How to Test Your Car’s Relay

A relay is a small electromagnetic switch that is responsible for sending electricity to various car accessories. The device is so called because it literally “relays” the electrical current on to the next device. Most relays channel a small amount of power in order to trigger another, larger device that requires more power. The relay is the transfer device that allows a computer to control devices that are too large to be controlled directly. Relays control everything from the fuel pump to the air conditioning, so if something stops working, a dead relay may be the problem. Read on for a step-by-step guide on how to test a relay to see if it is the problem.



Gather Your Supplies

You will need a few devices in order to test a relay. You will need a multimeter, which is a device used to check electrical current. You will also need a small wire, 14 to 20 gauge, and an automotive bulb and socket.


Locate the Relay

The car’s relays will be located in the fuse box. A car has multiple relays for different functions. If you are looking for a specific relay, locating the correct one may require consulting your owner’s manual.


Check the Fuses

Before checking the relays, you will want to first check your fuses. The fuses are responsible for powering the relays, so they could be the source of the problem. You can use your multimeter to check the fuses. If your fuses are okay, continue on to check the relays.


Turn On the Ignition

You will need a partner to help you out for this step. While the relay is still attached to the car, put your fingers on it and have someone turn the ignition key to the "on" position, and then turn the key all the way turning the engine on. At some point during this, you should feel a click beneath your fingers. If you do not, remove the relay and check the connections. They may be corroded or overheated, in which case you will need a new relay.


Check the Terminal Sockets

If you feel the clicking, but you’re still experiencing problems with your system, it’s time to take out your multimeter. Remove the relay, and use the probe to check all the terminal sockets in the relay connector. Two of them should have power. If only one or none have power, and all your fuses are okay, things will get a bit more complicated. You will need to consult a car repair manual to find a wiring diagram and trace it back to the power source. If two terminals do have power, continue to the next step.


Use the Wire to Bypass the Relay

Terminals 87 and 30 are the two responsible for transferring the power from one terminal to the next. Take your small wire and insert either end into the 87 and 30 block terminals. Turn the ignition key to "on." This bypasses the relay, and whatever the relay controls should now be turned on. For example, if you’re testing the relay for the air conditioning, the AC should now be on. If the device is not on, there is a problem with the connection between the device and the relay. If it does operate, continue to the next step.


Remove the Relay

Remove the relay and insert a small wire strand into the relay connector for the ground being tested.


Re-install Relay

Re-install the relay while keeping the wire strand secured in the relay terminal. Make sure it is clear of any other terminals or grounds.


Attach the Bulb and Socket

With the wire strand secured, attach one wire of an automative bulb and socket. It is ideal to use a car side marker bulb because it already has small wires attached. The remaining wire should be attached to power or ground, depending on what you are testing. The bulb will illuminate when the relay is in use, and will remain dark if the circuit has failed.


Replace Relay If Necessary

Over time, it will be necessary to replace certain relays. They can get burnt out from continuous use. If the bulb stays out, you will need to purchase a new relay to replace the old one.

Routinely checking your relays can help keep your car in the best shape possible. Your car is a complex machine, and if something is broken, there are many parts that could be the problem. Your fuses should always be checked first. If your fuses are working fine, test your relays to determine if they are in working condition. If your relays are the problem, it is time to replace them. If they are not the problem, knowing how to successfully test your relays can help you rule them out as the cause of the problem, which will allow you to narrow down the possible suspects and get your car in working order much sooner.