Late-model automatics have electronic/hydraulic controls operated by the Powertrain Control Module (PCM) or its own Transmission Control Module (TCM). Most late-model automatics have five or six gears (speeds), though some have as many as eight. Some automatics are Continuously Variable Transmissions (CVTs) that vary gear ratios depending on speed and load.
The most commonly sold product for automatic transmissions is ATF (Automatic Transmission Fluid). Most transmissions do not have a specified service interval. However, fluid changes may be recommended for preventive maintenance and to prolong the life of the transmission.
Different transmissions require different types of ATF. Using the wrong fluid (one that does not meet OEM specifications) may cause shift problems or damage the transmission. Refer to the vehicle owners manual, transmission dipstick or service literature to determine the correct type of ATF for the application.
ATF can be changed with a fluid exchange machine (best method) or by draining the transmission (which can leave several quarts of old ATF inside the torque converter). Service may also include replacing the ATF filter inside the transmission oil pan. A filter change will usually require a new pan gasket unless the filter is mounted on the outside of the transmission.
All vehicles have a transmission oil cooler (often a metal tube inside the radiator). Adding an aftermarket auxiliary ATF cooler is recommended for vehicles used for towing.
Transmission failure is often due to AFT breakdown or loss, and usually requires replacing the entire transmission with a remanufactured unit unless the problem is a defective solenoid valve or sensor, which can be replaced separately. Solenoid valves in the transmission valve body control shifting, while magnetic sensors monitor the speeds of the transmission input and output shafts and vehicle speed. The PCM or a separate Transmission Control Module (TCM) controls the overall operation of the transmission.
Troubleshooting automatic transmissions requires a scan tool for reading fault codes, sensor data and system functions.
The torque converter is a fluid coupling between the engine and transmission. Most also have a lockup clutch that engages above a certain speed to eliminate slippage for improved fuel economy. A torque converter failure can prevent the vehicle from accelerating normally. Replacing the torque converter requires separating the transmission or transaxle from the engine, and draining and refilling the transmission with new ATF.