things asthrottle position, coolanttemperature, engine speed,incoming air and the like.

%*��%8*�Sequential Fuel Injectionor Sequential ElectronicFuel Injection. A fuelinjection system using oneinjector for each cylinder.The injectors are mountedin the intake manifold. Theinjectors are firedindividually in the samesequence as the sparkplug firing sequence.

%!��1Used in computercontrolled transmissions.The solenoids (usuallytwo) are located in thetransmission housing andare controlled by the ECM.The ECM energizes thesolenoids individually, or incombination, to select aspecific gear. (Thesolenoids control the flowof hydraulic fluid to thetransmission shiftingvalves.) The ECM selectsthe appropriate gear ratioand shift point based uponengine operatingconditions.

%!��$��'A fault condition: anunwanted connection ofone electric circuit toanother causing a changein the normal current flowpath.

%��1A device to convert anelectrical signal tomechanical motion. Itconsists of a coil of wirewith a movable metal rodin the center. When poweris applied to the coil, theresulting electromagnetismmoves the rod andperforms some mechanicalaction. The ECM oftenuses solenoids to switchvacuum lines on and off.This allows the ECM tocontrol vacuum operateddevices such as an EGRvalve. Fuel injectors areanother type of solenoid.

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%��4��A special type of electricmotor with a shaft thatrotates in small “steps”instead of a continuousmotion. A certainsequence of frequencytype signals is required tostep the motor shaft. Adifferent signal sequencewill step the shaft in theopposite direction. Nosignals keeps the shaft stillin position. A constantsignal drive willcontinuously rotate theshaft. The shaft is usuallyconnected to a threadedassembly which movesback and forth to controlthings such as idle speedbypass air flow (see IAC).The engine computersends the correct signalsto the motor for control.

50�Throttle Body Injection. Afuel injection systemhaving one (or two)injectors mounted in acentrally located throttlebody, as opposed topositioning the injectorsclose to an intake valveport.

5 ��1Torque Convertor Clutchsolenoid. The ECM usesthis solenoid to control thelock-up clutch in thetransmission torqueconvertor. (Whenactivated, the lock-upclutch directly connects theengine to thetransmission.) Duringwarm engine cruiseconditions, the ECMenergizes this solenoid toeliminate transmissionslippage and increase fueleconomy. The ECMreleases the lock-up actionwhen driving conditionsrequire the transmission tooperate as normal.

5/ Top Dead Center. When apiston is at its uppermostposition in the cylinder -maximum compression.

5!�-��A resistor whoseresistance changes withtemperature. Thermistorsare used as sensors forvehicle coolant andmanifold air temperature.The resistance decreasesas temperature goes up.

5(%Throttle Position Sensor.This is a rotary typepotentiometer connectedto the throttle shaft. It hasa voltage signal output

which increases as the thethrottle is opened. Thissensor is used by the ECMfor idle speed, sparkadvance, fuel delivery,emission system andautomatic transmission(electronic type) control.

55%Transmission Temperaturesensor. This sensor is athermistor - a resistorwhose resistancedecreases as temperaturerises. It is mounted withinthe transmission housing.The ECM uses this sensorto monitor transmissionoperating temperature.

7%%Vehicle Speed Sensor.This sensor sends afrequency signal to theECM. The frequencyincreases as the vehiclemoves faster to give theECM vehicle speedinformation.

@#5Wide Open Throttle. Thevehicle operating conditionbrought about when thethrottle is completely (ornearly so) open. The ECMwill typically deliver extrafuel to the engine at thistime for accelerationpurposes. The ECM usesthe Throttle PositionSensor to identify the WOTcondition.

40

�� 67%��74��$�,��$��

Courtesy of General Motors Corp.

Front WheelSpeed Sensors

Electronic BrakeControl Unit

(ABS computer)

Rear WheelSpeed Sensors

HydraulicControl

Modulator

Master Cylinder

��"��0�� ,��

The following is an overview of Anti-Lock Brake Systems (ABS). There areseveral different types and versions.Refer to vehicle service manual forspecific details.

IMPORTANT: To service ABS systemssafely and effectively, you must obtaina service manual for your vehicle andcarefully follow all procedures.

:#� � ��;ABS is a safety feature designed tominimize accidents during braking.When engaged, ABS stops the vehiclein the shortest distance possible whilegiving the driver the greatest amountof steering control.

Heavy braking on non-ABS vehiclesoften causes wheels to lock up. Thisleads to a wheel skid conditionresulting in loss of maneuverability anda long stopping distance. The job ofABS is to prevent wheel lock-up.

�#� �� %��An ABS system combines a conven-tional hydraulic braking system alongwith additional components including:

– An ABS computer (separate fromthe engine computer)

– Wheel speed sensors

– Hydraulic control unit

The computer module controls theABS system. This module is calledElectronic Brake Control Module(EBCM), or similar. The computermonitors wheel speed, accelerationand deceleration using signals sent bythe wheel speed sensors. If thecomputer determines wheel lock-up islikely during braking, it will controlbrake pressure using the HydraulicControl Modulator. ABS componentsand braking operation will be detailedlater in this section. As a safetyfeature, the system reverts to normalhydraulic braking operation if the ABScomputer cannot operate.

41

�� %��

There are three basic types...

• RWAL (Rear Wheel Anti-Lock)This is the simplest ABS system(mostly used on small rear-wheel-drivetrucks). Only the rear wheels are ABScontrolled – not the front. This versionstops the vehicle in a straight line butdoes not allow the driver to maneuverbecause the front wheels may lock.

A single speed sensor (usuallymounted in the differential) monitorsdriveshaft rotation. The ABS computerexamines changes in shaft speed topredict rear wheel lock-up. Both rearbrakes are operated by a singlehydraulic channel. The ABS computercontrols rear hydraulic performance,when necessary, using a singlechannel Hydraulic Control Modulator(described later).

• 3 ChannelThis ABS system is a higher perfor-mance version of the RWAL versionjust described. Used on rear wheeldrive vehicles, this system deliversshort stopping distance and maneuver-ing control during heavy braking. Allfour wheels are ABS controlled.

Three hydraulic braking channels areused: right front wheel, left front wheeland a single channel for both rearwheels. The ABS computer uses athree channel Hydraulic ControlModulator (described later) to operatethe individual brake circuits asnecessary.

The two rear wheels are monitored witha single speed sensor (usuallymounted in the differential). This issimilar to the RWAL system describedbefore. Two more speed sensorsindividually monitor each of the fronttwo wheels.

• 4WAL (4 Wheel Anti-Lock) alsocalled “4 Channel”This ABS system is similar to the 3channel version previously described,but is designed for front wheel drivevehicles. The main difference is thatfour speed sensors are used, insteadof three, to individually monitor all four

wheels. The hydraulic portion is thesame as the 3 Channel system. TheABS computer uses a three channelHydraulic Control Modulator to controlthe right front wheel, the left frontwheel and both rear wheels.

��

Electronic Brake Control Module(EBCM)May also be called Electronic ControlUnit (ECU), or similar.

This is a computer module - the“brains” of the ABS system. Themodule is located either in thepassenger compartment, or close tothe ABS hydraulic controller in theengine area. The module monitorswheel speed sensors to determine iflock-up is about to occur when brakesare applied. If so, the module willoperate the ABS hydraulic solenoids tocontrol brake pressure and preventlock-up. (This process is describedlater. Refer to “How ABS ControlsBrakes.”) The EBCM also performschecks of itself, and other ABScomponents, during vehicle operation.If problems are found, the ABS systemis disengaged, the dashboard warninglight is energized and a diagnostictrouble code is stored in ABS computermemory.

Brake Light SwitchThis is the usual switch whichenergizes the rear brake lights whenthe brake pedal is applied. The ABScomputer is sometimes connected tothis switch - depends upon system.(Note: The engine control computermay also be connected to this switch.)Some ABS systems are activecontinuously. Other types wait for thebrake light switch to close beforeoperating.

Wheel Speed SensorThis is a reluctance sensor (a 2-wiretype). It consists of a permanentmagnet with a coil of wire wrappedaround it. Nearby the sensor is atoothed ring made of iron or steel(sometimes called a reluctor, sensorring, exciter ring, pick-up ring or tone

42

wheel). The ring is attached to thewheel, drive axle or transmission shaft.Whenever a tooth from the ring passesby the sensor, it attracts the magneticfield lines surrounding the magnet. Asthe field lines move, they pass throughthe wire coil and generate a smallvoltage pulse (magnetic inductionprinciple). Thus, a voltage pulse isgenerated every time a tooth passesby the sensor coil. This voltage signalis sent to the ABS computer.

The ABS computer determines wheelspeed by measuring how fast pulsesappear. The faster the wheel spins, themore quickly pulses will appear. Note:The voltage pulses get larger as thewheel speeds up. (The computerignores pulse size.) Values can rangefrom a fraction of a volt (low speed) toseveral volts (high speed).

Hydraulic Control ModulatorThis is an assembly containingsolenoid operated hydraulic valves. It

is usually mountedclose to the mastercylinder. (Somesystems combine thehydraulic controlmodulator and themaster cylinder intoone complete unit.)The valves areconnected in the brakelines between themaster cylinder andthe wheel caliper (orwheel cylinder). TheABS computer controlsbrake line pressure byoperating one, ormore, of thesesolenoid valves. (InABS systems, theprocess of varyingbrake pressure iscalled “modulation.”

This is why the solenoid assembly iscalled a “modulator.”)

Some modulator types use twosolenoid valves per brake circuit: an“isolation” valve and a “dump” valve.Other types use a special “two-stage”solenoid per brake circuit. This “twostage” solenoid provides the samebrake fluid control as the “isolation”and “dump” solenoids.

The “isolation” and “dump” solenoidshave two positions: coil off and coilfully energized. The “two stage”solenoid has an additional position:coil off, coil partially energized and coilfully energized. ABS computerscontrolling “two stage” type solenoidshave special built-in switching circuitsto energize the solenoid properly.

Refer to “How ABS Controls Brakes”(later in this section) for a descriptionof how the modulator is used.

Accumulator and Electric PumpThese two components work together.Depending upon system, their use (andconstruction) will differ a great deal.

• Low Pressure type: Accumulatorand pump only used during ABSoperation.

– Accumulator acts as a reservoir. Itcollects hydraulic fluid “bled” from

S N

{

:#�� 1��

Tocomputer

MagnetWire Coil

Airgap

Teeth Sensorring

Rotation

Wheelspeedsensor

Sensorring

Hub and rotorassembly

Sensorring

Axlehousing

Wheelspeedsensor

Typical Front Sensor Typical Rear Sensor

Hydraulic ControlModulator(Typical)

43

brake circuit during ABS operation.(ABS relieves brake line pressure toavoid wheel lock-up.) Four wheelABS systems use two accumulators:one for the rear wheel circuit and theother for the front wheel circuit.

The accumulator contains amoveable diaphragm whichseparates the inside into twochambers. One chamber collects thehydraulic fluid. The other sidecontains a spring pressing againstthe diaphragm. Because of thespring, accumulator hydraulicpressure is about 150 PSI.

– The electric pump operates toremove fluid from the accumulatorsand return it to the master cylinder.

– Some systems have no pump. Thespring driven diaphragm in theaccumulator pushes brake fluid backinto the master cylinder through acompensation port.

• High Pressure type: Accumulatorand pump used for both ABS andnormal brake operation.

– Accumulator stores fluid under veryhigh pressure (up to 2600 PSI). Theaccumulator contains a moveablediaphragm which separates theinside into two chambers. Onechamber holds the hydraulic fluid.The other side is filled with highpressure Nitrogen gas. The gas actsas a very strong spring keeping thehydraulic fluid under great pressure.The pressurized fluid is used duringnormal and ABS braking.

– The electric pump is a special highpressure hydraulic type. It runs asneeded to maintain high hydraulicpressure in the accumulator. Thepump is not controlled by the ABScomputer. A pressure activated switchthreaded into the accumulator fittingturns the pump on and off as required.

