How a Relay Works

A relay is nothing more than an electrically operated switch. Think of it this way — you have a light switch on the wall. Your finger is the control circuit. But that switch can only handle a certain amount of current before it burns up. A relay lets you use a small, lightweight control signal to switch a much heavier current load safely and reliably. The ignition switch triggers the relay. The relay does the heavy lifting.

Circuit 1 — the control circuit — goes through terminals 85 and 86. This is the electromagnet coil inside the relay. It draws very little current. Apply 12 volts to terminal 86 and a ground to terminal 85 and the electromagnet energizes. You hear a click. That click is the mechanical contacts snapping closed inside the relay.

Circuit 2 — the power circuit — goes through terminals 30 and 87. Terminal 30 is the high-current input from the battery. Terminal 87 is the output to the load. When the coil energizes and the contacts close, current flows from 30 through to 87 and powers the component. When the coil de-energizes, the contacts open and the load turns off.

85 and 86 — coil terminals, control circuit, low current. 30 — high current input, always connected to power. 87 — normally open output, connects to 30 when relay is energized. 87A — normally closed output on some relays, disconnects from 30 when energized.

Step 1 — Apply 12V to terminal 86 and a ground to terminal 85. You should hear a clear click as the contacts close. No click means the coil has failed or has no power or ground. Step 2 — With the coil energized, test continuity between terminals 30 and 87. Near zero ohms means the contacts closed correctly. Step 3 — Remove voltage from the coil. Continuity between 30 and 87 should now be open. A relay that clicks but has no continuity between 30 and 87 when energized has burned internal contacts. Replace it.