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Objectives (1 of 3)
Perform hot and cold transmission oil level
checks.
Identify the types of hydraulic fluid used in
truck automatic transmissions.
Change automatic transmission oil and filters.
Inspect transmission oil for signs of
contamination.
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Objectives (2 of 3)
Adjust the manual gear selector linkage,
mechanical modulator control linkage, and air
modulator control on a truck automatic
transmission. Perform a transmission stall test.
Perform engine speed and vehicle speed
shift point tests.
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Objectives (3 of 3)
Describe basic transmission test stand
procedure.
Test the transmission valve body.
Summarize some basic inspection and
troubleshooting procedures for automatic
transmissions.
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Low Oil Level
Air enters the system
When the transmission oil level is low, oil will not completelycover the oil filter. This pulls air into the pump inlet along with oilthat is then routed to the clutches and converter.
Aeration
The result of air in the hydraulic system is known as aeration.Because air is compressible, it severely affects the operation ofthe hydraulic system. The effects can be:
Converter aeration
Irregular shifting
Overheating
Poor lubrication
Aeration of transmission oil alters its viscosity and changes itsappearance to that of a thin, milky liquid.
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Shop Talk
It should be noted that a defective oil filler
tube seal ring will allow the oil pump to draw
air into the oil from the sump, which will result
in aeration of the oil.
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High Oil Level
Oil level should be below the rotating components.
At normal oil levels, the sump oil level should be slightly belowthe planetary gearsets.
When the oil level is maintained above the FULL mark on thedipstick, the oil level in the sump rises so that the planetary
gears run in oil, a condition that can cause foaming andaeration.
Aerated transmission fluid results in:
Converter aeration
Irregular shifting
Overheating
Poor lubrication
If accidental overfilling occurs during servicing, the excess oilshould be drained.
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Interpreting Oil Level Readings
Engine speed and oil temperature significantly
affect the oil level.
Both cold and hot level checks should be taken.
A cold level check is required to ensure there issufficient oil in the transmission until normal operating
temperature is reached.
The hot check is made when the transmission oil
reaches normal operating temperature (160
F200F) and is the more reliable of the two checks.
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Caution
You should check the transmission oil level at least
two times to ensure that an accurate reading is
made.
If the dipstick readings are inconsistent (some high,some low), check for proper venting of the
transmission breather or oil filler tube. A clogged
breather can force oil up into the filler tube and cause
inaccurate readings. If the filler tube is unvented, the
vacuum produced will cause the dipstick to draw oilup into the tube as it is pulled from the tube. Again,
the result will be an inaccurate reading.
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Shop Talk
The REF FILL (COLD RUN) level is an
approximate level and can vary with specific
transmissions.
To ensure proper operating levels, a hot oillevel check must be performed.
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Allison Dipstick
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Hot Check
The oil temperature should be between 160F and200F to make this test.
With the engine at idle and the transmission inneutral, wipe the dipstick clean and check the oil
level. If the oil level registers in the HOT RUN band(between ADD and FULL), the oil level is correct.
If the oil level registers on or below the bottom line ofthe HOT RUN band or the ADD line, add oil to bring
the level to the middle of the band. Note that one quart of oil will raise the level from the
bottom of the band to the top of the band in mosttransmissions (from the ADD line to the FULL line).
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Hydraulic Fluid Recommendations
Automatic transmissionmanufacturers mayrecommend: Dexron, Dexron II, Dexron III,
or Type C-4 (ATD-approvedSAE 10W or SAE 30)
Off-highway Type C-4 fluids are the only
fluids usually approved for usein off-highway applications.
Above 86F Type C-4 SAE 30 is specified
when the ambient temperatureis consistently above 86F.
Some but not all Dexron IIfluids also qualify as type C-4fluids.
C-4 compatibility Check that the materials used
in auxiliary equipment such astubes, hoses, external filters,and seals are C-4-compatible.
TranSynd
Allison currently recommendsthe use of TranSynd syntheticoil in all their transmissions.TranSynd can extend oil drainintervals by three times.
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Cold Startup
Dont operate cold. The transmission should not be operated in forward or
reverse gears if the transmission oil falls below acertain temperature.