Warning LightAll vehicles have a red “BRAKE” lighton the dashboard to warn of problemsin the normal braking system. SomeABS systems use this “BRAKE” light towarn of ABS problems as well. Othervehicles have a separate amber“ANTILOCK” dashboard light to warn of

ABS problems. The ABS computercontrols the warning light.

RelaysVarious relays are used by ABS -depends upon system. Refer to ABScircuit schematics in vehicle servicemanual. Typical relays include:

• ABS Power relay: Supplies powerto the electronic brake control module(ABS computer). Note: This relay mayhave built-in diodes to protect thecomputer against voltage surges orreverse voltage conditions.

• Electric Pump relay: Used by theABS computer to operate the electricpump. (This is the pump associatedwith the accumulator previouslydescribed.)

• ABS Solenoid relay: Connectsvehicle battery voltage to the solenoidcircuits in the Hydraulic ControlModule. Note: This does not turn onthe solenoids. The ABS computercontrols individual solenoids bycompleting each circuit as required.

Digital Ratio Adapter Controller(DRAC)This is a small electronic module usedon some rear wheel drive vehicles. Itworks with the speed sensor mountedin the differential. The DRAC receivesthe sensor signal, processes it, thensends it to the ABS computer,speedometer and cruise control (ifinstalled). The DRAC is matched to aspecific rear axle ratio and tire size.Any change to the rear axle ratio ortire size requires recalibrating (orreplacing) the DRAC.

Lateral Acceleration SensorThis sensor is only used on CorvetteABS systems. It is a small moduleused to monitor the amount ofsideways force exerted on the vehicleduring a hard turn. The module sendsa voltage signal to the ABS computer.The computer uses this signal tomodify ABS control of the rear brakes.

<�" �� Depending upon system, the ABScomputer will either:

44

– continuously monitor wheel speedor...

– wait for brake pedal applicationbefore monitoring wheel speed.

The ABS computer controls brakingaction when wheel speed accelerationor deceleration indicates lock-up isabout to occur.

Four-Wheel Drive Vehicle Note: TheABS computer will not allow ABSbraking during four-wheel driveoperation. (A switch in the four-wheeldrive powertrain sends a signal to theABS computer.)

The ABS computer controls brake linepressure by operating one, or more,solenoid valves in the HydraulicControl Modulator. Some systems usetwo solenoid valves per brake circuit:an “isolation” valve and a “dump” valve.Other systems use a special “two-stage” solenoid per brake circuit. Eithersystem controls brake line pressure inthe same way.

Normal Braking (no ABS action)

ABS computer sees normal wheelspeed changes. No ABS action isnecessary.

– Isolation valve is OPEN (notenergized). Brake circuit operatesnormally. Hydraulic pressure frommaster cylinder passes throughisolation valve to wheel caliper/cylinder.

– Dump valve is CLOSED (notenergized). This valve has no effecton the brake pressure when it isclosed.

(Systems using two-stage solenoid:Solenoid is not energized. Hydraulicflow same as described above.)

ABS Braking – Pressure Maintain

Wheel speed signals indicate lock-upis about to occur. ABS computer takesfirst step in brake control cycle: isolatewheel caliper/cylinder from mastercylinder – maintain fluid pressure towheel.

– Isolation valve is CLOSED(energized). Pressure flow betweenmaster cylinder and brake caliper/cylinder is blocked because ofclosed valve. Existing pressure towheel is maintained becausehydraulic fluid is trapped betweenisolation valve and wheel. Closedisolation valve also prevents anyincreases in master cylinderpressure from reaching wheelcaliper/cylinder.

– Dump valve is CLOSED (notenergized). This valve has no effecton the brake pressure when it isclosed.

(Systems using two-stage solenoid:Solenoid is partially energized.Hydraulic flow same as describedabove.)

HYDRAULIC PRESSURE FLOW

IsolationValveOPEN

DumpValve

CLOSED

FromMaster

Cylinder

To WheelCaliper orCylinder

(Accumulatoror Reservoir)


IsolationValve

CLOSED

DumpValve

CLOSED

(MasterCylinder)



45

ABS Braking – Pressure Decrease

Wheel speed signals indicate lock-upis still about to occur. ABS computertakes next step in brake control cycle:decrease hydraulic pressure to wheelcaliper/cylinder.

– Isolation valve is CLOSED(energized). Pressure flow betweenmaster cylinder and brake caliper/cylinder remains blocked because ofclosed valve. Closed isolation valvestill prevents changes in mastercylinder pressure from reachingwheel caliper/cylinder.

– Dump valve is OPEN (energized).Hydraulic pressure to wheel caliper/cylinder is reduced. The open dumpvalve relieves pressure by “bleeding”some fluid out of the wheel circuit.The fluid travels either: back to thebrake reservoir or to an accumulator- depends on system. Fluidcollected in the accumulator isreturned to the master cylinder.Some systems use an electric pumpto move the fluid. Other systemsbriefly pause the ABS brake cycle.Then, the spring driven diaphragminside the accumulator pushes thefluid back to the master cylinder.

(Systems using two-stage solenoid:Solenoid is fully energized. Hydraulicflow same as described above.)


IsolationValveOPEN

DumpValve

CLOSED

FromMaster

Cylinder



ABS Braking – Pressure Increase

Wheel speed signals indicate absenceof lock-up condition. ABS computertakes last step in brake control cycle:increase hydraulic pressure to wheelcaliper/cylinder.

– Isolation valve is OPEN (notenergized). Master cylinder isreconnected to wheel caliper/cylinder.Once again, hydraulic pressure frommaster cylinder passes throughisolation valve to wheel.

– Dump valve is CLOSED (notenergized). This valve has no effect onthe brake pressure when it is closed.

(Systems using two-stage solenoid:Solenoid is not energized. Hydraulicflow same as described above.)

The ABS system is capable ofrepeating the brake cycle at a rapidrate - up to 15 times a second.

1�#�� 6�On some vehicles, the components of ABSare shared with another system: Anti-SlipRegulation (ASR) also known as tractioncontrol. This system prevents wheel slipduring acceleration on slick road surfaces.

The ASR system is controlled by acomputer module. This ASR computer isconnected to the same wheel speedsensors and hydraulic control modulatorused by ABS. If one drive wheel slipsexcessively during acceleration, powerwill be transferred to the other drive wheelby applying brake pressure to the slippingwheel. (The ASR computer may try tostiffen throttle movement or retard enginetiming before applying brake action.)


IsolationValve

CLOSED

DumpValveOPEN

(MasterCylinder)

FromWheel

Caliper orCylinder

ToAccumulatoror Reservoir

46

��"�� "�� 1��#%��%��$�� ,��'��

WARNING: To avoid personal injury, DO NOT open a bleedervalve or loosen a hydraulic line while ABS is pressurized.Always follow vehicle manufacturer’s procedures to depres-surize ABS before servicing.• Always use specially designed ABS brake hoses and fittings

when replacing these parts.

• Always use ABS recommended brake fluids. DO NOT usesilicone brake fluids in ABS systems.

• Always wear approved eye protection.

• Always operate the vehicle in a well ventilated area. Do notinhale exhaust gases - they are very poisonous!

• Always keep yourself, tools and test equipment away from allmoving or hot engine parts.

• Always make sure the vehicle is in park (Automatic transmission)or neutral (manual transmission) and that the parking brake isfirmly set. Block the drive wheels.

• Never lay tools on vehicle battery. You may short the terminalstogether causing harm.

• Never smoke or have open flames near vehicle. Vapors fromgasoline or charging battery are highly explosive.

• Never leave vehicle unattended while running tests.

• Always turn ignition key OFF when connecting or disconnectingelectrical components, unless otherwise instructed.

• Always follow vehicle manufacturer’s warnings, cautions andservice procedures.

CAUTION: Some vehicles are equipped with safety air bags, alsoknown as Supplemental Inflatable Restraint (SIR) system. Youmust follow vehicle service manual cautions when working aroundthe air bag components or wiring. If the cautions are not followed,the air bag may open up unexpectedly, resulting in personal injury.

47

��)��8�� 9��#�%��

��*��,�� IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs. (Manual listings on page 4.)

• Do a thorough visual and “hands-on”inspection first. You can often find thecause of many problems by justlooking.

• ABS systems rely on accurate wheelsensor signals. Anything whichinterferes with the wheel sensor cancreate intermittent problems or settrouble codes. Note the following:

–Do not mix tire sizes. Rollingdiameter must be the same for allfour tires. Differently sized tires (orusing a “compact” spare) can causeinaccurate wheel speed sensoroperation.

–Do not tap the speed sensor ortoothed sensor ring. Tapping theseparts can disturb their magneticproperties and upset wheel speedsensor operation. Press (do nothammer) toothed sensor ring ontohub, if service is required.

–Do not overtighten wheel lug nuts.(Specifications are in vehicleservice manual.) Brake drum orrotor may bend causing inaccuratewheel speed sensor operation.

–Do not coat wheel speed sensorparts with grease. Refer to vehicleservice manual for recommendedmaterial.

–Check spacing between wheelspeed sensor and toothed ring,especially after servicing. Incorrectspacing can cause faulty sensoroperation. Specifications are invehicle service manual.

–Check for cracked or missing teethon the wheel speed sensor ring.

• Spinning tires (or axle) on astationary vehicle during a serviceprocedure may set ABS troublecodes.

• Radio transmitters can interfere withoperation of ABS computer. Keepantenna wiring from a CB radio orportable telephone away from ABSwiring

• Check ABS power circuits:

–Make sure vehicle alternator andvoltage regulator are workingproperly.

–Make sure vehicle battery is fullycharged.

–Make sure all ABS fuses, fusiblelinks and relays are good

• Inspect wiring for:

–Contactwith sharpedges.(Thishappensoften.)

–Contactwith hot surfaces, such as exhaustmanifolds.

–Pinched, burned or chafedinsulation.

–Proper routing and connections.

• Check electrical connectors for:

–Corrosion on pins.

–Bent or damaged pins.

–Contacts not properly seated inhousing.

–Bad wire crimps to terminals.

-�� "��# �� Inspect carefully. Note that someconnectors use a special grease onthe contacts to prevent corrosion. Donot wipe off! Obtain extra grease, if

48

needed, from your vehicle dealer. It isa special type for this purpose.

• Make sure any recommendedsystem flushing has been per-formed. Corrosion or dirt in ABSvalving can result in poor pedalperformance.

• Some body and chassis workprocedures can harm the ABScomputer:

– Disconnect vehicle computermodules when using electricwelding equipment.

– Do not expose the ABS computer tohigh heat for a long time (forexample, during vehicle painting).Keeping temperature below 185°F(85°C) for less than 2 hours isusually safe.

49

�� !�� 8�#� ��)� �� +��,��1) Use the chart to find the ABS system used on your vehicle.

2) Refer to the page listed where you will find...

• preliminary checks you need to make before reading codes,

• the code reading procedure,

• the procedure for erasing codes from ABS computer memory,

• a list of code definitions.

