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Dead Battery? Voltage Spikes? How to Diagnose a Car / Truck Alternator

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Hey Friends it's Len here from 1A Auto, today we've got a vehicle in the shop and we want to go over how to test your alternator. Let's get started. So let's say you notice a problem, maybe your vehicle doesn't want to start, you're going to want to check to see if it's charging or if it's just a battery issue, one of the first things you want to do, of course, is to pop the hood and take a look at that battery. If you see anything that looks like this, this is corrosion. Obviously, that's in very poor condition, you need to clean that. Once you've got it nice and clean, we need to check to make sure the battery is at a full charge. All right, so we've got these cleaned up fairly well here, it's important to make sure that you're wearing hand and eye protection anytime you're dealing with a battery like this. There's a couple different tools we can use, you can use something as simple as this, which is available at, and you would just connect it right on to the battery terminals. And it's going to give you a little instructions on what to do. It'll tell you your battery voltage, it'll tell you your alternator current. And it'll even tell you what your current is when you're starting the vehicle. This is great. If you don't have access to one of these, most people have one of these laying around their house somewhere maybe in the basement or in the shed or wherever your toolbox is. This is going to come in handy and we can test a lot of things with it. So this is what we're going to go with for today. Now if you're looking at your multimeter, you're going to see right along here, you've got the DC voltage range right there, you want to set this to DC voltage, and it needs to be something above 15 volts. So I'm going to go right here to this warning because that's definitely way up above where this is going to max out at.

I'm going to stand it up here so we can see it hopefully. And that'll take my test leads. And it really doesn't matter which way you connect them in but why not do red to the positive, black to the negative. We're just going to measure the voltage right up the battery. As you can tell, this battery looks great. It's at 12.6, you can't ask for any better than that. If it was at 11, or 11.5 it's a very weak battery, and there's probably something going on that's making it so it isn't charging. So now that we know that the battery is good, just sitting as it is, let's get inside the vehicle and we're going to turn on some of those loads. We'll turn on the headlights, we'll turn the key and we'll make sure that the wiper blades are going maybe even the blower motor, we're going to come out here and see where this reads it's going to dropdown, of course. Once we turn all that back-off, we're waiting for it to come right back up into that 12-volt range, even 12.2 or hopefully 12.4, or a little bit more like 12.6. Let's try it.

So as you can see, this is dropping down quite a bit. If it was to go down to about 10 or in the 10-volt range, you know you have a weak battery. This looks pretty decent right now it's holding at about 11.4, 11.5. So I think that that's fair. Now what we're going to do is go ahead and turn off all that stuff and we're going to come back out here and we want to watch this come back up into the 12s like I said.

So this is looking great. It's coming right back up, it's taking its time, but it's definitely up in the 12s. So now that we know we have a known good battery, we're doing all right. The next thing we need to do is check for charge going directly to the battery. So with the leads the same way that we had it set up originally, we're going to go ahead and start the vehicle, we're going to put on all the loads that we can. And we're also going to rev the engine up to approximately one and a half thousand RPMs, okay. You can go up to two if you need to. What we want to do is we want to make sure that the current right here on our multimeter is coming up to like 13.5, 14, maybe up to like 14.2. But you definitely don't want to be 14.7 because that's way too much voltage getting pushed into this battery and it's going to cause a major issue. So as long as we're between the 13.5 and the 14 like I said the 14.2. We're doing all right.

All right, get my loads on here. I'm going to rev it up. All right, so that came out great. So we know on this particular vehicle, we don't have a charging system issue at this time. But let's go with the assumption that maybe when we just ran that last test, this didn't change very much. Maybe it went up a teeny bit, but not really very much or even worse, maybe the voltage that is sitting out right now went down as the vehicle was running and the loads were on. In that case, it would mean that the alternator isn't pushing the current that it's supposed to be. So now that we know that the battery is good, we can move along. You need to make sure you have a known good battery before you can continue testing that alternator. So now we're going to continue on to testing right at the alternator. The best way to do that is take your two leads with your multimeter we're going to keep it on the same voltage range. I'm going to take my positive and I'm going to go right to this wire right here. This is the one that's going to go right to the battery. See if you can stick it in there. So it's definitely staying The next thing we want to do is ground this out, right at the housing. So I'm going to go right here. That's going to be a great ground point, I can tell because I have a ground wire that comes right to there. This looks good, I just need to make sure that this one's going to be making contact like it should. This is telling us that we still have the same battery voltage from earlier right at the battery. So this is looking great. The next thing that I want to do is start the vehicle. At that point, we want this to jump up 13 even into the 14s, but you want it to be very low 14s if it does. You don't want 14.7 because then your alternator is overcharging.

