What is a solenoid?

Your vehicle’s starter motor has the important job of starting the engine. But something also has to start the starter. And that something is the solenoid. In most automotive applications today, the solenoid is attached to the starter, with the two of them getting removed as a unit when necessary.

How a solenoid works

When running properly, and as long as it has a supply of fuel, the internal combustion engine continues to run by itself in an ongoing process from the inertia of the engine’s moving parts. But starting the engine is a separate process to get that inertia moving in the first place. This is the job of the starting system, whose main components include the:

  • Battery
  • Starter motor
  • Solenoid
  • Starter switch

Starting an engine: The first action

The process involves not one, but two separate electric currents – a stronger one and a weaker one. When activated, usually by turning a key in the ignition switch, the weaker current passes through the switch and to the solenoid. At that point, the current forces two large contacts to come together in the solenoid, which allows the stronger electric current to pass through the solenoid’s contacts. These contacts carry a current that requires heavy wiring cables directly from the battery. This current is heavy enough that it would be unwise to send it through a hand-operated switching mechanism. Hence, the need for the weaker current through the ignition switch.

The stronger current passes to the starter motor where it initiates two separate actions. The starter motor is designed so that the electric current activates a lever, forcing a small gear outward on a spring shaft. When extended, this gear, called a pinion, comes in contact with a toothed gear on the outer rim of a large flywheel on the end of the engine’s drive shaft. This large gear is called the starter ring gear.

Starting an engine: The second action

The second action in a direct current electric motor is the rotation of its central shaft, caused by the larger current passing through the motor. A motor transforms electric energy into the mechanical energy of the central shaft’s rotation. It does this because the electric current interacts with the magnetic field in the starter motor and results in the rotor on the shaft beginning to turn. By the time this turning action reaches the motor’s designed top speed, the pinion at the end of this shaft has already engaged the ring gear on the flywheel. The engine then starts running on its own and the starter’s safety features automatically disengage the pinion from the ring gear. The spring brings the pinion safely back to its resting position and the job of the starting mechanism is done.

When starting, if you hold the ignition key in the “start” position a little too long, you will encounter a problem. Here, too, is a spring that brings the switch back to the “on” position from “start” as soon as you release the key. If you fail to do so, you will hear the evidence of your mistake pretty quickly. The good news is that your mistake is not as bad as it sounds. The safety mechanism in the starter has already released the pinion from the ring gear. The bad news is if you do this often or for any extended period, you may drastically shorten the life of the starter motor.