!�::Model System Page

Blazer 7 84

C SeriesPickup Truck 7 84

K SeriesPickup Truck 7 84

Sierra 7 84

!�:�Model System Page

98 Regency 5 74

Astro 7 84

Blazer 7 84

Bonneville 5 74

Bonneville SSE 5 74


Delta 88 5 74

DeVille 5 74

Eldorado 6 79

Electra 5 74

Fleetwood 5 74

Jimmy 7 84


Park Avenue 5 74

Reatta 6 79

Riviera 6 79

S Series (2WD)Pickup Truck 7 84

Safari 7 84

Seville 6 79

Sierra 7 84

Toronado 6 79

50

!�� Model System Page

98 Regency 5 74

Astro 7 or 8 See note p. 50

Blazer 7 84

Bonneville 5 74

Bonneville SSE 5 74

Brougham 2 56


Corvette 1 51

Delta 88 5 74

DeVille 5 74

Eldorado 6 79

Electra 5 74

Fleetwood 5 74

G Series (RWD)Van 7 84

Jimmy 7 84


Park Avenue 5 74

R Series Truck 7 84

Reatta 6 79

Riviera 6 79

S SeriesPickup Truck 7 84

Safari 7 or 8 See note p. 50

Seville 6 79

Sierra 7 84

Suburban 7 84

T SeriesPickup Truck 7 84

Toronado 6 79

Trofeo 6 79

V Series Truck 7 84

!��!Model System Page


Blazer 7 or 8 See note p. 50

Bravada 8 88

Brougham 3 62


Caprice 2 56

Corvette 1 51

Custom Cruiser 2 56

Eldorado 3 62


Jimmy 7 or 8 See note p. 50


R Series Truck 7 84

Reatta 3 62

Riviera 3 62

Roadmaster 2 56

S SeriesPickup Truck 7 or 8 See note p. 50


Seville 3 62

Sierra 7 84

Sonoma 8 88

Suburban 7 84

Syclone 8 88

T SeriesPickup Truck 7 or 8 See note p. 50

Toronado 3 62

Trofeo 3 62

Typhoon 8 88

V Series Truck 7 84

51

!��5Model System Page



Bravada 8 88

Brougham 2 56

C SeriesPickup Truck 7 or 8 See note p. 50

Eldorado 3 62



K SeriesPickup Truck 7 or 8 See note p. 50

Riviera 3 62



Seville 3 62

Sierra 7 or 8 See note p. 50

Sonoma 8 88

Suburban 7 or 8 See note p. 50

Syclone 8 88


Toronado 3 62

Trofeo 3 62

Typhoon 8 88

Yukon 8 88

!��;Model System Page

Astro 8 88


Bravada 8 88

C SeriesPickup Truck 7 or 8 See note p. 50

Eldorado 4 12-38

G Series Van 8 88


K SeriesPickup Truck 7 or 8 See note p. 50

Riviera 3 62


Safari 8 88

Seville 4 12-38

Sierra 7 or 8 See note p. 50

Sonoma 8 88

Suburban 7 or 8 See note p. 50

Syclone 8 88


Typhoon 8 88

Yukon 8 88

52

!��6Model System Page

Astro 8 88

Blazer 7 or 8 See note below

Bravada 8 88

C SeriesPickup Truck 7 or 8 See note below

DeVille 4 68

Eldorado 4 68

G Series Van 8 88

Jimmy 8 88

K SeriesPickup Truck 7 or 8 See note below

S SeriesPickup Truck 7 or 8 See note below

Safari 8 88

Seville 4 68

Sierra 8 88

Sonoma 8 88

Suburban 8 88

T SeriesPickup Truck 7 or 8 See note below

Yukon 8 88

NOTE: Two different ABS systems were available for this vehicle. Eachsystem has a different code reading procedure. Do the following to identifywhich system is installed:

• Examine the Electro-Hydraulic Control Unit (EHCU). This assembly hasboth electrical and hydraulic connections. It is installed between themaster cylinder and the wheel caliper (or cylinder).

–If there are 5 hydraulic connections (2 inlet, 3 outlet), then vehicle hasSystem 8. Go to page 88.

–Less than 5 hydraulic connections means vehicle has System 7.Go to page 84.

53

��"#��$��%�!�� <� !��!��

-��$�� '�� /��Complete all steps before readingtrouble codes!

1) Check that all ABS systemgrounds are clean and tight

2) Check power sources whichsupply various parts of ABSsystem

• 1990, 1991Corvette:

–Check the AIRBAG fuse inthe underdashfuse block.

–Check the BRAKE fuse in theunderdash fuse block.

–Check the STOP/HAZ fuse in theunderdash fuse block.

–Check the CLSTR fuse in theunderdash fuse block.

–Check rust colored fusible link “J”on the positive junction block.

3) Check that pump motor relay,EBCM relay (Electronic BrakeControl Module), solenoid valverelay, and EBCM connectors areproperly installed (not loose)

This completes the visual inspection.Perform FUNCTIONAL CHECK beforereading codes!

9�� #�� A ��

1) Start the engine.The brakewarning lampshould comeON during engine cranking, andturn OFF shortly after the enginestarts. If it does not, check the brakewarning lamp circuit in accordancewith vehicle service manualinstructions.

2) Turn the engine OFF. Turn theignition key to the ON (RUN)position, but do not start the engine.The anti-lock lamp should illuminatefor about 2 seconds and then turnOFF, if no trouble codes arecurrently stored, however “history”trouble codes may be stored.Proceed to Step 3 below. If the anti-lock lamp stays ON, trouble codesare stored. Proceed directly toREADING ABS CODES, page 52. Ifthe anti-lock lamp does not turn ONat all or flashes very briefly (lessthan1/2 of a second), a problem exists inthe anti-lock lamp circuitry. See yourvehicle service manual.

3) Drive the vehicle at least 20 MPH. Ifthe anti-lock lamp turns ON,proceed to READING ABS CODES,page 52.

54

!�� A ��# 3�IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!

1) Take Safety Precautions

• Set parking brake, block drivewheels.



2) Have a Pencil and Paper Ready

This is for writing down all the codes.


• Servicemanualscall thisconnec-tor theAssem-bly Line Diagnostic Link (ALDL)connector. It may also be calledthe Assembly Line CommunicationLink (ALCL) or simply testconnector.

• The connecter is located under thedashboard on the driver’s side.

• The connector may be in full view,or it may be recessed behind apanel opening.

• The connector may have a slip-oncover labeled “DiagnosticConnector.” Remove cover fortesting. Replace cover aftertesting. Some vehicles require thiscover in place for proper operation.


5) Put the TEST switch on ABS

6) Plug theCodeScannerinto the testconnector

• The CodeScanner only fits ONE WAY intothe test connector.

• The Code Scanner will not harmthe vehicle computer.

Note: The Code Scanner does notuse all of the test connectorcontacts. This is normal.


WARNING: Stay away from theradiator cooling fan! It may turnon.

8) Get Codes fromthe Flashing“Anti-Lock”Light

• Count flashesto get trouble codes. (Flashesbegin after a few seconds.)

Code 12 looks like:




Code 23 looks like:


• Each code is flashed three (3)times before the next trouble codeis sent.

• After all codes are sent, the wholesequence is repeated. Thiscontinues until the ignition key isturned OFF (so you can doublecheck your code list).

OFF ON TEST

ABS ENGINE

TM

GM 1982 & higher - CP 9001


55



(longer pause)


(longer pause)


(even longer pause,then start over again)



(longer pause)


(longer pause)


(even longer pause,then go to next code)



(longer pause)



(longer pause)




• A code 12 is always sent evenwhen the computer sees noproblem. This tells you thecomputer diagnostic checks areworking properly.



10)Remove Code Scanner and Re-install Connector Cover, ifsupplied


11)Refer to ABS Code Meanings onpage 55. (Bosch 2S)




or,


IMPORTANT: Always follow vehicleservice manual procedures for anyABS repairs! (Manual listings onpage 4.)

56

TM

GM 1982 & higher - CP 9001


�� (��% A ��# 3�Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again. Note: Thecomputer will automatically erasecodes after several restarts (typically100) if the problem does not return.

Proceed as follows:

1) Turn the ignitionswitch to theOFF position.

Plug the Code Scanner (make surethe TEST switch is in the ABSposition) into the test connector.

2) Turn the ignitionswitch to the ONposition. Then...

• 1990 Corvette:

When the anti-lock lamp begins to flash codes,remove the Code Scanner from theconnector for about 1 second, thenreinstall the Code Scanner for aboutone second. Repeat this procedure

3 times within10 seconds.Leave theCode Scannerinserted in theconnector the3rd time.

• 1991 Corvette:

When the anti-lock lamp begins toflash codes, remove the CodeScanner from the connector forabout 1 second, then reinstall theCode Scanner for about 1 second.Repeat this procedure 4 timeswithin 10 seconds. Leave the CodeScanner inserted in the connectorthe 4th time.

OFF ON

TESTABS ENGINE

OFF

ON

TM

GM 1982 & higher - CP 9001


3) Wait 15 seconds, then observe theanti-lock indicator lamp. Code 12,the “diagnostic system operational”code should flash indicating that alltrouble codes have been erasedfrom memory.

Turn the ignitionswitch to the OFFposition.

Remove theCode Scanner,and reinstallthe testconnectorcover (ifused onvehicle).

OFF ON

57

�,��-��2�,��5�IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs!

(Manual listings on page 4.)

��Diagnostic SystemOperational.

��Right Front Wheel SpeedSensor circuit. Checks foropen, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH for thiscode to set.

��Right Front ToothedWheel Frequency Error.Checks for excessivelydirty, or damaged toothedwheel (sensor ring). Mis-matched tire sizes, or useof the temporary (mini)spare tire may set thiscode.

��Left Front Wheel SpeedSensor circuit. Checks foropen, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH for thiscode to set.

��Left Front Toothed WheelFrequency Error. Checksfor excessively dirty, ordamaged toothed wheel(sensor ring). Mis-matchedtire sizes, or use of thetemporary (mini) spare tiremay set this code.

��Right Rear Wheel SpeedSensor circuit. Checks foropen, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH for thiscode to set.

��Right Rear ToothedWheel Frequency Error.Checks for excessivelydirty, or damaged toothedwheel (sensor ring). Mis-matched tire sizes, or useof the temporary (mini)spare tire may set thiscode.

��Left Rear Wheel SpeedSensor circuit. Checks foropen, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH for thiscode to set.

��Left Rear Toothed WheelFrequency Error. Checksfor excessively dirty, ordamaged toothed wheel(sensor ring). Mis-matchedtire sizes, or use of thetemporary (mini) spare tiremay set this code.

��Right Front SolenoidValve circuit. This codewill set if the physicalposition of the subjectvalve does not match thecommanded position asgiven by the EBCM(Electronic Brake ControlModule).

��Left Front Solenoid Valvecircuit. This code will set ifthe physical position of thesubject valve does notmatch the commandedposition as given by theEBCM (Electronic BrakeControl Module).

��Rear Wheel SolenoidValve circuit. This codewill set if the physicalposition of the subject

valve does not match thecommanded position asgiven by the EBCM(Electronic Brake ControlModule).

��Pump Motor or MotorRelay circuit. This codewill set if the position of themotor relay contacts do notmatch the commandedposition of those contactsas given by the EBCM(Electronic Brake ControlModule). There is a motormonitoring circuit in theEBCM which will detect adefective relay or pumpmotor.

��Solenoid Valve Relaycircuit. This code will set ifthe position of the valverelay contacts do not matchthe commanded position ofthose contacts as given bythe EBCM (ElectronicBrake Control Module).There is a motor monitoringcircuit in the EBCM whichwill detect a defective relayor a failure in theassociated circuitry.

��EBCM (Electronic BrakeControl Module) failure.This code will set if there isan internal failure of theElectronic Brake ControlModule.

��Lateral Accelerometercircuit. This code will set ifthere is a short or opencircuit problem in theassociated circuitry.

��Lateral Accelerometersignal error. This code willset if the accelerometersignal indicates greaterthan 0.6g for 2 minutes ormore.

58

��%#��$��%��&'�� (

!�� ,��!��!�� /��/�+��!��5�,��


1) Check that all ABS systemgrounds are clean and tight.

• 1990 Brougham:

–Check the ground that is locatedon the right front corner of the rightfender.

–Check the ground that is locatednear the cruise control servo onthe fender.

• 1991 Caprice, Custom Cruiser,Roadmaster:

–Check the 2 grounds that arelocated near the left headlight. Oneis on the headlight support in frontof the vapor canister, the other isclose to the wiring harness.

–Check the ground that is locatednear the center of the left rearquarter panel.

–Check the ground that is locatednear the thermostat housing.

• 1992 Brougham:

–Check the ground that is locatedon the right front corner of the rightfender.

–Check the ground that is locatednear the cruise control servo on thefender.

2) Check power sources whichsupply various parts of ABSsystem.

• 1990Brougham:

–Check theGA-TRANSfuse in theunderdashfuse block.

–Check the ABS fuse in theunderdash fuse block.

–Check the STOP/HAZ fuse in theunderdash fuse block.

–Check the rust colored fusible linkson the positive junction block.

• 1991 Caprice, Custom Cruiser,Roadmaster:

–Check fuse number 3, 17 and 19 inthe main fuse block.

–Check the rust colored fusible linkD on the positive junction block.

