An automatic transmission valve body is a major component of a vehicle's automatic transmission system and directs pressurized transmission fluid to a network of valves that engage appropriate clutch packs for smooth gear shifting. These valves use pressure as hydraulic signals to determine which gear set, also called band servo, to select when the vehicle changes speed.
The valves inside the transmission valve body receive pressurized hydraulic fluid from the hydraulic pump, called the governor, to drive clutch and brake actuators for optimal band servo ratio. The most important of these valves is the manual valve. It is directly connected to the gear stick handle and instructs which passages should allow hydraulic fluid to pass through.
Modern automatic transmission systems incorporate sensors that monitor car speed, throttle position, brake pedal position and engine load to control how soft or firm the gear shift must be. The sensors send vital data to the onboard engine management computer, which directs electrically controlled solenoids that redirect hydraulic fluid to the correct clutch pack.
The incorporation of these sensors has made self-diagnostics possible on modern transmission systems. A malfunctioning transmission part is detected early by the engine management computer which, in turn, alerts drivers by activating a warning light on the vehicle's dashboard.