Starter solenoid
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A starter solenoid (or starter relay) is the part of an automobile which relays a large electrical current to the starter motor, which in turn sets the engine in motion.
[edit] Operation
The starter solenoid receives a large electrical current from the car battery and a small electrical current from the ignition switch. When the ignition switch is turned on, a small electrical current is sent to the starter solenoid. This causes the starter solenoid to relay a large electrical current to the starter motor, which in turn sets the engine in motion. The starter motor is an electric motor that initiates piston motion in a car's internal combustion engine before it can power itself.
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, it pushes out the drive pinion on the starter driveshaft and meshes the pinion with the ring gear on the flywheel of the engine.
The starter solenoid also closes high-current contacts for the starter motor and it starts to run. Once the engine starts, the key-operated switch is opened, a spring in the solenoid assembly pulls the pinion gear away from the ring gear, and the starter motor stops. The starter's pinion is clutched to its driveshaft through an overrunning sprag clutch which permits the pinion to transmit drive in only one direction. In this manner, drive is transmitted through the pinion to the flywheel ring gear. If the pinion remains engaged, because the operator fails to release the key as soon as the engine starts, the pinion will spin independently of its driveshaft. This prevents the engine driving the starter, for such backdrive would cause the starter to spin so fast that it would be seriously damaged.
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 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.
[edit] Problems
If a starter solenoid receives insufficient power from the battery, it will fail to start the motor, and may produce a rapid clicking or clacking sound. The lack of power can be caused by a low or dead 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 simply make a clicking sound, rather than setting the starter motor in motion.
To reduce the likelihood of starter solenoid failure, the battery connections should be cleaned and tightened at every oil change. Starter solenoid problems can be diagnosed by a mechanic at a service center by a test of the car's starting, charging and battery systems.
