Starter solenoid

A starter solenoid (or starter relay) is the part of an automobile which switches a large electric current to the starter motor, in response to a small control current, and which in turn sets the engine in motion. Its function is thus identical to that of a transistor, but using an electromagnetic solenoid rather than semiconductor to perform the switching. In many vehicles the solenoid also engages the starter pinion with the ring gear of the engine.