Minimum operating temperatures for recommendedfluids are as follows: TranSynd -10F
Dexron (I, II or III) -10F
Type C-4 SAE 10W 10F
Type C-4 SAE 30 32
F Preheat
When the ambient temperature is below the minimumfluid temperatures listed, preheat is required.
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Caution
Containers or transfer devices that have
been used for engine coolant solutions must
not be used for transmission fluid.
Antifreeze contains ethylene or propyleneglycol, which, if introduced into a
transmission, can cause the clutch plate to
fail.
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Governor Filter Change
Allison recommend that thegovernor filter be inspectedor replaced at every oil/filterchange.
A pipe plug can be used to
retain the governor oilscreen in older modelAllison transmissions, asshown.
If it is undamaged, clean it inmineral spirits and reinstall
it. If it is damaged, replaceit. Install the filter open endfirst into the transmissioncover and reinstall the pipeplug.
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Metal Particles
Metal particles in the oil or on the magnetic drainplug (except for those minute particles normallytrapped in the oil filter) may indicate transmissiondamage.
When larger metallic particles are found in thesump, the transmission should be disassembledand inspected to locate the source.
Beyond locating the cause, metal contaminationrequires a complete disassembly of the
transmission and cleaning of all internal andexternal circuits, cooler, and all other areas wherethe particles could lodge.
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Coolant Leakage
Engine coolant damages transmissions.
Remove all traces.
The transmission should be disassembled, inspected,
and cleaned. Test kits
Test kits can be used to detect traces of glycol in the
transmission oil.
You should note, however, that certain additives insome transmission oil can produce a positive reading.
In the event of questionable test results, therefore, use a
lab analysis of the oil.
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Auxiliary Filters
After a transmission failure
An auxiliary filter may be installed in the cooler returnline after debris or dirt has been introduced into the oilsystem because of failure.
Avoid a repeat failure The auxiliary filter, which should be installed before
the vehicle is placed back in service, prevents debrisfrom being circulated into the transmission and
causing a repeat failure. Most auxiliary oil filters are changed 5,000 miles
after their initial installation and at regular oilchange intervals thereafter.
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High-efficiency Filters
They are available for older transmissions.
They have no mileage limitations, but when used on
older transmissions, they should be changed when
they become clogged or at three-year intervals,whichever occurs first.
They have a differential pressure switch that
monitors pressure drop across the filter. It triggers a
dash-mounted warning light. Allison does not recommend the use of extended
service high-efficiency filters on any WT
transmissions.
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External Lines and
Oil Cooler Inspection Look for leaks
Inspect all lines for loose or leaking connections, worn ordamaged hoses or tubing, and loose fasteners.
Oil in the coolant
Examine the radiator coolant for traces of transmission oil. Thiscondition may indicate a defective heat exchanger.
High operating temperatures
Extended operation at high operating temperatures can causeclogging of the oil cooler and can lead to transmission failure.
Oil cooler
The oil cooler system should be thoroughly cleaned after anyrebuild work is performed on the transmission.
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Transmission Oil Myths (1 of 3)
Myth 1Oils are interchangeable.
Additive packages in lubricants made bydifferent companies vary.
Two oils with the same viscosity and basicproperties can have additives packages thatcan conflict and result in breakdown.
Some synthetic oil additives are incompatible
with mineral base stocks; therefore, withAllison, check if you are swapping from one tothe other.
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Transmission Oil Myths (2 of 3)
Myth 2Oil never wears out.
Oxidation limits the life of transmission oils,
and the extent to which the oil is oxidized
depends on running temperatures. Oil begins to oxidize at 65C and, for each
10C rise above that temperature, oil life is
reduced by half.
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Transmission Oil Myths (3 of 3)
Myth 3Snake oil additives improvelubrication.
Major OEMs have extensive research and
engineering capabilities when proofinglubricants and precisely balance the additivepackage to maximize operational life.
Tampering with the additive package by
dumping in additives of unknown chemistryunbalances the oil and can result in prematurefailure.
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Caution
Stall tests are usually specified for 15seconds.
Full stall should never be run for a period
exceeding 30 seconds at any one timebecause of the rapid rise in oil temperature.
Converter-out temperatures should notexceed 300F.
If the stall test has to be repeated, allow for acool-down period. Monitor the enginetemperature as well.
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Warning
To perform a transmission stall test, both the
vehicle parking and service brakes should be
properly adjusted and fully applied.
