A valve body assembly, or simply a valve body, is a mechanism in an automatic transmission that serves as a control center for the transmission. It operates by sending fluid through a maze-like structure which in turn operates the appropriate mechanisms to smoothly shift into the appropriate gear.
A valve body assembly is, at first sight, an intricate piece of equipment which features a number of channels and valves through which hydraulic fluid flows. The fluid activates valves which in turn activates whatever clutch pack or band servo that is required for the situation.
In automatic transmissions, the valve body assembly has a specific valve for each purpose within the assembly. For example, in order for a car to shift from second to third gear, the hydraulic fluid must flow through the maze-like structure to activate the specific value dedicated to that function, which would often be called the 2-3 valve. Similarly, the valve dedicated to shifting from third to second gear would be called the 3-2 valve.
The only valve in the valve body assembly that the driver or operator has direct control over is the manual valve. This valve is connected directly to the gear shift handle and covers or uncovers valves based on the handle's position. It also plays a central role in monitoring vehicle speed to determine the optimal force with which to shift from one gear to the next.