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How To Test A Relay On A Car

A fuse box with relays on a Jeep

Your automobile’s electrical system is a complex web of wiring that runs from bumper to bumper. Making it all work in harmony takes specialized components designed to do their job flawlessly. But when an electrical component fails, you may have trouble. You may think that when you turn on the headlights that you are directly closing the circuit using the headlight switch. But unless you are driving an antique car, you are actually triggering a headlight relay to supply power to the bulbs. That relay is designed to handle the switching function instead of a dial or rocker switch. But what if that relay doesn’t work? Here’s how to test a relay on a car to find out if you need a replacement.

What Is A Relay

In its simplest form a relay is a switch that is operated via electricity. A simple relay has two circuits, one acts as the trigger to operate the switch. Usually an electromagnet is used to operate the switch mechanism. When current is applied to the electromagnet circuit, it triggers the contacts of the switch mechanism to move together completing the circuit. Once the power is removed from the electromagnet circuit, the switch mechanism will open again and the power flow will be interrupted. There are all kinds of relays for specific situations. Some may have multiple circuit paths for triggering multiple circuits. A relay is a great way to control a higher voltage circuit using a lower voltage source.

How To Test A Relay On A Car

There are a few ways to test a relay. The first thing to do is locate the relay you want to test. Your owner’s manual should tell you the locations of the fuse boxes. On the cover of the fuse box there should be a chart that maps the layout of the fuses and relays found inside. Once you have located your suspect relay, search the fuse box for an exactly identical relay. For example, on a 2002 Toyota Camry the AC compressor clutch uses the exact same relay as the horn. Verify that the duplicate relay works (such as honking the horn). Now simply swap the suspect relay and the duplicate relay. The relays may be seated tightly in the socket, so pull firmly from the base of the relay itself. Once the relays are swapped, test the system of the suspect relay. If it works congratulations, you have now identified the suspect relay as being faulty. Put the duplicate relay back where it was and head to your local NAPA Auto Parts store for a new one.

If you cannot locate a duplicate relay to swap, you can still check if the relay is working. Locate the suspect relay and listen for a “click” when the system is activated. For example, have a helper turn the AC system off and on inside the vehicle while you listen to the relay plugged into the fuse box. Be careful to not let your body touch any moving underhood components. You should hear a “click” every time the system turns off and on. While not always a foolproof method, a silent mechanical relay can mean either a bad relay or a lack of power to the relay trigger circuit.

How To Test A Relay Off A Car

The next method is a bit more involved. You will need to remove the relay and take it somewhere like a workbench where you can test it. You will need a 12v power source, a multimeter, and two wires. You will need to reference either the vehicle wiring diagram or some relays have a wiring diagram printed on them. Here’s a typical relay:

relay wiring diagramIn this example terminals 85 and 86 are for the magnetic coil circuit. Terminals 87 and 30 are the actual switch that will be triggered to power whatever circuit is being powered (like the horn). When 12v is applied across terminals 85 and 86, the circuit between terminals 87 and 30 will be completed. You can test this by using the multimeter to check for resistance across terminals 87 and 30. When no power is applied to terminals 85 and 86, there will be no resistance measured between terminals 87 and 30 (an open circuit). Using the 12v power source, apply power across terminals 85 and 86. There should now be between resistance terminals 87 and 30 (a closed circuit). You should also hear an audible “click” when applying power. This is a very basic relay, but the same principle can be applied to almost all relays. Find the circuit terminals that trigger the relay and apply the appropriate input power, then check to see if the relay closed the circuit.

Here is a closeup of the relay terminals. The terminals are numbered to correspond to the diagram printed on the side of the relay.
Here is a closeup of the relay terminals. The terminals are numbered to correspond to the diagram printed on the side of the relay itself.

How To Find A Replacement Relay

Relays are designed for their specific uses so they are not one size fits all. Once you identify a bad relay simply remove it and bring it into your local NAPA Auto Parts store where they can find a replacement. The store can also look up the appropriate replacement by your vehicle year, make and model. Most relays have a part number printed or stamped on them. This number can be referenced to a replacement part that matches the same specifications as the old one. Pay attention to the amp rating, as using a lower rated relay can cause damage.

Check out all the electrical system products available on NAPAOnline or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on the car electrical system, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Pixabay.

Brian Medford View All

With an automotive writing career spanning over two decades, Brian has a passion for sharing the automotive lifestyle. An avid DIYer he can usually be found working on one of his many project cars. His current collection includes a 1969 Olds Delta 88 convertible, BMW E46 sedan, and a slant-6 powered 1975 Plymouth Duster.

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