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
The operator is responsible for first echelon (operator's) maintenance. The operator should not only be trained to know to look for the proper fluid level but also 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.
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.
The types of linkages found on an automatic transmission are gearshift selection and throttle kick-down. The system can be a cable or a series of rod 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. Lockup bands are always adjusted to the manufacturer's specifications. Bands are adjusted by loosening the locknut and tightening down the adjusting screw to a specified value. The band adjusting screw is backed off a specified amount of turns and the locking nut tightened down. NOT ALL BANDS ARE ADJUSTABLE. Always check the manufacturer's service manual before any servicing of the transmission.
Fluid replacement is to be performed according to the manufacturer's recommendations. These recommendations vary considerably for different makes and models. When 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 conditionsContinue Reading