• 1992 Brougham:

–Check the GA-TRANS fuse in theunderdash fuse block.


–Check the rust colored fusible linkE on the positive junction block.

3) Check that the over-voltage relay,ABS 6-way connector, and EBCM(Electronic Brake Control Module)connectors are properly installed(not loose).

4) Check that the parking brakeswitch is functioning properly.

This completes the visual inspection.

59

Perform FUNCTIONAL CHECK beforereading codes!

9�� #��Brougham

1) Start the engine.The brakewarning lamp should come ONduring engine cranking, and turnOFF shortly after the engine starts. Ifit does not, check the brake warninglamp circuit in accordance withvehicle service manual instructions.

2) Turn the engine OFF. Turn theignition key to the ON (RUN)position, but do not start the engine.The anti-lock lamp should illuminatefor about 4 seconds and then turnOFF, if no trouble codes arecurrently stored. However “history”trouble codes may be stored.Proceed directly to Step 3 below. Ifthe anti-lock lamp stays ON, troublecodes are stored. Proceed directly toREADING ABS CODES.

3) Drive the vehicle at least 20 MPH. Ifthe anti-lock lamp turns ON,proceed to READING ABS CODES.

Caprice, Custom Cruiser, Roadmaster

1) Start the engine. The brake warninglamp should come ON during enginecranking, and turn OFF shortly afterthe engine starts. If it does not,check the brake warning lamp circuitin accordance with vehicle servicemanual instructions.

2) Turn the engine OFF. Turn theignition key to the ON (RUN)position, but do not start the engine.The anti-lock lamp should illuminatefor about 2 seconds and then turnOFF if no trouble codes are currentlystored, however “history” troublecodes may be stored. Proceed toStep 3 below. If the anti-lock lampstays ON, trouble codes are stored.Proceed directly to READING ABSCODES. If the anti-lock lamp doesnot turn ON at all or flashes verybriefly (less than 1/2 of a second), aproblem exists in the anti-lock lampcircuitry. See your vehicle servicemanual.

3) Drive the vehicle at least 20 MPH. Ifthe anti-lock lamp turns ON,proceed to READING ABS CODES.

!�� )��# 36 �'�� IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!





2) Have a Pencil and Paper Ready.This is for writing down all thecodes.


• Service manuals call thisconnector the Assembly LineDiagnostic Link (ALDL) connector.It may also be called the AssemblyLine Communication Link (ALCL)or simply test connector.

• The connector is located under thedashboard on the driver’s side.


• The connector may have a slip-oncover labeled “DiagnosticConnector.”Removecover fortesting.Replacecover after

60

testing. Some vehicles require thiscover in place for proper operation.



6) Plug the Code Scanner into thetest connector

• The CodeScanneronly fitsONE WAYinto thetestconnector.



7) TurnIgnition Keyto ONPosition butDO NOTSTART THEENGINE


8) Get Codes fromthe Flashing“Anti-Lock”Light

• Count flashes to get trouble codes.(Flashes begin after a fewseconds.)

OFF ON TEST

ABS ENGINE

TM

GM 1982 & higher - CP 9001


OFF

ON

Code 12 looks like:




Code 23 looks like:


• Each code is flashed three (3)times before the next trouble codeis sent.


61



(longer pause)


(longer pause)


(even longer pause, then start overagain)



(longer pause)


(longer pause)


(even longer pause, then go to nextcode)



(longer pause)



(longer pause)




OFF ON






11) Refer to ABS Code Meanings onpage 61. (Bosch 2U, Version A)




or,


IMPORTANT: Always follow vehicleservice manual procedures for anyABS repairs! (Manual listings on page4.)

62

OFF ON

�� (��%) ��# 36 �'�� Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again. Note: Thecomputer will automatically erasecodes after several restarts (typically100) if the problem does not return.

Proceed as follows:

1) Turn the ignitionswitch to theON position.The anti-locklamp shouldturn OFF after 3to 4 seconds. If it does not, a failurestill currently exists which must becorrected before its correspondingtrouble code will clear.

2) Plug the Code Scanner (make surethe TEST switch is in the ABSposition) into the test connector.

TM

GM 1982 & higher - CP 9001


TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


OFF

ON

3) Remove theCodeScanner forabout 1second thenreinstall CodeScanner into the connector. Repeatthis procedure 4 times within 10seconds. Leave the Code Scannerinserted in the connector after the4th time.

4) Observe the anti-lock indicator lamp.Code 12, the “diagnostic systemoperational” code should flashindicating that all trouble codes havebeen erased from memory. Wait atleast 15 seconds before turning theignition switch OFF. Remove theCode Scanner, and reinstall the testconnector cover (if used on vehicle).

63

�,��-��=�,�� 5)� >'��?IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)

��Diagnostic SystemOperational. This code isalways sent.

��Right Front Wheel SpeedSensor circuit. Checksfor open, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH forthis code to set.


��Left Front Wheel SpeedSensor circuit. Checksfor open, shorted orintermittent condition insubject circuit. Vehiclespeed must be equal to orgreater than 4 MPH forthis code to set.

��Left Front ToothedWheel Frequency Error.Checks for excessivelydirty, or damaged toothedwheel (sensor ring). Mis-matched tire sizes, or useof the temporary (mini)spare tire may set thiscode.

��Rear Axle Speed Sensorcircuit. Checks for open,shorted or intermittentcondition in subject circuit.Vehicle speed must beequal to or greater than 4MPH for this code to set.

��Rear Axle Toothed WheelFrequency Error. Checksfor excessively dirty, ordamaged toothed wheel(sensor ring). Mis-matchedtire sizes, or use of thetemporary (mini) spare tiremay set this code.



��Rear Axle Solenoid Valvecircuit. This code will set ifthe physical position of thesubject valve does notmatch the commandedposition as given by theEBCM (Electronic BrakeControl Module).




64

��)#��$��%��&'�� (

��



• 1991 Brougham, Eldorado, Reatta,Riviera, Seville, Toronado, Trofeo:

–Check the ground that is locatedbehind the right headlight.

–Check the ground that is locatednear the parking brake pedal onthe left side of the dash.

• 1992 Eldorado, Seville:

–Check the ground that is locatedon the engine near the alternator.

–Check the ground that is locatedon the rear seat brace.

• 1992 Riviera, Toronado, Trofeo:


–Check the ground that is located inthe right front engine compart-ment.

• 1993 Riviera:


–Check the ground that is locatedon top of the left shroud.


• 1991 Brougham:

–Check the GA-TRANS fuse in theunderdash fuse block.


–Check the rust colored fusible linkE on the positive junction block.


–Check fuse number 7 in theinterior relay center.

–Check the black and blue coloredfusible links on the positivejunction block.

• 1991 Reatta, Riviera:



• 1991 Toronado, Trofeo:

–Check fuse number 18 in theunderdash fuse block.

–Check the black M and blue Ucolored fusible links on the positivejunction block.


–Check the A1 fuse in the trunkcompartment fuse block.

–Check fuse number 5 in theRIGHT MAXI fuse block.

–Check fuse number 6 in theRIGHT MAXI fuse block.

• 1992 Riviera:



65

• 1992 Toronado, Trofeo:

–Check fuse number 18 in theunderdash fuse block.


• 1993 Riviera:


–Check fuse number 6 and 9 in theunderdash fuse block.

–Check the black colored M and Pfusible links on the positivejunction block.

3) Check that all ABS/TCS connec-tors, and EBCTM (ElectronicBrake and Traction ControlModule) connectors are properlyinstalled (not loose).


5) Check that the brake pressuremodulator valve ground stud isclean and tight.

6) Check the over-voltage protectionrelay and its connector.


9�� #��1) Start the engine.

The brakewarning lampshould come ONduring enginecranking, and turn OFF shortly afterthe engine starts. If it does not,check the brake warning lampcircuit in accordance with vehicleservice manual instructions.

2) Turn the engine OFF. Turn theignition key to the ON (RUN)position, but do not start the engine.The anti-lock lamp should illuminatefor about 4 seconds and then turnOFF, if no trouble codes arecurrently stored. However “history”trouble codes may be stored.Proceed to Step 3 below. If the anti-lock lamp stays ON, trouble codesare stored. Proceed directly toREADING ABS CODES, page 64.

3) Drive the vehicle at least 20 MPH. Ifthe anti-lock lamp turns ON,proceed to READING ABS CODES,page 64.

66

!�� )��# 36 �'�� IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!











• The connectormay have aslip-on coverlabeled“DiagnosticConnector.” Remove cover fortesting. Replace cover aftertesting. Some vehicles require thiscover in place forproper operation.

4) Verify IgnitionKey is OFF

OFF ON

OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001







7) Turn Ignition Keyto ON Positionbut DO NOTSTART THEENGINE


8) Get Codes fromthe Flashing“Anti-Lock” Light

• Count flashes to get trouble codes.(Flashes begin after a few seconds.)

Code 12 looks like:




Code 23 looks like:




67



(longer pause)


(longer pause)





(longer pause)


(longer pause)


(even longer pause, then go to nextcode)



(longer pause)



(longer pause)




• A code 12 is always sent evenwhen the computer sees noproblem. This tells you the computerdiagnostic checks are workingproperly.



OFF ON



11)Refer to ABS Code Meanings onpage 67 (Bosch 2U, Version B)




or,


IMPORTANT: Always followvehicle service manual proce-dures for any ABS repairs!(Manual listings on page 4.)

68

�� (��%)��# 36 �'�� Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again. Note: Thecomputer will automatically erasecodes after several restarts (typically100) if the problem does not return.

Proceed as follows:

1) Turn the ignitionswitch to the ONposition. The anti-lock lamp shouldturn OFF after 3 to4 seconds. If it does not, a currentfailure still exists which must becorrected before its correspondingtrouble code will clear.

2) Plug the Code Scanner (makesure the TEST switch is in theABS position) into the testconnector until the anti-lock lampturns ON. Remove the CodeScanner. The anti-lock lamp willturn OFF.

OFF ON

OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


TM

GM 1982 & higher - CP 9001


3) Without turning the ignition switchOFF,repeat thesequenceoutlined inStep 2, 2more times.

4) All diagnostic trouble codes shouldnow be cleared. Confirm this byturning the ignition switch OFF,installing the Code Scanner, andturning the ignition switch back tothe ON position (do not start theengine). Code 12, the “diagnosticsystem operational” code should bethe only code displayed.

5) Turn the ignition switch OFF.Remove the Code Scanner, andreinstall the test connector cover (ifused on vehicle).

69

�,��-��=�,��5)� >'��,?IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)












��Rear Axle Solenoid Valvecircuit. This code will set ifthe physical position of thesubject valve does notmatch the commandedposition as given by theEBCM (Electronic BrakeControl Module).




70

��*#��$��%��&'�� (

!��;� ��/��

!��6� �0'��/��/��



–Check the ground that is locatedon the bottom left side of theengine block, near the transaxle.

–Check the ground that is locatedon the left rear seat brace.


–Check fuse A1(10 amp) inthe trunkcompartmentfuse block.This fuseprovides switched power to theEBCM.

–Check fuse B3 (20 amp) in theengine compartment fuse block.This fuse provides continuouspower to the brake light switch.

–Check fuse A3 (10 amp) in theengine compartment fuse block.This fuse provides switched powerto the amber anti-lock indicatorlamp.

–Check fuse # 5 (50 amp) in theright MAXI fuse block. This fuseprovides continuous power to therelays in the brake pressuremodulator valve assembly.

–Check the fusible links on thejunction block.

3) Check that all ABS/TCS connec-tors, and EBTCM (ElectronicBrake and Traction ControlModule) connectors are properlyinstalled (not loose).


5) Check that the brake pressuremodulator valve ground stud isclean and tight.


9�� #��

DeVille, Eldorado, Seville

1) Start the engine. The anti-lock lampshould come ON during enginecranking, and turn OFF shortly afterthe engine starts. If...

–The lamp does NOT turn OFFafter engine start, or

–The “ANTILOCK DISABLED”message appears, or

–The “TRACTION DISABLED”message appears,

then proceed to READING ABSCODES, page 69.

71

!�� ) ��# 36�'�� IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!








