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How to Know When a Car Relay Is Bad

Updated February 21, 2017

Whenever you operate a switch to an accessory in your car that carries a lot of current--such as the lights, wipers or horn--you run a small amount of current through a relay which, in turn, closes contacts to operate the accessory. If one of your accessories doesn't come on and you suspect the relay is bad, you can perform a few simple tests. Look for the location of your relay in your owner's manual or in the fuse panel inside the car or under the bonnet.

Open the bonnet and/or fuse compartment so that your assistant can access the relay.

Turn the key to the "Accessories" position and instruct your friend to touch the relay while you operate the switch for the accessory that isn't working. Your friend should feel a click in the relay when it operates. If not, you have a bad relay.

Pull the relay out of its socket with the ignition off and insert a short piece of wire into the two connections of the relay that make contact when the electromagnetic coil pulls them together. Look on the relay for a diagram of the pins. They should be the ones labelled "Common" (C) and "Normally Open" (NO). On a standard Bosch automotive relay, the pins are 30 and 87.

Operate the switch again to see if the accessory works. If it now works, the relay is bad.

Tip

Most car relays pull out of a socket. Sometimes the relay vibrates out or works loose. Try pulling it out and pushing it back in to fix the problem. On a standard Bosch relay there is also a pin labelled 87a. This is not the pin you want. Make sure it's labelled 87.

Warning

If you can't find a diagram for your relay, don't guess on the pins with your piece of wire. You could short across the coil windings and blow a fuse when you operate the switch. Also, don't have your friend work under the bonnet with the engine running.

Things You'll Need

  • Short piece of 14- to 20-gauge wire, stripped at both ends
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About the Author

Richard Asmus was a writer and producer of television commercials in Phoenix, Arizona, and now is retired in Peru. After founding a small telecommunications engineering corporation and visiting 37 countries, Asmus studied broadcasting at Arizona State University and earned his Master of Fine Arts at Brooklyn College in New York.