Automatic Transmission Service

Automatic transmission service can be easily divided into the following areas: preventive maintenance, troubleshooting, and major overhaul. Before you perform maintenance or repair on an automatic transmission, consult the maintenance manual for instructions and proper specifications. As a floor mechanic, however, your area of greatest concern is preventive maintenance. Preventive maintenance includes the following:

• Checking the transmission fluid daily.

• Adjusting the shifting and kickdown linkages.

• Adjusting lockup bands.

• Changing the transmission fluid and filter at recommended service intervals.

Checking the Fluid

The operator is responsible for first echelon (operator’s) maintenance. The operator should be trained not only to know to look for the proper fluid level but also to know how to look for discoloration of the fluid and debris on the dipstick. Fluid levels in automatic transmissions are almost always checked at operating temperature. This is important to know since the level of the fluid may vary as much as three quarters of an inch between hot and cold. The fluid should be either reddish or clear. The color varies due to the type of fluid. (For example, construction equipment using OE-10 will be clear). A burnt smell or brown coloration of the fluid is a sign of overheated oil from extra heavy use or slipping bands or clutch packs. The vehicle should be sent to the shop for further inspection.

CAUTION

Not all transmission fluids are the same. Before you add fluid, check the manufacturer’s recommendations first. The use of the wrong fluid will lead to early internal parts failure and costly overhaul. Overfilling the transmission can result in the fluid foaming and the fluid being driven out through the vent tube. The air that is trapped in the fluid is drawn into the hydraulic system by the pump and distributed to all parts of the transmission. This situation will cause air to be in the transmission in place of fluid and in turn cause slow application and burning of clutch plates and facings. Slippage occurs, heat results, and failure of the transmission follows.

Another possible, but remote, problem is water, indicated by the fluid having a "milky" appearance. A damaged fluid cooling tube in the radiator (automotive) or a damaged oil cooler (construction) could be the problem. The remedy is simple. Pressure-test the suspected components and perform any required repairs. After repairs have been performed, flush and refill the transmission with clean, fresh fluid.

Linkage and Band Adjustment

The types of linkages found on an automatic transmission are gearshift selection and throttle kickdown. The system can be a cable or a series of rods and levers. These systems do not normally present a problem, and preventive maintenance usually involves only a visual inspection and lubrication of the pivot points of linkages or the cable. When adjusting these linkages, you should strictly adhere to the manufacturer’s specifications.

If an automatic transmission is being used in severe service, the manufacturer may suggest periodic band adjustment. Always adjust lockup bands to the manufacturer’s specifications. Adjust the bands by loosening the locknut and tightening down the adjusting screw to a specified value. Back off the band adjusting screw with a specified number of turns and tighten down the locking nut. Not all bands are adjustable. Always check the manufacturer’s service manual before any servicing of the transmission.

Fluid Replacement

Perform fluid replacement according to the manufacturer’s recommendations. These recommendations vary considerably for different makes and models. Before you change automatic transmission fluid, always read the service manual first. Service intervals depend on the type of use the vehicle receives. In the NCF, because of the operating environment, more than a few of the vehicles are subjected to severe service. Severe service includes the following: hot and dusty conditions, constant stop and-go driving (taxi service), trailer towing, constant heavy hauling, and around-the clock operations (contingency). Any CESE operating in these conditions should have its automatic transmission fluid and filter changed on a regular schedule, based on the manufacturer's specifications for severe service. Ensure the vehicle is on level ground or a lift and let the oil drain into a proper catchment device. The draining of the transmission can be accomplished in one of the following three ways:

1. Removing the drain plug.

2. Loosening the dipstick tube.

3. Removing the oil pan.

CAUTION

Oil drained from automatic transmissions contains heavy metals and is considered hazardous waste and should be disposed of according to local instructions. Once the oil is drained, remove the pan completely for cleaning by paying close attention to any debris in the bottom of the pan. The presence of a high amount of metal particles may indicate serious internal problems. Clean the pan, and set it aside. All automatic transmissions have a filter or screen attached to the valve body. The screen is cleanable, whereas the filter is a disposable type and should always be replaced when removed. These are retained in different ways: retaining screws, metal retaining clamps, or O-rings made of neoprene. Clean the screen with solvent and use low-pressure air to blow-dry it. Do not use rags to wipe the screen dry, as they tend to leave lint behind that will be ingested into the hydraulic system of the transmission. If the screen is damaged or is abnormally hard to clean, replace it. Draining the oil from the pan of the transmission does not remove all of the oil—draining the oil from the torque converter completes the process. To do this, remove the torque converter cover and remove the drain plug, if so equipped. For a torque converter without a drain plug, special draining instructions may be found in the manufacturer’s service manual. Before performing this operation, clear it with your shop supervisor.

Refilling the Transmission Reinstall the transmission oil pan, the oil plug, and the fill tube. Fill the transmission with the fluid prescribed by the manufacturer to the proper level. With the brakes applied, start the engine and let it idle for a couple of minutes. Move the gear selector through all gear ranges several times, allowing the fluid to flow through the entire hydraulic system to release any trapped air. Return the selector lever to park or neutral and recheck the fluid level. Bring the fluid to the proper level. Run the vehicle until operating temperature is reached, checking for leaks. Also, recheck the fluid and adjust the level as necessary.