How to Diagnose a Failed Relay In Your Car Truck or SUV

In this video, we're going to be talking about how to diagnose a faulty relay. A lot of vehicles are different, but most of the time you find the relays right where the fuses are. So, these big boxes, these are relays. These smaller ones are relays also, and then these are fuses, even these are fuses. So, here is a common four-prong relay, and on the side of it, most of them have a little key that explains the wiring diagram for the relay, and the little pins are numbered on the bottom. And this picture represents how this relay works. So, these two terminals represent 86 and 30, and there is battery positive on those terminals all the time. And then the other two terminals, this terminal represents 87 and this terminal right here represents 85. We'll talk more about the numbers later. We're using a horn circuit because it's very simple. It's easy to understand. What we have is we have the horn grounded on this side. When you actually push the button, that closes the circuit so that you get the ground on this side of the relay. That is going to activate this electromagnet and it's going to move this lever over, and then you're going to have battery positive going through the relay. It's going to go down here through a fuse and then it'll activate the horn.

So, why do we have a relay? The reason why is because you have a high amperage circuit, so it takes a lot of amperage to get the horn to work, and then you have it go through a fuse. And if you had a switch, let's say you just wired the horn right into the switch of the horn, you're going to have a lot of amperage going through the horn switch and it's going to burn out a lot faster. So the relay is going to prevent that switch from burning out. The relay could burn out over time, although they're made for this. They're made to have a lot of amperage go through them, and then you're going to have the lower amperage circuit for the button. It doesn't take a lot of amps to get the relay to click, and that will make that work, and that's why you have a relay. Here is a relay with the cover off. You can see the windings for the electromagnet, and this is the lever right here. So, if you pulled the cover off and had this hooked up and you pushed on that, that would activate that lever right there, and that's how it works. So, for the item that you're looking for that's not working, we're checking out the horn circuit, that's the relay for the horn circuit. So, quick and easy way to check the relay, see if it's even working on the switch side is have someone push the horn and actually feel the relay and see if it's clicking. And we can actually hear it clicking there and you'd be able to feel it too.

So, we know this side of the relay is working when we push the button. The relay is clicking, so we know this side is actually working. So, something is wrong on this side, whether there's no power here, whether there's bad contacts in the relay, the fuse, or the horn, or even the ground. Now, what if your relay wasn't clicking when you were touching the horn button? Well, something on this side of the circuit is not working, whether you don't have battery positive, whether the relay internals are broken, or even a wire on this side, the horn switch is bad or even the ground side, so something in that circuit is bad. So, a quick and easy way to test if this relay is bad, what we could do is find another one of these relays and just match the numbers up. So, this is 8866, and this one matches over here, 8866. So, you wouldn't want to switch it with one of these, 8567, although the internals are probably very similar, it's not worth the risk. So, let's swap the relay, put this one right here. And it'd be good to check the circuit of this one before you did this. So if this was fog lights, make sure your fog lights work. And put this in. And so before, we were just getting a click. Let's see if the horn works.

So, that was a quick and easy way to test it. Let's try another test. Here's a relay tester. You can test a bunch of different types of relays. This can be a four-prong or a five-prong, and then the different configurations. What you do is take the four-prong relay, stick it in here. You have it attached to battery positive and negative, and then you hit the test button. It's going to test the relay's electronics. And the relay is good. Now, let's test our relay. Right off the bat, it's bad right away. Still clicks, but it's not making a good contact between the terminals 30 and 87. And you could use a relay kit where you actually put the relay in the top of it, and then you have the tests on the side. When you plug this in, this is where the numbers are important. You can see the numbers right there for which circuit is which. This is the relay tester we're going to use.

So, relay is going to go in like this, and there's our number 30 and 87. Then we can take a test light, and we can see if the circuit number 30 is getting power. And we are getting power there. If we were to ground 85, then we should hear the relay click, and you can just ground it with the test light because it's a low-voltage circuit, and see the relay clicks but the horn is not going off. If we had a good relay, and we did the same thing, you're going to get the horn to go off. Or you could also take a fused jumper harness, and if you jump the two terminals, terminal 30 and 87, you're going to hear the horn, or at least hopefully, you hear the horn if everything else is working in the circuit, then you know the relay is bad.

So, when I flip the relay over, remember that the terminals are going to be opposite of what you see. So, I see terminal 30 is right here, which is actually going to be the terminal that's located right here, right there, and then 87 is going to be over here. They cross, the terminals...even though the wiring diagrams that we have showed you, it shows they're next to each other, they're not really next to each other. That's why you got to pay attention to the numbers, so 30 is here, 87 here. So, we'll start with 87 because there's no power on 87. Put the jumper harness in there and then put this side in 30 and just do it quick. And there you go.

So, now you know a little bit more on how a relay works and how to diagnose it. If you enjoyed this video, make sure you subscribe to our channel, ring that bell, turn on all notifications so you don't miss any of our videos.