• Service manuals call this connectorthe Assembly Line Diagnostic Link(ALDL) connector. It may also becalled the Assembly Line Commu-nication Link (ALCL) or simply testconnector.

OFF ON

OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001




• The connectormay have aslip-on coverlabeled“DiagnosticConnector.” Remove cover fortesting. Replace cover after testing.Some vehicles require this cover inplace for properoperation.







7) Turn IgnitionKey to ONPosition butDO NOT STARTTHE ENGINE


8) Get Codes from the Flashing“Anti-Lock” Light

• Count flashes to get trouble codes.(Flashes begin after a few seconds.)

Code 12 looks like:




Code 23 looks like:




72



(longer pause)


(longer pause)





(longer pause)


(longer pause)


(even longer pause,then go to next code)



(longer pause)



(longer pause)









11) Refer to ABS Code Meanings onpage 72 (Bosch 2U, Version C)




or,


IMPORTANT: Always follow vehicleservice manual procedures for anyABS repairs! (Manual listings on page4.)

73

�� (��%) ��# 36 �'�� Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again. Note: Thecomputer will automatically erasecodes after several restarts (typically100) if the problem does not return.

Proceed as follows:

1) Turn the ignitionswitch to the ONposition. The anti-lock lamp shouldturn OFF after 3to 4 seconds. If itdoes not, a current failure still existswhich must be corrected before itscorresponding trouble code willclear.

2) Install theCode Scanner(make surethe TESTswitch is inthe ABS position)into the testconnector until theanti-lock lampturns ON. Remove the CodeScanner. The anti-lock lamp will turnOFF.

3) Without turning the ignition switchOFF, repeatthe sequenceoutlined inStep 2, 2 moretimes.

4) All diagnostic trouble codes shouldnow be cleared. Confirm this byturning the ignition switch OFF,installing the Code Scanner, andturning the ignition switch back tothe ON position (do not start theengine). Code 12, the “diagnosticsystem operational” code should bethe only code displayed.

5) Turn the ignitionswitch OFF.Remove theCode Scanner,and reinstall thetest connectorcover (if usedon vehicle).

OFF ON

OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


TM

GM 1982 & higher - CP 9001


74

�,��-��=�,��5)� >'��?IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)




��Right Front Wheel SpeedSensor Continuity fault.Checks for open, orshorted condition insubject circuit. The ignitionmust be on, and thevehicle not moving for thiscode to set.



��Left Front Wheel SpeedSensor Continuity fault.Checks for open, orshorted condition in subjectcircuit. The ignition must beon, and the vehicle notmoving for this code to set.



��Right Rear Wheel SpeedSensor Continuity fault.Checks for open, orshorted condition in subjectcircuit. The ignition must beon, and the vehicle notmoving for this code to set.



��Left Rear Wheel SpeedSensor Continuity fault.Checks for open, orshorted condition insubject circuit. The ignitionmust be on, and thevehicle not moving for thiscode to set.


��Right Front TractionControl System PilotValve fault. This code willset if the physical positionof the subject valve doesnot match the commandedposition as given by theEBTCM (Electronic Brakeand Traction ControlModule).


��Left Front TractionControl System PilotValve fault. This code willset if the physical positionof the subject valve doesnot match the commandedposition as given by theEBTCM (Electronic Brakeand Traction ControlModule).

75

��Right Rear SolenoidValve circuit. This codewill set if the physicalposition of the subjectvalve does not match thecommanded position asgiven by the EBCM(Electronic Brake ControlModule).

�� Left Rear Solenoid Valvecircuit. This code will set ifthe physical position of thesubject valve does notmatch the commandedposition as given by theEBTCM (Electronic Brakeand Traction ControlModule).

�� Rear ABS Solenoid Valvecircuit. This code will set ifthe physical position of thesubject valve does notmatch the commandedposition as given by theEBCM (Electronic BrakeControl Module).



��Brakelight Switch circuit(Vehicles with TractionControl). This code will setif the brakelight switchsignal is not received.

��EBCM or EBTCM(Electronic Brake Controlor Traction ControlModule) failure. This codewill set if there is aninternal failure of theElectronic Brake ControlModule.

��EBTCM (Electronic Brakeand Traction ControlModule) PWM (PulseWidth Modulated) Signalfailure (4.6 Liter engineonly). This code will set ifthere is a desired torquelevel signal communicationfailure between the EBTCM(Electronic Brake andTraction ControlModule)and the PCM(Powertrain ControlModule).

��Low Brake Fluid Level(Vehicles with TractionControl). This code will setif the brake fluid level islow, or there is a problemwith the brake fluid levelsensing system.

76

��+#�,��-��&'�� (!�:� ,��/�,��/�0�� ::/�0'��/

��/� 1��#��/� �:�+��/��$��

!�� ,��/�,��/�0�� ::/�0'��/��/� 1��#��/� �:�+��/��$��


1) Check brake fluid level

2) Release the parking brake if it isset

3) Check all ABSsystem fuses

4) Check systemelectrical connections

–Wheel speed sensor connectors.

–EBCM (Electronic Brake ControlModule) connectors.

–System relay connectors.

–System grounds.


9�� #��1) Turn the ignition key to the ON

position, but do not start the engine.

2) The anti-lock lampshould illuminatefor at least 3seconds. If it doesnot, refer to the proper diagnosticchart in the vehicle service manual.

3) Observe the anti-lock and brakelamps while starting the engine(ignition key is in the crankingposition). Both the anti-lock andbrake lamps should be ON duringcranking. If they are not, refer to theproper diagnostic chart in thevehicle service manual.

4) When the engine starts, allow it torun for 30 seconds, and then turn itOFF for 10 seconds.

5) Return the ignition key to the ONposition (do not start the engine)and wait 10 seconds. Observe thecondition of the anti-lock and brakewarning lamps:

–If the anti-lock lamp is ON, and thebrake warning lamp is OFF,proceed to READING ABSCODES, page 75.

–If the anti-lock lamp is OFF, andthe brake warning lamp is ON,refer to the proper diagnostic chartin the vehicle service manual.

77

–If the anti-lock lamp is ON, and thebrake warning lamp is ON, refer tothe proper diagnostic chart in thevehicle service manual.

–If the anti-lock lamp is OFF, and thebrake warning lamp is OFF, thenthis indicates normal systemoperation, or an intermittentproblem. Intermittent trouble codesmay or may not be stored.

!�� ) �� (�� //�'�� IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!

1) Safety First!





2) Verify Codes are Stored

• Turn the ignition key to the ONposition, but do not start theengine.

• Wait 30 seconds.

• Observe anti-locklight...

– Light ON:Codes are stored. Proceed withtesting.

– Light OFF: No codes are stored.Stop testing.





• Service manuals call this connectorthe Assembly Line Diagnostic Link(ALDL) connector. It may also becalled the Assembly Line Commu-

OFF ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


• The connector may be in full view,or it may be recessed behind apanel. An opening in the panelallows access to recessedconnectors.

• The connectormay have aslip-on coverlabeled“DiagnosticConnector.” Remove cover fortesting. Replace cover after testing.Some vehicles require this cover inplace for proper operation.


6) Put TEST switchon ABS




Note: The Code Scanner does notuse all of the test connectorcontacts. Also, one Code Scannerpin may plug into an empty testconnector position. This is normal.



nication Link (ALCL) or simply testconnector.


78

9) Get a TroubleCode fromFlashing “Anti-Lock” Light


• Count flashes to get troublecodes. Flashes start after about a4 second delay.

– The first digit is flashed, then...

– there is a 3 second pause,then...

– the second digit is flashed,then...

– the anti-lock light turns ON andstays ON. Do not count thissteady light as a “flash.”

• EXAMPLES:

Code 12 looks like:

❊ PAUSE ❊❊FLASH (3 second pause) FLASH

FLASH

(then light stays on.)



Code 23 looks like:

❊❊ PAUSE ❊❊❊FLASH FLASH (3 second pause)

FLASH FLASH FLASH


10)Get More Codes (if any) fromFlashing “Anti-Lock” Light

• Do this step after first code hasbeen flashed and anti-lock light isON steadily.

• Do NOT turn ignition switch off.

– RemoveCodeScanner fromtest connec-tor. Then...

– Re-install Code Scanner intotest connector.

– The next ABS trouble code (if

TM

GM 1982 & higher - CP 9001


any) will be flashed in the samemanner as the first code.

• Repeat this step until all troublecodes have been read. The ABScomputer can store up to 7 troublecodes.

11)Turn Ignition KeyOFF

12)Remove CodeScanner andRe-installConnectorCover, ifsupplied

The computer system is now back tonormal operation.

13)Refer to ABS Code Meanings onpage 77 (Teves Mark ll, Version A)




or,



�� (��%) �� (�� // �'�� Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again.

1) IMPORTANT! - Trouble codescannot be cleared until they havebeen read! Refer to READING ABSCODES, page 75.

2) Drive the vehicle at a speed ofgreater than 18 MPH. Troublecodes should automatically clear.

OFF ON

79

�,��-��=��-��$� ((� >'��?IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)

��EBCM (Electronic BrakeControl Module) Failure.In most cases, this codeindicates an EBCM failure.Follow the appropriatechart to check the groundcircuit.


��Main Valve. Checks foropen, shorted orintermittent condition inthe main valve solenoidand its circuitry.

��Left Front Inlet Valve.Checks for open, shortedor intermittent condition inthe left front inlet valvesolenoid and its circuitry.

��Left Front Outlet Valve.Checks for open, shortedor intermittent condition inthe left front outlet valvesolenoid and its circuitry.

��Right Front Inlet Valve.Checks for open, shortedor intermittent condition inthe right front inlet valvesolenoid and its circuitry.

��Right Front Outlet Valve.Checks for open, shortedor intermittent condition inthe right front outlet valvesolenoid and its circuitry.

��Rear Inlet Valve. Checks foropen, shorted or intermittentcondition in the rear inletvalve solenoid and itscircuitry.

��Rear Outlet Valve. Checksfor open, shorted orintermittent condition in therear outlet valve solenoidand its circuitry.

��Left Front Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circuit.

��Right Front Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circuit.

��Right Rear Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circuit.

��Left Rear Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circuit.









��Left Front Wheel SpeedSensor Signal Missing.Checks for missing signalin the subject circuit

��Right Front Wheel SpeedSensor Signal Missing.Checks for missing signalin the subject circuit.

��Either Rear Wheel SpeedSensor Signal Missing.Checks for missing signalin either of the rear wheelspeed sensor circuits.(Note that it is impossibleto determine which of therear wheel speed circuits iscausing the problem.) Thiscode will set if the frontwheels spin while the rearwheels are stationary.

��Any three Wheel SpeedSensor Signals Missing.There are missing signalsin three (3) of the four (4)wheel speed sensorcircuits.

80

��Left Front WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 71, which hasthe same meaning.

��Right Front WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 72, which hasthe same meaning.

��Right Rear WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 73, which hasthe same meaning.

��Left Rear WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 74, which hasthe same meaning.





��Shorted Low Fluid, orHydraulic PressureSwitch, or shorted ABSdiode, or associatedcircuitry. This code will setif the low fluid switch, thehydraulic pressure switch,or the ABS diode or any ofthe associated circuitryexhibits a short circuitcondition.

��Left Front WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note:This code may also be sentalong with Code 51, whichhas the same meaning.

��Right Front WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note:This code may also be sentalong with Code 52, whichhas the same meaning.

��Right Rear WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note: Thiscode may also be sentalong with Code 53, whichhas the same meaning.

��Left Rear WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note:This code may also be sentalong with Code 54, whichhas the same meaning.



��Right Rear Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circuit


81

��.#�,��-��&'�� (!�:��/�+��/�+��/��/��!�� /�+��/�+��/��/��/��


1) Check brake fluid level

2) Release the parking brake if it isset

3) Check all ABSsystem fuses

4) Check systemelectrical connections

–Wheel speed sensor connectors.

–EBCM (Electronic Brake ControlModule)connectors.

–System relay connectors.

–System grounds.


9�� #��1) Turn the ignition

key to the ONposition, but donot start theengine.

2) The anti-locklamp should illuminate for at least 3seconds. If it does not, refer to theproper diagnostic chart in the vehicleservice manual.

