Chapter 19 Bennet

8/10/2019 Chapter 19 Bennet

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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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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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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.

S


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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.

S


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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.

Chapter 19 Bennet

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