The wheels over at least two axles must bechocked as an extra precaution. It also
makes sense to chain the vehicle to the floor
anchors. No one should stand either in frontof or behind the vehicle during the test.
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Stall Test Results
Because of operating condition variables, stall speed deviationsof up to 150 rpm from specification can be viewed as withinnormal range.
Over 150 rpm low = Engine stall speed is more than 150 rpmbelow the OEM specification: indicates an engine problem.
Over 150 rpm high = Engine stall speed is more than 150 rpmabove the OEM specification: indicates a transmission-basedproblem.
Extremely low = An extremely low stall speed, such as 30% ofthe specified engine stall rpm, with no engine tattletales such assmoking, could indicate a freewheeling torque converter stator.
Normal but too hot = If the engine stall speed conforms tospecification but the transmission oil overheats, perform thecool-down check. If the oil does not cool down during the 2-minute cool-down check, a stuck torque converter stator couldbe indicated.
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Shift Point Engine
Speeds for Select Transmissions
See Table 19-1 on page 568 of the textbook.
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Adjusting Shift Points (1 of 2)
Shift speeds can be altered bychanging the positions of theadjusting rings.
The adjusting ring is held inthe shift signal valve bore by apin that is press-fit through the
valve body housing. Adjustment of the valves is
performed using a valve ringadjusting tool.
When the ring is depressed bythe adjusting tool, the slots on
the ring that engage the pinare released.
The adjusting ring can then beturned to adjust springpressure.
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Adjusting Shift Points (2 of 2)
Clockwise rotation increasesspring pressure.
The slots on the adjusting ringthat engage the pin are on asloping ramp around thecircumference.
This alters the spring tensionas the slots reengage the pinin the new ring position.
Each notch in the adjustmentring will alter the shift point byan incremental rpm value.
For example, one notch mightbe equal to 25, 35, 40, or 50rpm of engine speed, basedon the particular modeltransmission.
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Speedometer Method
Begin the test by checking the top speed thevehicle can achieve in each gear before a shiftoccurs. Some electronically managed engines may have
programming that makes these values soft.
When the top vehicle speed has been recorded foreach gear, accelerate the vehicle at full throttle froma standing start and note the speed at which eachupshift occurs.
Compare the upshift rpms with the selected shiftspeeds recorded in the first part of the test drive.
The same general adjustment principles apply as inrpm adjustment.
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Test Stand Calibration
The valve body is bolted onto
a manifold that resembles thelower portion of thetransmission housing.
This manifold is drilled andtapped to mate to all the ports
and hoses required to route oilproperly through the valvebody for testing.
The test stand can now beused to check five principalvalve body functions:
Governor pressure Modulator pressure
Hold regulator pressure
Shift points (up-down/inhibit)
Trimmer regulator operation
Shift P i t ith t M d l t d
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Shift Points without Modulated
or Adjustable Lockup
See Table 19-2 on page 571 of the textbook.
Automatic Transmission
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Automatic Transmission
Troubleshooting
See Table 19-4 on pages 572-573 in the
textbook.
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Summary (1 of 4)
Automatic transmissions should be cleaned
with a power washer to make inspection and
servicing easier. Special care should be
taken to avoid forcing water through thetransmission breather.
Inspect the transmission for loose bolts,
loose or leaking oil lines, oil leakage, and the
condition of the control linkage and cables.
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Summary (2 of 4)
Maintaining the specified oil level in an
automatic transmission is important, as either
low or high oil levels can cause aeration of
the transmission fluid. At each oil change, examine the oil that is
drained for evidence of dirt or water.
Metal particles in the oil or on the magneticdrain plug (except for the minute particles
normally trapped in the oil filter) indicate that
damage has occurred in the transmission.
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Summary (3 of 4)
If engine coolant leaks into the transmissionoil system, immediate action should be taken
to prevent serious damage. Antifreeze
destroys the clutch materials used inautomatic transmissions.
There are three methods of testing shift
points in automatic transmissions. Adjusting
shift points is part of routine preventive
maintenance.
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Summary (4 of 4)
Truck technicians should be able toundertake first-level troubleshooting of Allison
transmissions using diagnostic charts.
During initial troubleshooting, the engine andtransmission should be regarded with equal
attention, as engine problems can often be
misinterpreted as transmission problems, and
vice versa.