Automatic Transmission Valves
At the heart of the automatic transmission's hydraulic control system is the valve body. It consists of several parts: the separator plate or transfer plate, the valves, and the valve body itself. A gasket and a separator plate separate the upper and lower halves. It's installed onto the transmission as a single unit and can be constructed of iron or on most transmissions aluminum.
The valve body contains different valves for different purposes: 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 acts as a port; it either blocks or allows 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. They are 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 a reaction area. It's the area where fluid pressure, linkage, or a spring acts as a force to activate the valve.