3) Observe the anti-lock and brakelamps while starting the engine(ignition key is in the cranking

OFF

ON

position). Both the anti-lock andbrake lamps should be ON duringcranking. If they are not, refer to theproper diagnostic chart in thevehicle service manual.

4) When the engine starts, allow it torun for 30 seconds, and then turn itOFF for 10 seconds.

5) Return the ignition key to the ONposition (do not start the engine)and wait 10 seconds. Observe thecondition of the anti-lock and brakewarning lamps:

–If the anti-lock lamp is ON, and thebrake warning lamp is OFF,proceed to READING ABSCODES, page 80.

–If the anti-lock lamp is OFF, andthe brake warning lamp is ON,refer to the proper diagnostic chartin the vehicle service manual.

–If the anti-lock lamp is ON, and thebrake warning lamp is ON, refer tothe proper diagnostic chart in thevehicle service manual.

–If the anti-lock lamp is OFF, andthe brake warning lamp is OFF,then this indicates normal systemoperation, or an intermittentproblem. Intermittent trouble codesmay or may not be stored.

82

!�� )�� (�� / �'�� IMPORTANT: Perform all steps inPRE-DIAGNOSTIC VISUAL INSPEC-TION and FUNCTIONAL CHECKbefore reading ABS codes!

1) Safety First!





2) Verify Codes are Stored


• Wait 30 seconds.

• Observe anti-locklight...

– Light ON: Codes are stored.Proceed with testing.

– Light OFF: No codes are stored.Stop testing.







• The connector may be in full view,

OFF ON

OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


or it may be recessed behind apanel. An opening in the panelallows access to recessedconnectors.

• The connector may have a slip-oncover labeled “DiagnosticConnector.” Remove cover fortesting. Replace cover aftertesting. Somevehiclesrequire thiscover in placefor properoperation.


6) Put TEST switch on ABS

7) Plug the Code Scanner into theTest Connector



Note: The Code Scanner does notuse all of the test connectorcontacts. Also, one Code Scannerpin may plug intoan empty testconnectorposition. This isnormal.



9) Get a TroubleCode fromFlashing “Anti-Lock” Light


83

TM

GM 1982 & higher - CP 9001


• Count flashes to get trouble codes.Flashes start after about a 4 seconddelay.– The first digit is flashed, then...– there is a 3 second pause, then...– the second digit is flashed, then...– the anti-lock light turns ON andstays ON. Do not count this steadylight as a “flash.”

• EXAMPLES:

Code 12 looks like:

❊ PAUSE ❊❊FLASH (3 second pause) FLASH

FLASH




Code 23 looks like:

❊❊ PAUSE ❊❊❊FLASH FLASH (3 second pause)

FLASH FLASH FLASH


10)Get More Codes (if any) fromFlashing “Anti-Lock” Light

• Do this step after first code hasbeen flashed and anti-lock light isON steadily.

• Do NOT turnignition switchoff.

– Remove CodeScanner fromtest connector. Then...

– Re-install Code Scanner into testconnector.

– The next ABS trouble code (ifany) will be flashed in the samemanner as the first code.

• Repeat this step until all troublecodes have been read. The ABScomputer can store up to 7 troublecodes.

OFF ON

11)Turn IgnitionKey OFF

12)Remove CodeScanner andRe-installConnectorCover, ifsupplied

The computersystem is now back to normaloperation.

13)Refer to ABS Code Meanings onpage 82 (Teves Mark ll, Version B)



• Have your vehicle professionallyserviced. Trouble codes indicateproblems found by the computer.or,


IMPORTANT: Always followvehicle service manual proce-dures for any ABS repairs!(Manual listings on page 4)

�� (��%)�� (�� // �'�� Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again.

1) IMPORTANT! - Trouble codescannot be cleared until they havebeen read! Refer to READING ABSCODES, page 80.

2) Drive the vehicle at a speed ofgreater than 20 MPH. Troublecodes should automatically clear.

84

�,��-��=��-��$� ((� >'��,?IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)



��Main Valve. Checks foropen, shorted orintermittent condition in themain valve solenoid and itscircuitry.

��Left Front Inlet Valve.Checks for open, shortedor intermittent condition inthe left front inlet valvesolenoid and its circuitry.

��Left Front Outlet Valve.Checks for open, shortedor intermittent condition inthe left front outlet valvesolenoid and its circuitry.

��Right Front Inlet Valve.Checks for open, shortedor intermittent condition inthe right front inlet valvesolenoid and its circuitry.

��Right Front Outlet Valve.Checks for open, shortedor intermittent condition inthe right front outlet valvesolenoid and its circuitry.

��Rear Inlet Valve. Checksfor open, shorted orintermittent condition in therear inlet valve solenoidand its circuitry.

��Rear Outlet Valve. Checksfor open, shorted orintermittent condition in therear outlet valve solenoidand its circuitry.












��Left Rear Wheel SpeedSensor circuit. Checks foran open, short orintermittent condition insubject circui

��Left Front Wheel and One(1) Rear Wheel SpeedSensor Signal Missing.Checks for missing signalin the subject circuits.

��Right Front Wheel andOne (1) Rear WheelSpeed Sensor SignalMissing. Checks formissing signal in thesubject circuits.

��Either Rear Wheel SpeedSensor Signal Missing.Checks for missing signalin either of the rear wheelspeed sensor circuits.(Note that it is impossibleto determine which of therear wheel speed circuits iscausing the problem.) Thiscode will set if the frontwheels spin while the rearwheels are stationary.

85

��Any three Wheel SpeedSensor Signals Missing.There are missing signalsin three (3) of the four (4)wheel speed sensorcircuits.


��Right Front WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 72, which hasthe same meaning.

��Right Rear WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 73, which hasthe same meaning.

��Left Rear WheelHydraulic PressureReduction fault. Thiscode is the result of anincorrect response to ahydraulic circuit pressurereduction command assent by the EBCM(Electronic Brake ControlModule). Note: This codemay also be sent alongwith Code 74, which hasthe same meaning.

��Shorted Low Fluid, orHydraulic PressureSwitch, or shorted ABSdiode, or associatedcircuitry. This code willset if the low fluid switch,the hydraulic pressureswitch, or the ABS diode orany of the associatedcircuitry exhibits a shortcircuit condition.


��Right Front WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note:This code may also be sentalong with Code 52, whichhas the same meaning.

��Right Rear WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note:This code may also be sentalong with Code 53, whichhas the same meaning.

��Left Rear WheelHydraulic PressureReduction fault. This codeis the result of an incorrectresponse to a hydrauliccircuit pressure reductioncommand as sent by theEBCM (Electronic BrakeControl Module). Note: Thiscode may also be sentalong with Code 54, whichhas the same meaning.

86

OFF ON

��/#�01234�2��

��

�� !"��#��

��$ �� %��& !"�'�(��&� �'�� #��)�)�(

�� %��& !"�'�(��&� �'�� #��)�)�(

�� %��& !"�'�(�� #��

��* ��

��+ ��

$�� #��Complete all steps before readingtrouble codes!

1) Block the wheels, and release theparking brake. Do not step on theservice brake!

2) Turn the ignition key to the ONposition, but do not start the engine.Observe theinstrument panelbrake warning light:

• If the brake lightturns ON, and then OFF after about2 seconds, the self-diagnosticcircuitry has found NO currentproblems with the ABS system.

– DO NOT perform ABS codereading procedure! A false code 9will be stored in ABS computermemory if the code reading stepsare followed when no faults currentlyexist. (This is a quirk of the RWALABS system.)

– Do this additional check: Step onthe service brake. If the brake

warning light comes ON, there is aproblem with the combination valve.(This valve is part of the normalbraking system, not part of the anti-lock braking system.) Release theservice brake.

• If the brake light turns ON and staysON (or turns ON after the bulbcheck), proceed to the diagnosticprocedure outlined in your vehicleservice manual for the brake lightON symptom.

• If the brake light stays OFF,proceed to the diagnosticprocedure outlined in your vehicleservice manual for the brake lightOFF symptom.

• If the brake light is FLASHING,proceed to READING ABSCODES, page 85.

3) Turn the ignition keyto the OFF position.

87

!�� )B��%�<%� !:��

IMPORTANT: Perform all steps inDIAGNOSTIC CIRCUIT CHECKbefore reading ABS codes!





2) Verify ABS Problem CurrentlyExists


WARNING: Stay away from theradiator cooling fan! It may turnon.• Wait 5 seconds, and then observe

the brake warning light.

– Brake light OFF: ABS computerdoes NOT detect a currentproblem. Do not proceed with thistest! A false code 9 will be storedin ABS computer memory if thecode reading steps are followedwhen no faults currently exist! Turnthe ignition key OFF and stoptesting.

– Brake light ON: An ABS problemcurrently exists. At least onetrouble code is stored in computermemory. Go to step 3 and continuecode reading procedure.

3) Find the Computer Test Connec-tor

• Service manuals call this connectorthe Assembly Line Diagnostic Link(ALDL) connector. It may also becalled the Assembly Line Communi-cation Link (ALCL) or simply testconnector.


OFF

ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001



• The connectormay have aslip-on coverlabeled“DiagnosticConnector.”Remove cover for testing. Replacecover after testing. Some vehiclesrequire this cover in place forproper operation.

4) Verify IgnitionKey is ON andEngine is OFF


6) Plug the Code Scanner into theTest Connector




7) Get the Code from the Flashing“Brake” Light

• Count flashes to get the troublecode.

– Flashes begin after 20 seconds,or more.

– The code starts with one long flashand is followed by several shortflashes. Count the long flash alongwith the short flashes to get the codenumber. After a pause, the code isrepeated.

88

OFF ON

Code 3 looks like:

starts over pause

SHORT

Code 3

SHORT

FLASH

L O N G

FLASH❊ ❊

FLASH❊

Code 5 looks like:

starts over pause

SHORT

Code 5

SHORT

FLASH

SHORTSHORTL O N G

FLASH❊ ❊

FLASH❊

FLASH❊

FLASH❊

• The same code is flashed overand over again. Note that the longflash helps you tell when the codeis being repeated.

• Important: The code may beflashed incorrectly the first time.The code will be flashed correctlythe rest of the time. Count the flashsequences a few times to verifythe code.

• Code flashing continues until theignition key is turned OFF or theCode Scanner is unplugged.

8) Turn Ignition KeyOFF

9) Remove CodeScanner andRe-installConnectorCover, ifsupplied


10)Refer to ABS Code Meanings onpage 87 (Kelsey-Hayes RWAL)



• Have your vehicle professionallyserviced. Trouble codes indicateproblems found by the computer.or,


IMPORTANT: Always follow

OFF ON

vehicle service manual proce-dures for any ABS repairs!(Manual listings on page 4.)

NOTE (multiple trouble code storage):

This ABS system is only capable ofdisplaying one code at a time. Repairthe failure which generated thedisplayed trouble code, and thenrepeat the READING ABS CODESprocedure to see if any more codesare stored. Continue repeating theprocedure until all codes are dis-played, diagnosed, and repaired.

�� (��%) B��%�<%� !:��Erase codes from memory when youcomplete all repairs or to see if aproblem will occur again.

Proceed as follows:

1) Turn the ignitionkey to the OFFposition.

2) Remove the indicated fuse fromthe fuse block and wait at least 10seconds.

• 1988–93 C & K Trucks:Remove the STOP/HAZ fuse.

• 1989–92 Astro/Safari:Remove the HORN/DM fuse.

• 1989 S Series Trucks:Remove the ECM B fuse.

• 1990–92 G Series RWD Vans:Remove the TAIL LPS fuse.

• 1990–91 R & V Series (Suburbansetc.):Remove the STOP/HAZ fuse.

• 1990–93 S & T Series Trucks:Remove the ECM B fuse.

3) Replace fuse. Trouble codes arenow erased from computer memory!

89

�,��-��=�9��78��+%�4IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)

�ECM (Electronic ControlModule) Failure. Thiscode indicates an anti-lockbrake system controlmodule failure, or a mis-read code.

