How to Test Relays

by Lawrence Stephens
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12 volt relais, relay image by Sascha Zlatkov from

The relays in your car are electrical switches that are used to control different electrical components, such as the air conditioner, the fuel pump, and the radiator fans. Relays can malfunction and fail over time. Testing a relay to see if it is still functioning properly may seem like a daunting task to someone who is unfamiliar with testing electronic components. The testing procedure is fairly straightforward, however, and it can be done with the use of a standard multimeter.

Step 1

Locate the relay that you want to test. Refer to your owner's manual for relay locations. They are usually under the dashboard or under the hood, depending on the circuit that the relay controls.

Step 2

Turn the ignition switch to the "On" position, but do not crank the engine.

Step 3

Use a high impedance test light to test that the relay is receiving power. Connect the alligator clip from the light to a suitable ground. Probe the wire from the relay to the component it controls. If the bulb lights, then there is power leaving the relay, and it is working properly.

Step 4

Use the same probing procedure on the wires that are receiving voltage from the power source. If the bulb fails to light, then power is not coming into the relay. The power source should be tested.

Step 5

Turn the ignition off and remove the relay from the connector. Be careful not to break the locking tabs on the case.

Step 6

Set the multimeter to the "Ohms" setting and test the continuity of the relay's power terminals. The terminals are usually labeled on the case of the relay. The multimeter should have an infinite, or "OL," reading. If there is continuity, replace the relay.

Step 7

Attach a jumper wire to the positive terminal of the battery and one of the control terminals. Attach the other jumper cable to the opposite control terminal and a suitable ground. You should hear a click as the connection is made. If you do not, reverse the control terminal connections. If there is still no click, you will need to replace the relay.

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