Rebuilding an automatic transmission involves removing the transmission from the vehicle and servicing each part of the transmission as necessary. Some parts of an automatic transmission often only need cleaning during a rebuild, but others may require more intricate maintenance or replacement.
A transmission rebuild requires significant mechanical knowledge and a number of tools. Those without the necessary gear or skills should take the vehicle to a certified mechanic.
Major parts of an automatic transmission serviced during a rebuild include the torque converter, transmission oil pan, valve body, front pump and clutches. While specific procedures vary from model to model, in most cases the transmission assembly itself must be removed first, followed by removing the torque converter, oil pan and valve body from the transmission itself. The pumps, servos and clutches are also removed. The mechanic should have a diagram of the transmission mechanism handy to assist in parts recognition and proper reassembly while performing a transmission rebuild.
Many parts of an automatic transmission, such as the valve body and planetary gears, only require a thorough cleaning during a typical automatic transmission rebuild, except in cases where those parts are heavily damaged. Other mechanisms, such as the torque converter, require replacement more frequently depending on their condition. Transmission fluid, lubricants, seals and O-rings are almost always replaced in a rebuild.