Starter solenoids are electrical components found in most cars, trucks, and other vehicles. The main purpose of a starter solenoid is to isolate an ignition switch from the high level of amperage necessary to operate a starter motor. When an ignition switch is turned in one of these systems, battery voltage is provided to a starter solenoid. That causes the solenoid to activate and bridge a contact between the starter and battery. Most starter motors have integrated solenoids, though in some cases the starter solenoid is a separate component.
Most internal combustion engines found in cars and trucks require an electric motor to get started. The purpose of this starter motor is to rotate the engine until the internal combustion process can take over. Since these electric motors often take several hundred amperes to operate, it is necessary to isolate the starter circuit from the ignition key. This is one of the primary uses of a starter solenoid.
A solenoid is an electrical component that consists of a metal slug with a coil wound around it. When the coil receives electricity, a magnetic field is generated and the core is forced to move. This action is used in a starter solenoid to act as a sort of relay. These solenoids are typically connected to battery voltage on one side and a starter motor on the other, and are activated by a switch. When the switch is turned, the solenoid engages and provides battery voltage directly to the starter.
When starter solenoids are installed directly on starter motors, they often perform another useful action. The linear movement of the solenoid core can be used to move the starter gear, causing it to engage the flywheel. If the starter solenoid is not installed directly on the motor, then some other method needs to be used to perform this action. In most cases that involves a separate solenoid, the starter gear shaft will have a helical groove. This causes the rotational movement of the starter shaft to force the gear into place instead of the linear motion of a solenoid.
In applications where solenoids are separate components, they are typically simple to replace. These solenoids are usually installed somewhere in the engine compartment, and typically have four electrical connections. Integrated starter solenoids are usually more difficult to reach, and when they fail it is common to replace or rebuild the entire starter. In both cases, it is necessary to disconnect the battery before attempting to work on the solenoid, since they have live connections that can easily short circuit during a repair operation.