Chapter 19 Automatic Transmission Maintenance Objectives 1 of

Chapter 19 Automatic Transmission Maintenance

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.

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.

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.

Low Oil Level • Air enters the system – When the transmission oil level is low, oil will not completely cover the oil filter. This pulls air into the pump inlet along with oil that 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 of the hydraulic system. The effects can be: • Converter aeration • Irregular shifting • Overheating • Poor lubrication – Aeration of transmission oil alters its viscosity and changes its appearance to that of a thin, milky liquid.

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.

High Oil Level • Oil level should be below the rotating components. – At normal oil levels, the sump oil level should be slightly below the planetary gearsets. – When the oil level is maintained above the FULL mark on the dipstick, the oil level in the sump rises so that the planetary gears run in oil, a condition that can cause foaming and aeration. • Aerated transmission fluid results in: – Converter aeration – Irregular shifting – Overheating – Poor lubrication • If accidental overfilling occurs during servicing, the excess oil should be drained.

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 is sufficient oil in the transmission until normal operating temperature is reached. – The hot check is made when the transmission oil reaches normal operating temperature (160°F– 200°F) and is the more reliable of the two checks.

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 oil up into the tube as it is pulled from the tube. Again, the result will be an inaccurate reading.

Shop Talk • The REF FILL (COLD RUN) level is an approximate level and can vary with specific transmissions. • To ensure properating levels, a hot oil level check must be performed.

Allison Dipstick

Hot Check • The oil temperature should be between 160°F and 200°F to make this test. • With the engine at idle and the transmission in neutral, 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 of the 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 most transmissions (from the ADD line to the FULL line).

Hydraulic Fluid Recommendations • Automatic transmission manufacturers may recommend: – Dexron, Dexron III, or Type C-4 (ATD-approved SAE 10 W or SAE 30) • Off-highway – Type C-4 fluids are the only fluids usually approved for use in off-highway applications. • Above 86°F – Type C-4 SAE 30 is specified when the ambient temperature is consistently above 86°F. – Some but not all Dexron II fluids also qualify as type C-4 fluids. • C-4 compatibility – Check that the materials used in auxiliary equipment such as tubes, hoses, external filters, and seals are C-4 -compatible. • Tran. Synd – Allison currently recommends the use of Tran. Synd synthetic oil in all their transmissions. Tran. Synd can extend oil drain intervals by three times.

Cold Startup • Don’t operate cold. – The transmission should not be operated in forward or reverse gears if the transmission oil falls below a certain temperature. – Minimum operating temperatures for recommended fluids are as follows: • • Tran. Synd -10°F Dexron (I, II or III) -10°F Type C-4 SAE 10 W 10°F Type C-4 SAE 30 32°F • Preheat – When the ambient temperature is below the minimum fluid temperatures listed, preheat is required.

Caution • Containers or transfer devices that have been used for engine coolant solutions must not be used for transmission fluid. • Antifreeze contains ethylene or propylene glycol, which, if introduced into a transmission, can cause the clutch plate to fail.

Governor Filter Change • Allison recommend that the governor filter be inspected or replaced at every oil/filter change. • A pipe plug can be used to retain the governor oil screen in older model Allison transmissions, as shown. • If it is undamaged, clean it in mineral spirits and reinstall it. If it is damaged, replace it. Install the filter open end first into the transmission cover and reinstall the pipe plug.

Metal Particles • Metal particles in the oil or on the magnetic drain plug (except for those minute particles normally trapped in the oil filter) may indicate transmission damage. • When larger metallic particles are found in the sump, the transmission should be disassembled and inspected to locate the source. • Beyond locating the cause, metal contamination requires a complete disassembly of the transmission and cleaning of all internal and external circuits, cooler, and all other areas where the particles could lodge.

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 in some transmission oil can produce a positive reading. • In the event of questionable test results, therefore, use a lab analysis of the oil.

Auxiliary Filters • After a transmission failure – An auxiliary filter may be installed in the cooler return line after debris or dirt has been introduced into the oil system because of failure. • Avoid a repeat failure – The auxiliary filter, which should be installed before the vehicle is placed back in service, prevents debris from 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 oil change intervals thereafter.

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.

External Lines and Oil Cooler Inspection • Look for leaks – Inspect all lines for loose or leaking connections, worn or damaged hoses or tubing, and loose fasteners. • Oil in the coolant – Examine the radiator coolant for traces of transmission oil. This condition may indicate a defective heat exchanger. • High operating temperatures – Extended operation at high operating temperatures can cause clogging of the oil cooler and can lead to transmission failure. • Oil cooler – The oil cooler system should be thoroughly cleaned after any rebuild work is performed on the transmission.

