Editing Automatic transmission (section)

====Hydraulic controls====
The aforementioned friction bands and clutches are controlled using [[automatic transmission fluid]] (ATF), which is pressurized by a pump and then directed to the appropriate bands/clutches to obtain the required gear ratio.<ref name="awnc"/> The ATF provides lubrication, corrosion prevention, and a hydraulic medium to transmit the power required to operate the transmission. Made from petroleum with various refinements and additives, ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.

The ''main pump'' which pressurises the ATF is typically a gear pump mounted between the torque converter and the planetary gear set. The input for the main pump is connected to the torque converter housing, which in turn is bolted to the engine's flexplate, so the pump provides pressure whenever the engine is running. A disadvantage of this arrangement is that there is no oil pressure to operate the transmission when the engine is not running, therefore it is not possible to [[push start]] a vehicle equipped with an automatic transmission with no rear pump (aside from several automatics built prior to 1970, which also included a rear pump for towing and push-starting purposes). The pressure of the ATF is regulated by a ''governor'' connected to the output shaft, which varies the pressure depending on the vehicle speed.

The ''valve body'' inside the transmission is responsible for directing hydraulic pressure to the appropriate bands and clutches. It receives pressurized fluid from the main pump and consists of several spring-loaded valves, check balls, and [[servomechanism|servo]] pistons. In older automatic transmissions, the valves use the pump pressure and the pressure from a [[centrifugal governor]] on the output side (as well as other inputs, such as throttle position or the driver locking out the higher gears) to control which ratio is selected. As the vehicle and engine change speed, the difference between the pressures changes, causing different sets of valves to open and close. In more recent automatic transmissions, the valves are controlled by [[solenoids]]. These solenoids are computer-controlled, with the gear selection decided by a dedicated [[transmission control unit]] (TCU) or sometimes this function is integrated into the [[engine control unit]] (ECU). Modern designs have replaced the centrifugal governor with an electronic speed sensor that is used as an input to the TCU or ECU. Modern transmissions also factor in the amount of load on an engine at any given time, which is determined from either the [[throttle]] position or the amount of intake manifold vacuum.

The multitude of parts, along with the complex design of the valve body, originally made hydraulic automatic transmissions much more expensive and time-consuming to build and repair than manual transmissions; however mass-production and developments over time have reduced this cost gap.