Automatic transmissions is now in the very base of the automobile industry and is basically divided into two types: those used in front−engine, front−wheel drive (FF) vehicles and those used in front−engine, rear−wheel drive (FR) vehicles.Transmissions used in front−wheel drive vehicles are designed to be more compact than transmissions used in rear−wheel drive vehicles because they are mounted in the engine compartment. They are commonly referred to as a "transaxle." The differential is an integral part of the front−wheel drive transmission, whereas the differential for the rear−wheel drive transmission is mounted externally. The external differential is connected to the transmission by a driveshaft.The basic function and purpose for either front or rear drive automatics are the same. They share the same planetary gear train design which is used in all Toyota automatic transmissions and the majority of automatics in production today.

The automatic transmission is composed of three major components:
• Torque converter
• Planetary gear unit
• Hydraulic control unit
For a full understanding of the operation of the automatic transmission, it is important to understand the basic role of these components.The torque converter provides a means of power transfer from the engine to the input shaft of the transmission. It acts like an automatic clutch to engage engine torque to the transmission and also allows the engine to idle while the vehicle is standing still with the transmission in gear.The planetary gear unit provides multiple gear ratios in the forward direction and one in reverse. The design includes two simple planetary gear sets and a common sun gear. These ratios are provided by use of holding devices which hold members of the planetary set. These
holding devices can be multiplate clutches or brakes, brake bands or one−way clutches. The hydraulic control unit regulates hydraulic pressure and shift points based on vehicle speed and throttle position. It is made up of a highly precision housing and spool valves which are balanced between spring tension and hydraulic pressure. The spool valves in turn control hydraulic passages to holding devices and regulate pressure.