Transmission Oil Myths (1 of 3) • Myth 1–Oils are interchangeable. – Additive packages in lubricants made by different companies vary. – Two oils with the same viscosity and basic properties can have additives packages that can conflict and result in breakdown. – Some synthetic oil additives are incompatible with mineral base stocks; therefore, with Allison, check if you are swapping from one to the other.

Transmission Oil Myths (2 of 3) • Myth 2–Oil 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 65°C and, for each 10°C rise above that temperature, oil life is reduced by half.

Transmission Oil Myths (3 of 3) • Myth 3–Snake oil additives improve lubrication. – Major OEMs have extensive research and engineering capabilities when proofing lubricants and precisely balance the additive package to maximize operational life. – Tampering with the additive package by dumping in additives of unknown chemistry unbalances the oil and can result in premature failure.

Caution • Stall tests are usually specified for 15 seconds. • Full stall should never be run for a period exceeding 30 seconds at any one time because of the rapid rise in oil temperature. • Converter-out temperatures should not exceed 300°F. • If the stall test has to be repeated, allow for a cool-down period. Monitor the engine temperature as well.

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 be chocked as an extra precaution. It also makes sense to chain the vehicle to the floor anchors. No one should stand either in front of or behind the vehicle during the test.

Stall Test Results • Because of operating condition variables, stall speed deviations of up to 150 rpm from specification can be viewed as within normal range. – Over 150 rpm low = Engine stall speed is more than 150 rpm below the OEM specification: indicates an engine problem. – Over 150 rpm high = Engine stall speed is more than 150 rpm above the OEM specification: indicates a transmission-based problem. – Extremely low = An extremely low stall speed, such as 30% of the specified engine stall rpm, with no engine tattletales such as smoking, could indicate a freewheeling torque converter stator. – Normal but too hot = If the engine stall speed conforms to specification but the transmission oil overheats, perform the cool-down check. If the oil does not cool down during the 2 minute cool-down check, a stuck torque converter stator could be indicated.

Shift Point Engine Speeds for Select Transmissions • See Table 19 -1 on page 568 of the textbook.

Adjusting Shift Points (1 of 2) • Shift speeds can be altered by changing the positions of the adjusting rings. • The adjusting ring is held in the shift signal valve bore by a pin that is press-fit through the valve body housing. • Adjustment of the valves is performed using a valve ring adjusting tool. • When the ring is depressed by the adjusting tool, the slots on the ring that engage the pin are released. • The adjusting ring can then be turned to adjust spring pressure.

Adjusting Shift Points (2 of 2) • Clockwise rotation increases spring pressure. • The slots on the adjusting ring that engage the pin are on a sloping ramp around the circumference. • This alters the spring tension as the slots reengage the pin in the new ring position. • Each notch in the adjustment ring will alter the shift point by an incremental rpm value. – For example, one notch might be equal to 25, 35, 40, or 50 rpm of engine speed, based on the particular model transmission.

Speedometer Method • Begin the test by checking the top speed the vehicle can achieve in each gear before a shift occurs. – Some electronically managed engines may have programming that makes these values soft. • When the top vehicle speed has been recorded for each gear, accelerate the vehicle at full throttle from a standing start and note the speed at which each upshift occurs. • Compare the upshift rpms with the selected shift speeds recorded in the first part of the test drive. • The same general adjustment principles apply as in rpm adjustment.

Test Stand Calibration • The valve body is bolted onto a manifold that resembles the lower portion of the transmission housing. • This manifold is drilled and tapped to mate to all the ports and hoses required to route oil properly through the valve body for testing. • The test stand can now be used to check five principal valve body functions: – – – Governor pressure Modulator pressure Hold regulator pressure Shift points (up-down/inhibit) Trimmer regulator operation

Shift Points without Modulated or Adjustable Lockup • See Table 19 -2 on page 571 of the textbook.

Automatic Transmission Troubleshooting • See Table 19 -4 on pages 572 -573 in the textbook.

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 the transmission breather. • Inspect the transmission for loose bolts, loose or leaking oil lines, oil leakage, and the condition of the control linkage and cables.

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 magnetic drain plug (except for the minute particles normally trapped in the oil filter) indicate that damage has occurred in the transmission.

Summary (3 of 4) • If engine coolant leaks into the transmission oil system, immediate action should be taken to prevent serious damage. Antifreeze destroys the clutch materials used in automatic transmissions. • There are three methods of testing shift points in automatic transmissions. Adjusting shift points is part of routine preventive maintenance.

Summary (4 of 4) • Truck technicians should be able to undertake first-level troubleshooting of Allison transmissions using diagnostic charts. • During initial troubleshooting, the engine and transmission should be regarded with equal attention, as engine problems can often be misinterpreted as transmission problems, and vice versa.