Solenoid valve is an electromechanical device that uses an electromagnetic coil to control the flow of fluid, typically transmission fluid, within the gearbox.

The primary function of a solenoid valve in a gearbox is to regulate the engagement and disengagement of the various gear clutches and bands. By opening and closing the valve, the solenoid controls the application of hydraulic pressure, which in turn selects the appropriate gear for the driving conditions.