How to Know When a Car Relay Is Bad

Written by richard asmus
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How to Know When a Car Relay Is Bad
Your car relay may look like this. (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.

Skill level:
Easy

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Things you need

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

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Instructions

  1. 1

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

  2. 2

    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.

  3. 3

    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.

  4. 4

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

Tips and warnings

  • 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.
  • 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.

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