Electrical Relay
An electromagnetic switch that uses a small electrical current to control a much larger current in another circuit.
An electrical relay is an electromagnetic switch used throughout a vehicle's electrical system to allow a low-current control circuit to turn a high-current operating circuit on and off. This design allows delicate switches on the dashboard or steering column, as well as sensitive electronic modules (like the ECU), to control high-draw components—such as the starter solenoid, radiator cooling fans, fuel pump, and headlights—without running heavy-gauge wiring directly to the passenger cabin.
Inside a standard automotive relay, there is a small electromagnet coil and a set of spring-loaded contact points. When a low-current signal is sent to the relay coil, it creates a magnetic field that pulls the contact points together, closing the high-current circuit and allowing power to flow to the accessory. Relays are typically housed in the vehicle's fuse boxes and are easily pluggable for quick replacement.
Relays can fail in two ways: the internal contacts can become burned and pitted over time, sticking in the closed position (causing the accessory, like a fuel pump or cooling fan, to run continuously even when the key is off), or the coil can burn out, leaving the contacts open (meaning the accessory will not turn on at all).
Symptoms of a bad relay include the failure of a major component to operate, combined with a lack of the characteristic clicking sound that a functioning relay makes when activated. Mechanics test relays using a multimeter to check the resistance of the control coil and verify continuity across the switch contacts when energized.