1. Locate the relay you wish to test. Relays could be located in the fuse box in the engine compartment or in the interior of your vehicle, or they may be mounted separately from the fuse box in the engine compartment or elsewhere. Your owners manual can help you find the location of the relay specific to the circuit in question. Now you can remove the relay from the vehicle. This may involve unplugging a wiring connector and/or unscrewing or disconnecting the relay from its mount, or you may be able to simply pop it out of the fuse box.
Once you have the relay removed, handle it carefully and avoid dropping it. This could cause internal shorting that could harm your vehicle if the damaged relay were reinstalled. Take a look at the connector pins on your relay and the wiring connector. If you see any signs of corrosion or if they are especially dirty, using an electronics cleaner and a small brush may be everything needed to restore the connection. This is also a good opportunity to swap the relay for a known-good relay with the same specifications and tolerances, but this could also damage the new or known-good relay if the issue is in the wiring of the vehicle.
Relays on your vehicle are a switched electrical component that allow high-current power to be sent to an electrical accessory. They use a smaller-gauge, low-current switching wire to activate an electromagnet inside the relay and connect power across the posts to activate things like your power locks, horn, headlights, fuel pump, air conditioning compressor, and more. Along with fuses, relays help to deliver the power needed to keep the electrical system in your vehicle functioning as it should. Though relays are more difficult to test than a fuse, which can usually be visually assessed, being able to test relays yourself can save you time and money when attempting to diagnose your own vehicle.
What Does a Relay Do?
A relay can perform a variety of functions, but every relay is essentially a switch that turns electrical power on and off as needed. Many relays are controlled by the engine or body control module, and rely on power being sent from a controller or switch to connect or disconnect the power being sent to an accessory circuit. Relays are often used to avoid running heavy-gauge wire over long distances, and allow a small-gauge wire to do most of the heavy lifting when designing vehicle wiring harnesses. Your starter solenoid, for example, is a type of relay that temporarily connects power from a heavy-gauge battery cable to the starter using an incoming signal from the ignition switch. When a relay fails, it can prevent accessories from operating when they need to, or it may allow them to continue running even after they should have turned off. This can cause damage to on-board systems, and could even be dangerous in some cases.
What are the Symptoms of a Bad Relay?
Because relays control so many different systems on the vehicle, it’s hard to pinpoint the symptoms of a failing relay or to separate them from symptoms of a failing component in some cases. The main symptom you may experience if a relay is failing is a lack of power to one or more systems on your vehicle, but more specifically this could cause issues like:
- Failure to turn the engine over (starter relay)
- No fuel pressure and a no-start condition (fuel pump relay)
- Lack of air conditioning compressor activation (A/C clutch relay)
- Non-working or poorly-timed windshield wipers (wiper relay)
- Non-working or always-on turn signals or hazard lights (turn signal flasher)
- Headlights stuck on low- or high-beams, or not working at all (headlight relay)
- Non-working horn, or a horn that honks continuously until disconnected (horn relay)
These are only an example of common relay failures, but in reality, modern vehicles commonly have 20 or more relays that each control their own portion of the on-board systems. This can make diagnosing electrical system failures difficult, but testing fuses and relays is often a great place to start to narrow down the cause of the problem. You can also learn more about what causes fuses to blow with our helpful article.
How to Test a Relay
First, make sure you’ve got your multimeter, a 9V or 12V battery, and some jumper wires handy. We built our jumper wires using blade connectors and clips to ensure they wouldn’t fall off the relay during testing, and this is a good idea to prevent any risk of electrical shorting during your testing.
2. If you’re ready to test your relay, set the relay with the connectors visible on your workbench and hook up your jumper wires. If you want to build your own jumper wires, learn more about how to crimp electrical connectors to easily build relay connectors using basic wire and crimp-on blade connectors. It is usually best to color-code your jumper wires to be specific to the pins of the relay, which should be marked or at least in the standard orientation below. Keep in mind this only applies to four-pole relays common in automotive applications.
- Terminal 30 to battery (incoming power)
- Terminal 87 to accessory (outgoing power)
- Terminal 86 to trigger source (switched power)
- Terminal 85 to ground
Now you’re ready to use your multimeter to start testing. If you need a refresher on how to perform common readings, check out our helpful article on how to use a multimeter. Set it to ohms (resistance) and connect terminals 86 and 85 (switch and ground). It should read between 50 and 120 ohms, but if it is outside these readings or is open (no reading is displayed), it likely means the relay needs to be replaced.
3. If the relay passed the first test, with your multimeter set to read ohms or continuity, probe terminal 30 and 85. With the relay powered down (not connected to a battery), you should not read any continuity, or if you’re set to ohms, should read 0L. This indicates that the relay is not connecting power to the accessory circuit, and is the correct reading for a normally-open relay. This test would be reversed for normally-closed relays, which are designed to consistently provide power and turn off power to the accessory when they are activated, but these are considerably less common in automotive applications. If a normally-open relay has continuity or reads any amount of resistance during this test, it has failed and should be replaced.