So I'm going to start up the vehicle, I'm going to put on all the loads, like I said before, the blower motor, the headlights, the wiper blades, pretty much anything that I can come up with, that's going to put a load on that battery. And it's going to make the voltage regulator on the alternator kick up and it's going to say start pushing some voltage to the battery. Here we go. All right, now I'm going to rev it up to approximately one and a half thousand RPMs.That's great. Let's turn off all of our loads. So that went up exactly to where it's supposed to be. That means that the alternator is pushing current the way that it should. If for some reason it didn't jump up to that 13/ almost 14 range, you know that there's an issue with the charging system at the alternator.

Now if your alternator turns out that it isn't pushing any current, there's a couple things that you want to look for, you want to actually make sure that this pulley right down here is spinning the way that it should when your vehicle is running. Obviously, the vehicle's running, you don't want to go anywhere near this with your fingers or anything else because the belts going to be spinning and it could potentially, you know, hurt you in some way. But make sure that that's spinning, you can visually see it spinning. You can also take a look all inside here, where your winding is, you can see pretty much all around in the alternator if it looks like it's full of gunk, or even a whole lot of dust more than likely the brushes on the inside of your alternator are no good. If that's the case, it's not going to generate the electricity as it should.

If you were to look at the back of your alternator, you're probably going to see something that looks a lot like this. It has the terminal coming off of it right there. And then it's going to have the area right there. You take this cover off, and now you can see your voltage regulator. These areas right here are the diodes, those are super important, you need to make sure that those are in good condition and they're not corroded or falling apart. If you see something that looks a lot like this, obviously, that's probably on its way out. That's going to be an issue for your charging system. You can turn it around, take a peek, this all looks fairly decent. We have a little bit of corrosion and stuff here but it's not really too much to worry about. Now we can look inside the alternator, here you have all your copper lining, you have the area where this is going to be rubbing across and creating a magnetic field which is going to generate that electricity. And then it's going to send it through these wires to that voltage regulator and the voltage regulator is going to turn it in to direct current instead of alternating current.

If you were to continue on by looking at this right here, you can see your bearings right here. That's super important, all right. And you have this copper insulator right there. This is the area where these brushes are going to ride on as it spins around. These are going to get worn down over time. And when they're brand new, they're actually much longer than this. But over time they wear down. And the spring you see right here my fingers actually just keeps pushing it out so that it stays up against this and it remains in contact. Once these get worn down too much, they don't make great contact anymore and you're going to have poor generating ability. Looking at the front of your alternator you have your nut there, you have your pulley. You get those out, you're going to have a shaft coming out and of course, you can see your bearing. You just pull those apart, you can have a little washer here. Super important. Now you've got the shaft area.

While your alternator is inside the vehicle and the vehicle's running, this is going to be spinning due to your serpentine belt, all right. You're going to have your belt on there, it's going to be spin, spin, spinning, it's going to spin this around in circles at whatever RPM you're engine's at. As this spins around, it's going to be inside here and it's going to be creating a magnetic field and generating energy just like that with this pulley.

Now when testing your charging system on your vehicle, it's important to remember older vehicles are going to have a generator, newer vehicles are going to have an alternator and they're two very different systems. Another thing that's super important to remember is just because you have a newer vehicle and it comes with an alternator, the alternator doesn't necessarily always have the voltage regulator inside of it or as part of it, it could also be regulated by the PCM of the vehicle. Now with a newer vehicle like this with the alternator, as I said with the actual voltage regulator as part of it or even in the PCM if you were to disconnect that negative battery terminal, you're going to cause detrimental damage to your charging system more than likely the voltage regulator itself.

Okay, friends so that pretty much wraps it up for us. If you saw something you like make sure you smash on that like button. If there was something thing that you wanted to say maybe something I missed, go ahead and leave it in the comment section. I'd love to hear from you. While you're at it, why don't you go ahead and click on that subscribe button and ring the bell that way then you'll be kept up with all of our latest content. Thanks.

Thanks for watching. Visit for quality auto parts shipped to your door, the place for DIY auto repair. And if you enjoyed this video, please click the subscribe button.

Mm-hmm, and this one. Mm-hmm and this one. All right, let's put it back in the car.

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