A starter solenoid (or starter relay) is the part of an automobile which relays a large electric current to the starter motor, which in turn sets the engine in motion. In many vehicles the solenoid also engages the starter pinion with the ring gear of the engine.
The starter solenoid receives a large electric current from the car battery and a small electric current from the ignition switch. When the ignition switch is turned on, a small electric current is sent to the starter solenoid. This causes the starter solenoid to close a pair of heavy contacts, thus relaying a large electric current to the starter motor, which in turn sets the engine in motion.[1]
The starter motor is a series-wound direct current electric motor with a solenoid switch (similar to a relay) mounted on it. When low-current power from the starting battery is applied to the starter solenoid, usually through a key-operated switch, the solenoid closes high-current contacts for the starter motor and it starts to run. Once the engine starts, the key-operated switch is opened and the solenoid opens the contacts to the starter motor.
Most modern starters also rely on the solenoid to engage the starter pinion with the ring gear of the flywheel. When the solenoid is energized, it operates a plunger or lever which forces the pinion into mesh with the ring gear. The pinion incorporates a mechanism such that when the engine starts and runs faster than the starter motor the pinion is forced to unmesh.
Some older starter designs, such as the Bendix drive, used the rotational inertia of the pinion to force it along a helical groove cut into the starter driveshaft, and thus no mechanical linkage with the solenoid was required.[2]
The starter solenoid can be located under the hood of a car by following the positive (red) cable from the battery, which usually leads directly to the solenoid. Then, the solenoid will have another cable of similar or equal weight which will go down to the starter, which is normally accessed from the bottom of the vehicle. The solenoid will also have a third (smaller) wire, which comes in from the starter switch. Starter solenoids can also be built into the starter itself, often visible on the outside of the starter.
If a starter solenoid receives insufficient power from the battery, it will fail to start the motor, and may produce a rapid clicking sound. The lack of power can be caused by a low battery, by corroded or loose connections in the battery cable, or by a damaged positive (red) cable from the battery. Any of these problems will result in some, but not enough, power being sent to the solenoid, which means that the solenoid will only begin to push the engagement gear, making the metallic click sound. Starter solenoid problems are best diagnosed by an experienced auto-electrician.