With manual transmission vehicles you work the clutch yourself to shift gears. With an automatic transmission system, shifting is controlled by a complicated hydraulic system, which uses control solenoids to direct the transmission fluid.
Automatic transmissions use clutch packs to engage and disengage different gear sets. The flow of transmission fluids to different clutch packs determines which gears will engage.
Control solenoids are cylindrical devices that can be opened or closed to regulate the flow of fluid through them. When specific control solenoids are opened, they allow transmission fluid to flow to specific clutch packs, which changes the gears engaged with the engine.
The opening and closing of control solenoids is controlled by the computer on the solenoid pack, the unit on which all the control solenoids are mounted. By directing certain control solenoids to open and others to close, the computer directs the flow of transmission fluid. This translates into controlling which clutch packs will be engaged.
