The main function of transmission fluid is to provide the hydraulic (fluid) power used by an automatic transmission to perform its basic functions. All automatic transmission functions--gear changing, transmission engagement and disengagement, and downshifting--require the force provided by transmission fluid. Unlike manual transmissions, which use manually operated clutches, gear shifts, and linkages, an automatic transmission uses transmission fluid to operate those transmission components. Pressurized transmission fluid provides the physical force, or pressure, to perform automatic transmission functions.
As transmission fluid flows throughout an automatic transmission, it lubricates the transmission gears and bands and helps to keep transmission operating temperatures reduced. Just like an engine uses motor oil to lubricate and cool its inner parts, a transmission uses automatic transmission fluid to perform the same functions. Heat and friction are natural by-products of automatic transmission gear and band function; transmission fluid reduces heat and friction and ultimately increases automatic transmission life.
When a vehicle is not running and its automatic transmission is not in use, all the transmission fluid flows into the transmission pan, a square metal holding basin located directly underneath the car. The transmission pan is similar to a vehicle's oil pan. Both pans serve as nothing more than catch basins and reservoirs. When transmission fluid is added to a vehicle, the fluid is poured directly into the transmission dipstick, which funnels the fluid directly into the transmission pan. When a vehicle is started, the automatic transmission engages and all the stored automatic transmission fluid stored in the transmission pan begins to circulate throughout the entire transmission.