�Open Isolation valve ordefective ECM(Electronic ControlModule). This codeindicates an open circuit inthe isolation valve orassociated wiring. Theanti-lock brake systemcontrol module may alsohave failed.

�Open Dump valve ordefective ECM(Electronic ControlModule). This codeindicates an open circuit inthe dump valve orassociated wiring. Theanti-lock brake systemcontrol module may alsohave failed.

�Grounded anti-lock valveReset Switch. This codeindicates a groundedcircuit in the anti-lock valvereset switch or associatedwiring

�Excessive actuation ofthe Dump valve duringan anti-lock stop. On 2wheel drive vehicles, thiscode indicates probablefailure of the isolation/dump valve assembly. On4 wheel drive vehicles, thiscode indicates eitherpossible failure of theisolation/dump valveassembly, or a failure inthe front axle transfer caseswitch or its associatedwiring and/or connectors.

�Erratic Vehicle SpeedSensor signal. This codeindicates either a defectiveor erratic vehicle speedsensor (check for enginediagnostic trouble code24), defective DRAC(Digital Ratio AdapterController) or its circuitry,or any associated wiring.An open battery feed fuseto the ABS electroniccontrol module could alsocause this code to set.

�Shorted Isolation valveor defective ECM(Electronic ControlModule). This codeindicates a shorted circuitin the isolation valve orassociated wiring. Theanti-lock brake systemcontrol module may alsohave failed.

�Shorted Dump valve ordefective ECM(Electronic ControlModule). This codeindicates a shorted circuitin the dump valve orassociated wiring. Theanti-lock brake systemcontrol module may alsohave failed.

�Open circuit to theVehicle Speed Sensorsignal. This code indicatesa defective vehicle speedsensor (check for enginediagnostic trouble code24), defective DRAC(Digital Ratio AdapterController) or its circuitry,or any of its associatedwiring. An open batteryfeed fuse to the ABSelectronic control modulecould also cause this codeto set.

��Stop (Brake) Light circuit.This code indicates amissing stop light signalfrom the stop light switch.The stop light switch maybe mis-adjusted ordefective, or there may bea problem with itsassociated wiring.

��ECM (Electronic ControlModule) Failure. Thiscode indicates an anti-lockbrake system controlmodule failure, or a mis-read code.

��ECM (Electronic ControlModule) Failure. Thiscode indicates an anti-lockbrake system controlmodule failure, or a mis-read code.

��ECM (Electronic ControlModule) Failure. Thiscode indicates an anti-lockbrake system controlmodule failure.



90

��5#�01234�2*��

��

�� !�"�#$�%�� !#��!%��

�� &��'��"�#$�%�� !��"�#$�%��(��(��!#��!%��)�$��

�� &��'��"�#$�%�*��+��

�� !��"�#$�%��(��(��!#��!%��)�$��

��, ��&��'��"�#$�%�*��+��

�� !��"�#$�%��(��(��)�$��

$�� #��Complete all steps before readingtrouble codes!

1) Block the wheels, and release theparking brake. Do not step on theservice brake!

2) Turn the ignition key to the ONposition, but do not start the engine.Observe the instrument panelamber anti-lock light:

• If the anti-lock lightturns ON,and thenOFF after about 2 seconds, thediagnostic circuit check issuccessful. Proceed to Step 3.

• If the anti-lock light turns ON andstays ON, or turns ON after thebulb check, proceed to the

diagnostic procedure outlined inyour vehicle service manual for theanti-lock light ON symptom.

• If the anti-lock light stays OFF,proceed to the diagnosticprocedure outlined in your vehicleservice manual for the anti-locklight OFF symptom.

3) Turn the ignitionkey to the OFFposition. Proceedto READINGABS CODES,page 89.

OFF ON

91

!�� )B��%�<%� 5:��

IMPORTANT: Perform all steps inDIAGNOSTIC CIRCUIT CHECKbefore reading ABS codes!

Note (1990, 1991 only): Codes 21, 22,25, 26, 31, 32, 35, and 36 will disableABS system, but are not stored incomputer memory. They are erasedwhen ignition is turned off. All othercodes are stored in memory when theyoccur.








• Service manuals call this connectorthe Assembly Line Diagnostic Link(ALDL) connector. It may also becalled the Assembly Line Commu-nication Link (ALCL) or simply testconnector.



• The connectormay have a slip-on cover labeled“DiagnosticConnector.”Remove cover for testing. Replacecover after testing. Some vehiclesrequire this cover in place forproper operation.


TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


OFF ON


6) Plug theCodeScanner intothe TestConnector




7) Turn IgnitionKey to ONPosition but DONOT START THEENGINE


8) Get Codes from the Flashing“Anti-Lock” Light

• Count flashes to get trouble codes.(Flashes begin after a fewseconds.)

Code 12 looks like:




Code 23 looks like:


OFF

ON

92






11) Refer to ABS Code Meanings onpage 91 (Kelsey-Hayes 4WAL)




or,



�� (��%) B��%�<%� 5:��Erase codes from memory wheneveryou complete a repair or to see if aproblem will occur again.

Proceed as follows:

• 1990, 1991:

1) Turn the ignitionswitch to the ONposition.

OFF ON

2) Put the CodeScanner TESTswitch in the ABSposition.

3) Plug the Code Scanner into the testconnector for 2 seconds. Removethe CodeScanner for 2seconds.Repeat thisprocedure 5more times.

Important: A false code 65 may becreated if the plug/unplug procedureis only done twice. You mustcomplete all of step 3 in order toerase trouble codes.

4) Turn the ignitionswitch to the OFFposition. Reinstalltest connector cover(if used on vehicle).

• 1992 and newer:

1) Turn the ignitionswitch to the ONposition.

2) Put the CodeScanner TESTswitch in the ABSposition.

3) Plug the CodeScanner into thetest connectorfor 2 seconds.Remove theCode Scannerfor 1 second. Repeat this procedureonce more. Trouble codes areerased when the anti-lock and brakelights turn on, then turn off. Repeatthe plug/unplug procedure ifnecessary.

4) Turn the ignitionswitch to the OFFposition. Reinstalltest connector cover(if used on vehicle).

TM

GM 1982 & higher - CP 9001


OFF

ON

OFF ON

TESTABS ENGINE

TM

GM 1982 & higher - CP 9001


OFF

ON

OFF ON

TESTABS ENGINE

93

�,��-��=�9��78�� 6%�4IMPORTANT: Always follow vehicle service manual procedures for any ABSrepairs! (Manual listings on page 4.)

��ABS System functional,2WD, service brake notapplied. This code indicatesthat the anti-lock brakesystem as installed on a 2wheel drive vehicle isfunctioning normally. Asindicated above, this codeshould appear without theservice brakes applied.

��ABS System functional,2WD, service brake applied.This code indicates that theanti-lock brake system asinstalled on a 2 wheel drivevehicle is functioningnormally. As indicated above,this code should appear withthe service brakes applied. If,with the service brakesapplied, the anti-lock lampcontinues to indicate a code12, it is likely that there is aproblem with the stop (brake)lamp switch circuit.Assistance is available underthe diagnostic chart fordiagnostic trouble code 81.

��ABS System functional,4WD, service brake notapplied. This code indicatesthat the anti-lock brakesystem as installed on a 4wheel drive vehicle isfunctioning normally. Asindicated above, this codeshould appear without theservice brakes applied.

��ABS System functional,4WD, service brake applied.This code indicates that theanti-lock brake system asinstalled on a 4 wheel drivevehicle is functioningnormally. As indicated above,this code should appear withthe service brakes applied. If,with the service brakesapplied, the anti-lock lampcontinues to indicate a code14, it is likely that there is aproblem with the stop (brake)lamp switch circuit.Assistance is available underthe diagnostic chart fordiagnostic trouble code 81.

��Right Front Wheel SpeedSensor Circuit resistanceis incorrect. This code isindicative of high resistancein this particular wheel speedsensor circuit. Theresistance of the wheelspeed sensor may be out ofspecification, or there maybe loose, corroded, or dirtyconnections in this circuit.

��Right Front Wheel SpeedSensor Voltage Output isincorrect. This codeindicates a problem in thewheel speed sensor or itsassociated tone (toothed)wheel. Check for the properair gap between the wheelspeed sensor and tonewheel, missing or brokenteeth on the tone wheel, aloose, improperly mounted,or mis-adjusted wheel speedsensor. An intermittentconnection in the associatedwiring can also set this code.

��Right Front Wheel SpeedSensor Voltage Output iserratic. Erratic voltage outputusually indicates a loose, dirty,or corroded connection. Checkthe connections at the wheelspeed sensor connector, thewheel speed sensor harness,and the EHCU (Electro-Hydraulic Control Unit). Theremay also be a problem in thewheel speed sensor or itsassociated tone (toothed)wheel. Check for the proper airgap between the wheel speedsensor and tone wheel,missing or broken teeth on thetone wheel, a loose,improperly mounted, or mis-adjusted wheel speed sensor.

��Left Front Wheel SpeedSensor Circuit resistance isincorrect. This code isindicative of high resistance inthis particular wheel speedsensor circuit. The resistanceof the wheel speed sensor maybe out of specification, or theremay be loose, corroded, ordirty connections in this circuit.

��Left Front Wheel SpeedSensor Voltage Output isincorrect. This codeindicates a problem in thewheel speed sensor or itsassociated tone (toothed)wheel. Check for the properair gap between the wheelspeed sensor and tonewheel, missing or brokenteeth on the tone wheel, aloose, improperly mounted,or mis-adjusted wheel speedsensor. An intermittentconnection in the associatedwiring can also set this code.

��Left Front Wheel SpeedSensor Voltage Output iserratic. Erratic voltage outputusually indicates a loose,dirty, or corroded connection.Check the connections at thewheel speed sensorconnector, the wheel speedsensor harness, and theEHCU (Electro-HydraulicControl Unit). There may alsobe a problem in the wheelspeed sensor or itsassociated tone (toothed)wheel. Check for the properair gap between the wheelspeed sensor and tone wheel,missing or broken teeth onthe tone wheel, a loose,improperly mounted, or mis-adjusted wheel speed sensor.

��One or Two Wheel SpeedSensor Voltage OutputSignals are erratic. Troublecodes 23, 27, 33, or 37 maybe set along with code 28. Ifone or more of these 4codes are present, use thatcode(s) to diagnose thefailure. If trouble code 28 isthe only code set, it isadvisable to drive the vehiclean additional period of timeuntil one of the 4 listedcodes does set, and thenuse that code chart fordiagnosis. If code 28persists as the only troublecode, it indicates that anintermittent connectionexists somewhere in thesystem. All connections willhave to be checked.

94

��All Wheel Speed SensorVoltage Output Signalsare erratic. Trouble code 29is usually caused by animproperly seated 8 wayconnector at the EHCU(Electro-Hydraulic ControlUnit). Check for corrosion ordirty contacts, and that theconnector is properlyinstalled in the EHCU.

��Right Rear Wheel SpeedSensor Circuit resistanceis incorrect. This code isindicative of high resistancein this particular wheelspeed sensor circuit. Theresistance of the wheelspeed sensor may be out ofspecification, or there maybe loose, corroded, or dirtyconnections in this circuit.

��Right Rear Wheel SpeedSensor Voltage Output isincorrect. This codeindicates a problem in thewheel speed sensor or itsassociated tone (toothed)wheel. Check for the properair gap between the wheelspeed sensor and tonewheel, missing or brokenteeth on the tone wheel, aloose, improperly mounted,or mis-adjusted wheel speedsensor. An intermittentconnection in the associatedwiring can also set thiscode.

��Right Rear Wheel SpeedSensor Voltage Output iserratic. Erratic voltageoutput usually indicates aloose, dirty, or corrodedconnection.Check theconnections at the wheelspeed sensor connector, thewheel speed sensorharness, and the EHCU(Electro-Hydraulic ControlUnit). There may also be aproblem in the wheel speedsensor or its associatedtone (toothed) wheel. Checkfor the proper air gapbetween the wheel speedsensor and tone wheel,missing or broken teeth onthe tone wheel, a loose,improperly mounted, or mis-adjusted wheel speedsensor

��Left Rear Wheel SpeedSensor Circuit resistanceis incorrect. This code isindicative of high resistancein this particular wheelspeed sensor circuit. Theresistance of the wheelspeed sensor may be out ofspecification, or there maybe loose, corroded, or dirtyconnections in this circuit.

