How to Wire an Automotive Relay

by K.K. Lowell
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Photos and graphics by KKLowell

Every circuit added to a modern car should be run through an automotive relay. The factory wiring in modern cars is light gauge, and is not designed for additional loads. A relay can be used to power a new circuit with very little added current draw on the original wiring. Using a relay can also shorten the distance that high-current wiring needs to run.

Step 1

Connect relay terminal 87 to the vehicle body or battery negative terminal. Use a piece of 18-gauge primary wire with a solder-less female spade connector on one end, and a solder-less ring terminal on the other.

Step 2

Connect relay terminal 85 to an automotive circuit breaker. Connect the circuit breaker's other terminal to the battery's positive terminal. Use primary wire that's large enough to handle the current load of the new device.

Step 3

Connect relay terminal 86 to the device that needs power. Use the same gauge wire as that used for terminal 85.

Step 4

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Run an 18-gauge primary wire to a switch in the driver's area. Power the switch from a fused circuit inside the vehicle. Connect the wire to relay terminal 30 with a solder-less connector.

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