Automatic Transmission Valves
At the heart of the automatic transmission's hydraulic control system is the valve body. It usually consists of several parts: the separator plate or transfer plate, the valves and the valve body itself. The typical valve body consists of two parts; the upper and lower halves separated by a gasket and a separator plate. It’s installed onto the transmission as a single unit and can be constructed of iron or on most transmissions aluminum.
Inside the valve body you will find different types of valves: the poppet, the check ball, and the spool valve.
The poppet valve can be a ball or a flat disc. It typically has a stem that acts as a guide for the valve’s opening or closing. This valve, like so many others, acts as a port to either block or allow the flow of fluid.
A check ball valve sits in a seat in the valve body. It allows or blocks fluid flow according to the direction of flow. These check balls can be either normally open or closed depending on the application and manufacturer.
The spool valve is the most common valve found in automatic transmissions. They resemble a spool of thread with large areas used to block fluid flow called lands and stemmed areas called valleys. At each end of the valve is located a reaction area. This is the area where fluid pressure, linkage, or a spring acts as a force to activate the valve.