Next, connect the positive terminal of your battery to the jumper wire for terminal 86, and connect the negative terminal to terminal 85. This energizes the electromagnetic coil inside the relay, and in many cases will make an audible clicking noise if the relay is working properly. Even if you don’t hear the relay click, it may not mean that the relay has failed, but you can connect and disconnect the terminals a few times to make sure you can’t hear it click before moving on.
4. Leave the battery connected, and you can now connect terminal 30 to the battery’s positive terminal. This simulates incoming power that should be sent across the relay to terminal 87 when the relay is activated. Using your multimeter set to VDC (DC voltage) or a test light, check for power at terminal 87 by touching both the terminal wire and the battery’s negative terminal. If you read approximately the amount of power coming from your battery (9 to 12 volts), or if your test light illuminates, you can assume you are getting power across the relay and that it is working as it should. Disconnecting the power going to terminal 86 or the ground going to 85 should cause power to no longer reach terminal 87, as this powers down the electromagnetic coil and opens the relay.
If your relay passed all of these tests, it may be time for more intensive diagnostics, and you should check the wiring connector to determine if power is coming both from the switched source and the battery using your multimeter set to VDC or your test light. If there is a fuse in the circuit, this would also be a good time to check the fuse if you haven’t already. You can learn more about how to check fuses with our article. If you have the appropriate power and ground all of the connectors on the relay’s plug and across the fuse, it could point to a problem in the wiring, the component the relay powers, or a variety of other factors that may require professional diagnosis.
Adding and Wiring a Relay for Aftermarket Accessories
Though your vehicle is equipped from the factory with relays to perform a variety of tasks, if you are adding electrical accessories like aftermarket lighting, horns, or air suspension upgrades, you will typically need to wire in a relay to control these add-ons. Most accessories that require a relay include one, as well as installation instructions, but if that is not the case, we have prepared a step-by-step guide to help you add a relay and aftermarket accessory to your electrical system.
To start with, determine what type of relay you need. For most electrical accessories, a 4-pin normally-off relay is appropriate, but 5-pin changeover relays may also be used if some part of the accessory should always be powered. Even if you need a 4-pin relay, a 5-pin relay can typically be used and one terminal not connected, keeping in mind that you will need to choose the appropriate switched terminal during installation.
Once you have your relay, you will need to tap into an appropriate source of power. This can be a constant power source or an accessory power source, depending on which type of power would be appropriate for your installation. Constant power is present even when the key is in the off position, and is often used for things like station presets and audio settings memory on your radio, horns, and lights. Accessory power will only be available for use by the relay when the key is in the run or accessory position. It may help to have a wiring diagram for your vehicle to determine where you might be able to locate these power sources, or it may be beneficial to use a fuse holder to add a completely new circuit. A good failsafe is to use accessory power for your relay switch and constant power for the incoming power source. This ensures that when you turn your key off, you don’t accidentally forget to turn off the accessory and drain your battery.
Once you have found your power source(s), you will need to locate or make a suitable ground. This could include tapping into an existing ground wire or making a ground wire using a ring terminal, wire, and self-tapping screw attached to the metal body of your vehicle. At this point it is usually a good idea to disconnect your battery to prevent any short-circuiting while you work. If you are using a switch other than the ignition switch for items like off-road lights or aftermarket horns, connect the switch to incoming accessory power and then to terminal 86 on your relay. You can then connect terminal 30 to your constant power source and terminal 85 to your ground wire.
The final step is to connect the accessory itself to terminal 87, the outgoing power terminal. Then, if necessary, find a good spot to mount your relay and switch, and ensure that all wires coming to and from the relay are secured and away from any obstructions or areas that could cause damage or a short. Now you can reconnect your battery and test your new accessory. If it is still not working, it may be necessary to use a test light or multimeter to double-check your incoming and outgoing power, ground, and continuity, but testing the relay itself may be as simple as turning the ignition or auxiliary switch on and off and listening for a click.
Electrical System Diagnosis and Repair
As we’ve discussed throughout this article, testing and installing or wiring a relay is not nearly as complicated as it may seem. But the nature of the electrical system means that relays aren’t the only thing that may need to be checked if you’re having trouble with your accessory circuits. Fuses, wiring, connectors, and other modules and controllers can lead to a loss of power, and it can sometimes be very difficult to trace where the problem might have originated so it can be fixed. This is increasingly true as modern vehicles continue to feature more and more on-board electrical accessories, requiring even more complicated wiring harnesses and circuit designs. If you’re not comfortable taking on electrical system repairs on your vehicle by yourself, we are always happy to recommend a local repair shop. You can also check out our video on how to install a light bar on your vehicle to see the procedure for installing accessory lighting, switches, and relays. Hopefully, this has helped you better understand a small part of the electrical system on your car – but if you need additional help, just visit your local O’Reilly Auto Parts for the advice and parts you need to get the job done.