How to Know When a Car Relay Is Bad

12 volt relais, relay image by Sascha Zlatkov from Fotolia.com

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.

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