Function of a Starter Solenoid

The starter is an electric motor that, with the aid of the battery and the starter solenoid, turns the engine once the ignition is engaged. If the starter motor is not signaled to activate by the starter solenoid -- if the solenoid does not switch on the starter motor -- the engine will not start.

The starter solenoid serves primarily as an electric switch for the starter motor. When the ignition is engaged, the solenoid completes the circuit between the vehicle's battery and the starter motor. At the same time, an electrically activated lever inside the solenoid engages a drive pinion and clutch assembly inside of the starter motor. With the starter motor activated and its clutch moved into the start position, a large amount of energy and torque is produced within the starter motor to turn a flywheel that cranks the engine. Once the engine has started, the solenoid switches off the starter motor.

Samarins.com notes that problems with starter solenoids are actually quite rare and that if there is a problem with a vehicle's starting system, it is more than likely related to the battery or the starter motor rather than the solenoid.