��Left Rear Wheel SpeedSensor Voltage Output isincorrect. This codeindicates a problem in thewheel speed sensor or itsassociated tone (toothed)wheel. Check for the properair gap between the wheelspeed sensor and tonewheel, missing or brokenteeth on the tone wheel, aloose, improperly mounted,or mis-adjusted wheel speedsensor. An intermittentconnection in the associatedwiring can also set thiscode.

��Left Rear Wheel SpeedSensor Voltage Output iserratic. Erratic voltageoutput usually indicates aloose, dirty, or corrodedconnection. Check theconnections at the wheelspeed sensor connector, thewheel speed sensorharness, and the EHCU(Electro-Hydraulic ControlUnit). There may also be aproblem in the wheel speedsensor or its associatedtone (toothed) wheel. Checkfor the proper air gapbetween the wheel speedsensor and tone wheel,missing or broken teeth onthe tone wheel, a loose,improperly mounted, or mis-adjusted wheel speedsensor.

��Speed Sensor VoltageSignal Output is erratic.When this code appears, it isusually accompanied withcode 23, 27, 33, or 37. If it isaccompanied with one ormore of the listed codes, usethe trouble chart for that codeto diagnose the system. Iftrouble code 38 appears byitself, all connectionsbetween the wheel speed

sensors and the EHCU(Electro-Hydraulic ControlUnit) will need to be checkedfor looseness, dirt, corrosion,etc

��Right Front Isolation ValveSolenoid, open circuit.Double check to make surethat the code has beenproperly read. If it has, clearall trouble codes by followingthe ERASING ABS CODESFROM COMPUTERMEMORY procedure onpage 12-83. Thoroughly roadtest the vehicle. Check fortrouble codes again. If thiscode returns, the EHCU(Electro-Hydraulic ControlUnit) is defective and shouldbe replaced. The EHCU is acostly part! Your dealer mayoffer an exchange policy inwhich case your defectiveunit has value (core credit)towards the purchase of thereplacement. Securelyattach a tag to your defectiveunit clearly indicating thetrouble code(s) reported bythe defective EHCU.

��Right Front Pulse WidthModulation ValveSolenoid, open circuit.Refer to the explanationunder trouble code 41.

��Right Front Isolation ValveSolenoid, short circuit.Double check to make surethat the code has beenproperly read. Carefullycheck all power and groundconnections at the EHCU. Ifthe code has been properlyread, and there are noproblems in the power orground circuits, clear alltrouble codes by followingthe ERASING ABS CODESFROM COMPUTERMEMORY procedure onpage 12-83. Thoroughly roadtest the vehicle. Check fortrouble codes again. If thiscode returns, the EHCU(Electro-Hydraulic ControlUnit) is defective and shouldbe replaced. The EHCU is acostly part! Your dealer mayoffer an exchange policy inwhich case your defectiveunit has value (core credit)towards the purchase of thereplacement. Securely

95

attach a tag to your defectiveunit clearly indicating thetrouble code(s) reported bythe defective EHCU.

��A��A��A��A��A��A�B�� !�� -��-�If all of these codes appearsimultaneously, carefullycheck all power and groundconnections at the EHCU(Electro-Hydraulic ControlUnit).

��Right Front Pulse WidthModulation ValveSolenoid, short circuit.Refer to the explanationunder trouble code 43.

��Left Front Isolation ValveSolenoid, open circuit.Refer to the explanationunder trouble code 41.

��Left Front Pulse WidthModulation ValveSolenoid, open circuit.Refer to the explanationunder trouble code 41.

��Left Front Isolation ValveSolenoid, short circuit.Refer to the explanationunder trouble code 43.

��Left Front Pulse WidthModulation ValveSolenoid, short circuit.Refer to the explanationunder trouble code 43.

��Rear Isolation ValveSolenoid, open circuit.Refer to the explanationunder trouble code 41.

��Rear Pulse WidthModulation ValveSolenoid, open circuit.Refer to the explanationunder trouble code 41.

��Rear Isolation ValveSolenoid, short circuit.Refer to the explanationunder trouble code 43.

��Rear Pulse WidthModulation ValveSolenoid, short circuit.Refer to the explanationunder trouble code 43.

��Right Front Reset Switch,open circuit. Refer to theexplanation under troublecode 41.

��Left Front Reset Switch,open circuit. Refer to theexplanation under troublecode 41.

��Rear Reset Switch, opencircuit. Refer to theexplanation under troublecode 41.

��Pump Motor Relay, opencircuit. Refer to theexplanation under troublecode 41. IMPORTANT -This code may be FALSELYset by an improperlyperformed trouble codeclearing procedure.

��Pump Motor Relay, shortcircuit. Refer to theexplanation under troublecode 41.

��Pump Motor Circuit, opencircuit. This code indicatesan open circuit in theconnection between thepump motor, and the EHCU(Electro-Hydraulic ControlUnit).

��Pump Motor Circuit,shorted circuit. This codeindicates a shorted circuit inthe connection between thepump motor, and the EHCU(Electro-Hydraulic ControlUnit). Carefully check allpower and groundconnections at the EHCU.

��EHCU (Electro-HydraulicControl Unit) has a RAM(Random Access Memory)error. Refer to theexplanation under troublecode 41.

��EHCU (Electro-HydraulicControl Unit) has a ROM(Read Only Memory)error. Refer to theexplanation under troublecode 41.

��EHCU (Electro-HydraulicControl Unit) has aninternal circuit error.Refer to the explanationunder trouble code 41.

��EHCU (Electro-HydraulicControl Unit) has aninternal circuit errorwhich is causingexcessive isolation time.Refer to the explanationunder trouble code 41.

��Brake (Stop Lamp) switchcircuit is shorted or open.This code indicates amalfunction in the stoplamp switch circuit. Notethat this code can be set bya driver who rides the brakepedal.

��Anti-lock indicator lampcircuit is open. This codeif present, will be flashed bythe brake warning lamp,not the anti-lock lamp, andindicates an open circuit inthe anti-lock lamp circuit.

��Anti-lock indicator lampcircuit is shorted. Thiscode if present, will beflashed by the brakewarning lamp, not the anti-lock lamp, and indicates ashort circuit in the anti-locklamp circuit.

��Brake warning indicatorlamp circuit is shorted.This code indicates a shortcircuit in the brake warningindicator lamp circuit.

96

��

��

* Code Scanner is only applicable if vehicle DOES NOT HAVE a climate control computer

CenturyElectraElectra WagonEstate WagonLe SabreLe Sabre WagonPark AvenueReatta *RegalRegal Grand NationalRiviera *RoadmasterSkyhawkSkylarkSomerset

BUICKCimarron

CADILLACBerettaCamaroCapriceCavalierCelebrityChevetteCitationCorsicaCorvetteEl CaminoImpalaLuminaMonte Carlo

CHEVROLETAchievaCalaisCustom CruiserCutlass CalaisCutlass CieraCutlass CruiserCutlass Cruiser WagonCutlass SupremeCutlass Supreme ClassicDelta 88Eighty-EightFirenzaNinety-EightOmegaToronado *Touring SedanTrofeo *

OLDSMOBILE60006000 STEBonnevilleFieroFirebirdGrand AmGrand PrixJ2000LeMansParisiennePhoenixSafariSafari WagonSunbirdT1000

PONTIAC

All models

All models

SATURN TRUCKS& VANS

All gasolineburning vehicles1 ton capacityor less

All gasolineburning vehicles1 ton capacityor less

1982-93

1994Roadmaster 5.7L Camaro 3.4L, 5.7L

Cavalier 3.1LLumina 3.1L

All models1995

Caprice 4.3L

Firebird 3.4L, 5.7LSunbird 2.0L, 3.1L

97

MAKE

Buick

Cadillac

Chevrolet

Oldsmobile

Pontiac

CARSYEAR1989-901989-901989-911989-931991

1990-921989-901989-931989-901989-93

19911990-91

1989-9019911989-901989-921990-92

1989-901989-90

MODELElectraPark AvenueReattaRivieraRoadmaster

BroughamDeVilleEldoradoFleetwoodSeville

CapriceCorvette

98 RegencyCustom CruiserDelta 88ToronadoTrofeo

BonnevilleBonneville SSE

MAKE

Chevrolet

GM

GMC

Oldsmobile

TRUCKSYEAR1988-931990-93

1988-941988-941990-911989-941990-941990-91

1989-931989-931988-931991-931991-931992-931992-93

1991-93

MODELBlazerSuburban

C SeriesK SeriesR SeriesS SeriesT SeriesV Series

JimmySafariSierraSonomaSycloneTyphoonYukon

Bravada

MAKEChevroletGM Van

VANSYEAR1989-931990-93

MODELAstroG Series

Vehicles not listed may be tested for engine/transmission trouble codes only.

��

98

ONE (1) YEAR LIMITED WARRANTY

Actron Manufacturing Company (“Actron”) warrants to the original purchaser that this prod-uct will be free from defects in materials and workmanship for a period of one (1) year fromthe date of original purchase. Any unit that fails within this period will be replaced or repairedat Actron’s discretion without charge. If you need to return product, please follow the instruc-tions below. This warranty does not apply to damages (intentional or accidental), alterationsor improper or unreasonable use.

DISCLAIMER OF WARRANTYACTRON DISCLAIMS ALL EXPRESS WARRANTIES EXCEPT THOSE THAT APPEARABOVE. FURTHER, ACTRON DISCLAIMS ANY IMPLIED WARRANTY OF MERCHANT-ABILITY OF THE GOODS OR FITNESS OF THE GOODS FOR ANY PURPOSE. (TO THEEXTENT ALLOWED BY LAW, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR OFFITNESS APPLICABLE TO ANY PRODUCT IS SUBJECT TO ALL THE TERMS AND CON-DITIONS OF THIS LIMITED WARRANTY. SOME STATES DO NOT ALLOW LIMITATIONSON HOW LONG AN IMPLIED WARRANTY LASTS, SO THIS LIMITATION MAY NOT APPLYTO A SPECIFIC BUYER.)

LIMITATION OF REMEDIESIN NO CASE SHALL ACTRON BE LIABLE FOR ANY SPECIAL, INCIDENTAL OR CONSE-QUENTIAL DAMAGES BASED UPON ANY LEGAL THEORY INCLUDING, BUT NOT LIM-ITED TO, DAMAGES FOR LOST PROFITS AND/OR INJURY TO PROPERTY. SOMESTATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF INCIDENTAL OR CON-SEQUENTIAL DAMAGES, SO THIS LIMITATION OR EXCLUSION MAY NOT APPLY TO ASPECIFIC BUYER. THIS WARRANTY GIVES YOU SPECIFIC LEGAL RIGHTS, AND YOUMAY ALSO HAVE OTHER RIGHTS WHICH VARY FROM STATE TO STATE.

TO USE YOUR WARRANTYIf you need to return the unit, please follow this procedure:1. Call Actron Tech Support at (800) 253-9880. Our Technical Service representatives aretrained to assist you.2. Proof of purchase is required for all warranty claims. Please retain your sales receipt.3. In the event that product needs to be returned, you will be given a Return MaterialAuthorization number.4. If possible, return the product in its original package with cables and accessories.5. Print the RMA number and your return address on the outside of the package and send tothe address provided by your Customer Service representative.6. You will be responsible for shipping charges in the event that your repair is not covered bywarranty.

OUT OF WARRANTY REPAIRIf you need product repair after your warranty has expired, please call Tech Support at(800) 253-9880. You will be advised of the cost of repair and any freight charges.

All information, illustrations and specifications contained in this manual are based on the latest informationavailable from industry sources at the time of publication. No warranty (expressed or implied) can be madefor its accuracy or completeness, nor is any responsibility assumed by Actron or anyone connected with it forloss or damages suffered through reliance on any information contained in this manual or misuse ofaccompanying product. Actron reserves the right to make changes at any time to this manual oraccompanying product without obligation to notify any person or organization of such changes.

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