How Does a Starter Relay Work?

by Richard Rowe
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Like most other motorized or powerful electric devices on your car, the starter doesn't draw its power directly from the switch used to trigger it. Instead, the ignition switch triggers a sort of secondary switch, known as a relay or -- in the case of a starter -- the solenoid.

Starting System Basics

When you turn the ignition key on your car, the key closes a gap between the positive wire on your battery and a positive terminal on the starter solenoid, When the circuit closes on the solenoid, a switch inside of the solenoid closes and closes another circuit that links the battery and the electric motor on your starter. Hypothetically, you could just run a wire directly from the battery to the ignition switch and then to the motor, but that would require a lot of very heavy-gauge wire and a heavy-duty switch.

How a Solenoid Works

When electrical current from the switch enters the solenoid, it energizes a small electromagnet inside the solenoid. This electromagnet pulls on an metal arm attached to the battery's positive terminal against a metal plate attached to the starter terminal. In this way, the starter solenoid mimics the behavior of the ignition switch; you can think of the starter solenoid as a second ignition switch that sits in line between the battery and the starter.

Divorced vs Integrated

Modern starters generally use a starter solenoid integrated into the body of the starter itself, while older cars typically use a divorced solenoid mounted on the wheel well or firewall. The integrated solenoid and starter package is cheaper to produce and install because it reduced the number of connections in the electrical system and the machine processes necessary to install a separate solenoid mechanism in the engine bay. To locate your starter solenoid -- be it divorced or integrated -- simply follow the thick, red cable connected to your positive battery terminal.

Failure and Troubleshooting

A clicking starter solenoid usually indicates low battery power. The main starter circuit closing draws a great deal of power from the battery, which temporarily deprives the ignition switch of the amperage that it needs to trigger the solenoid. When the main circuit opens, power returns to the electromagnet, the circuit closes with a sharp click and the cycle starts over again. In this scenario, divorced solenoids have a slight advantage over integrated solenoids, since a driver can simply jump the exposed battery and starter terminals on the solenoid with a screwdriver. This circumvents the whole on-off-on cycle, possibly allowing a driver to start the car when he might otherwise end up stranded. This may be technically possible with some integrated solenoids, but the integrated solenoid is by nature harder to get to.

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