[How Does] An Automotive Relay Work

What's up guys? Oscar Gomez here from Master Automotive Training, smartautotraining.com. As always guys, bettering the automotive industry one technician at a time. So today's video guys, we're gonna be talking about relays. So I get asked quite often, especially during my electrical courses, what is a relay and how does it work? So what we've done is we've put together this quick video for you guys just to share with you guys how a relay works, how it's set up, how you can diagnose it, how you can repair it or replace it if necessary. So go ahead and grab your tools guys, let's get ready, let's hit it. So when we're looking at a relay, at this relay that we have here up on the screen, you can notice that we have five pins to it. So we have, just to name a few, we have pin 85, pin 86, pin 30, pin 87 and pin 87A. So when we're talking about a relay, a relay is an electrical magnetic device that allows you to use low current to control a high current component. So I'll say that again, it uses low current to control a high current device. So how does that work? So by looking at pin 85, pin 85 usually would be a low current power supply. So this would be something either from a switch, a computer control, a fuse, but it's gonna be low amperage because all it's really controlling is the magnetic side of the actual relay. Now that we're talking about electricity, we are using conventional theory, which states it goes from positive to negative. So that would mean that pin 86 on our relay would be our ground circuit. So our ground circuit typically on a relay is the one that controls or turns the relay on or off. Also notice in between pin 85 and 86, we have a check valve, not necessarily an actual physical check valve, but this is a diode. So this allows electricity or current to flow in one direction, but not the other. So looking at how the base of this diode is, this would mean that current is only going to flow from pin 85 down to pin 86. And it won't allow it to go back up. So now that we have those two pins out of the way, pin 85 and pin 86. Pin 85, pin 86 have to work together in order for them to create a magnetic field. That magnetic field is gonna come into play in a couple of minutes. So now looking at pin 30, pin 30 usually is going to be a fused battery positive. So this can either come from a fuse in the engine compartment if you're wiring a separate electrical circuit directly from the battery, then it would have its own fuse for that circuit, make sure that it's rated properly. So that's gonna be giving you power throughout that entire circuit. So notice using this diagram, we see that pin 87A in its normal configuration is going to have power based off the way that it's set up. So notice how we have power here because we have a fused power feed. So power will be going to pin 87A. Usually 87A is not used because the relay is in its default position in an off position. So the current or the electricity flow from pin 30 just goes to pin 87 back to ground. Then whenever we have a power feed at pin 85 and a ground feed at pin 86, we create a magnetic field inside the windings of the actual relay. When that happens, we then switch pin 30 to pin 87. So then that means that pin 87 is going to have a power feed. Whatever the voltage would be from pin 30 is exactly what should be at pin 87. So that's basically how a relay works. We need power at pin 85, ground at pin 86, that creates our magnetic field, which then closes our contact on between pin 30 and pin 87. Then pin 87 allows current to flow to whatever component it's actually controlling. All right, so if you're diagnosing a relay-controlled circuit there's a few things you want to pay attention to. First things first is you want to make sure that you have power at pin 85. Why? Because pin 85 has to have power, pin 86 needs ground in order for it to create a magnetic field. So you want to make sure that you have undisturbed power at pin 85. Then your next step is to verify that you have power at pin 30. Why? Because pin 30 closes to pin 87 and then that gives you electricity to control whatever component is on that circuit. So those two you want to make sure that you have current or power. Then if you have a computer-controlled relay circuit, you would want to use your scan tool to turn on the actual relay while monitoring pin 86. So pin 86 should come alive with ground, that's going to create the magnetic field for you, then that should cause the relay to close and then pin 87 should at that point have power close to as possible whatever came out of pin 30. So if you're doing the diagnosis, you determine you have power at pin 85, power at pin 30, when you command your scan tool to close the relay at pin 86 all you're seeing is power, then that would mean that your computer is not grounding the circuit. So you can either have an open in that circuit or the computer driver itself got burned out. Or another situation, if you are commanding the relay closed, you have power at pin 85 and you have ground at pin 86, the relay's closing, but you have zero volts coming out of pin 87, that would indicate that you have a problem at pin 30. Okay, so those are some of the things that you want to pay attention to when it comes to diagnosing an actual relay-controlled circuit. All right, so on the next picture that we're looking at here we're looking at a cutaway of the same relay. So here we can see we have pin 30, 85, 86, and 87. So again, pin 30 is going to have our battery positive, pin 85, pin 86 are gonna be power and ground, and then pin 87 is going to have the power needed to control whatever is on that specific circuit. So notice what happens once we have the ground completed to activate the actual relay. So now that we have power and ground at our coil, notice how the coil itself has become active. Now that's going to give us a power feed through the actual relay contacts, down the contacts, and out pin 87. Okay, so that's going to give you the power feed you need to control whatever component you're doing. So let's say that this is for the fuel pump. Okay, so this is a fuel pump relay. If we have pin 86 and pin 85 complete, that creates enough magnetic field to then close the contacts and allow current to flow from pin 30 to pin 87. Okay, so now taking a look at our relay, notice how now we no longer have power and ground at the coil, so the coil doesn't have any magnetic, hasn't created a magnetic field. Because of that, notice how our contacts are still open. So this would mean that any current flow at pin 30 is being stopped right here at the actual contact. So it's not allowing it to close this gap, so then there is no flow of electricity down to pin 87. So at this point, you would have a non-functional fuel pump because this is a fuel pump relay like we were talking about earlier. So because of this, we know that the fuel pump will not work because we don't have power and ground to create the magnetic field that we need to close our contact. And that's it, guys, that's that simple. The relay uses only power and ground at pin 85 and 86. Those two together create the magnetic field to close the contact from pin 30 to pin 87. Then that's gonna give you the current you need to control the high current motor, pump, whatever it is that is on that circuit in order to allow them to work. It's not as complicated as we make it feel. It's super, super, super easy. As long as you guys follow the steps that I just outlined for you, relays are nothing to be afraid of. Always make sure when you guys are testing them, check them for power and ground. If one of the two is missing, you're not gonna have a fully functional relay, and that could be giving you all the problems that you might be facing. As always, guys, if you guys like these videos, make sure you guys give me a thumbs up. If you don't like them, who cares? Give me a thumbs down, but put in the comments why you didn't like it. If this video helped you guys understand a little bit better about relays, put in the comments what specifically made more sense to you, and always make sure that you guys are sharing these videos. Let other people know about these videos. I wanna make sure that everybody who watches these videos becomes the best. The only way I can do that is by doing it one technician at a time. As always, guys, a good technician is always learning. Signing off here, Oscar Gomez, smartautotraining.com. ♪ Yeah ♪