How to Test an Alternator

hey guys chrisfix here today I'm going to show you how to test your alternator and make sure it's working right some symptoms you might have with a bad battery or a bad alternator is that your battery light might be on the - the engines cranking slowly or won't crank at all the battery seems weak interior lights or headlights seem weak rumbling or squeaking noises coming from the alternator which indicates that there could be a bad bearing these are symptoms of a bad alternator or battery to see how to test a battery check out my other video the link is in the description below this video is specifically for the alternator and I'm going to show you everything that you need to know about how to test the alternator so when you're doing this test you want to set up your multimeter to DC v which is DC volts and you want to set to something above 15 so 20 in this case here's the negative terminal and the positive terminal on the battery you want to make sure these terminals are clean so that you'll get a reading when you do this test so I'm going to put my black to my negative and red to the positive here and you can see you want 12 point 6 volts or something around that it could be a little bit lower a little bit higher that's not a big deal but twelve point six is the magic number to shoot for now to see if the alternator is working properly we have this attached and we're going to start the car and ideally what should happen is this should go between fourteen point two and 14 point 7 volts and that means your alternator is running correctly and charging the battery up correctly if you're getting over fourteen point seven volts and that means that your alternator is overcharging the battery and that could cause damage to the battery so you don't want that and then under fourteen point two depending on how much under let's just say thirteen point two that means your alternator isn't strongly charging the battery it'll still charge it because it's still above this twelve point six volts but it's not going to be enough to charge it when you run accessories such as your headlights and radio let's start the car so I could show you what I'm talking about so you can see it's running about 14.2 and that's exactly where we want to be there's a voltage regulator in the alternator that regulates the voltage that this should be at and fourteen point two to fourteen point seven is exactly where we want to be and that's spot on again if this was bad it'll be either below the fourteen point to fourteen point seven range or above now we're going to put a load on the battery and alternator so we're gonna turn the lights on we're going to turn the radio on I have off-road lights I'm going to turn those on and all that stuff together is going to put a load on this and what we should see is this shouldn't drop too much we want to make sure the voltage doesn't go below 13 volts now right now I have a lot of things on I have four 100-watt off-road lights on I have my headlights I have fog lights on and have the radio on and I have a fan on and with all that stuff the voltage is still almost 14 which is great we don't want this to drop below twelve point seven with all that stuff on but as you can see my my alternator is running great and now I'm going to shut off everything I turned on you'll see this will spike back up so now when we started I had 12 point 6 volts when I shut the car off it should be higher than 12 point 6 and that means that the alternator's charging the battery so we just shut it off and we could say that it's twelve point nine volts this is the surface charge eventually this will drop back down to twelve point six so if while doing these tests something doesn't check out your voltage doesn't go up when the car is running or when you turn all the accessories on the voltage drops below 13 volts if that's the case then we're gonna check out a couple of things first off you want to check out the connections here take the leads off the terminals and send it and get a really good connection because sometimes the connection isn't good and that'll cause the alternator to have a hard time charging the battery another spot to look at is the back of the alternator make sure all the wires are plugged in and secured all of mine are plugged in obviously but make sure yours are and follow this and make sure that this goes all the way to the battery and isn't disconnected at all check for any corrosion any damage to the wires any frayed wires any kinked wires all that stuff could cause a problem also your alternator might have an external voltage regulator check the wires to the external voltage regulator make sure that it's going there make sure it's clipped in and make sure the external voltage regulator is working and finally as the car is running make sure that the alternator is spinning make sure that the belt is tight you want to make sure your belt is tight on the alternator because that's how you get your power now the last test I'm going to show you guys is a voltage drop test for the negative and positive sides of the battery first we'll test the voltage drop on the negative side of the battery to do this test go start the car turn on a bunch of accessories you want your headlights on you want your blower fan on your radio on and then I'll come back out here and I'll show you we're going to connect the black lead to the negative side of the battery and then we're going to touch the red lead to the alternator case you can see I have a bunch of accessories on off-road lights headlights fog lights blower motor radio get your black lead attach it to the grounds and then take your red lead and you're going to touch it to the alternator case the outside of the case make sure you have a good connection when you get a good connection and you can see a reading what you want to do is you want to go and raise your rpms to about 1500 that right there is about 1,500 rpms and you can see the reading is 0.05 you don't want to see a reading that is point one or two in that range or greater if it's point one or two or greater then you have a problem this is 0.05 so I'm good now let's just say that you get a reading that's point one volts right so what you want to do next is take this and touch it to the bracket and if you see a substantial voltage drop that means the connection between the alternator and alternator bracket needs to be clean because the ground isn't good now if you don't find your problem between the connection of the alternator and the alternator bracket then you should try the engine block so you want to be careful because the fan is running so I know it's hard to see but I'm connecting it to just like a bare metal part on the engine block and you can see it's point o4 so again I don't have a problem it's not point one it's point o4 but if you had a problem and you saw a substantial decrease in voltage from your alternator case to this then that would be where your problem is at that point I would clean the connection between your mounting bracket and your engine block and now if you don't find your problem here the less but I would check is your chassis ground so you can see here's mine it connects to the firewall what you'll do is you'll take your red lead and connect it to the ground like that and you can see 0.06 still the same and remember to bring the RPMs up to about 1500 when you do all these tests the idea is that keep testing the ground connections until you can find where the connection is bad once you find the bad connection sand it get it clean so that the connection is good and then hopefully that will fix your problem now we're going to test the voltage drop across the alternator and the positive side of the battery to do this again it called the accessories on car running you'll bring it up to 1500 rpms connect the red to the red and then we're going to connect our black to the B+ post on the alternator the B+ post has a nut and a post in there so make a connection right on the end like that remember to rev up the engine to about 1500 rpms when you check this and now we're gonna look here you don't want this to be Oh above point three bolts so you can see mines 2.8 so that's below point three so I'm good now let's just say you're above point three the next place you want to check is the nut right here and if you see a substantial voltage drop you know the connection between the nut and the stud needs to be cleaned I'm good still and there's no significant voltage drop anyway and the next place you're going to want to check is the lead that's coming from the alternator into that wire there's a metal piece you can see it's in between the nut and the red there on the alternator so just touch it in there and then we'll come around and see still below three and there's no significant drop so I'm good that's good to go I know I don't have any problems with this but that just shows you how to diagnose the positive side as well for a voltage drop again the idea of doing this is to keep testing all the connections until you can find where the connection is bad and then once you find that connection just sand it so it's clean and good and you're good to go hopefully this video was helpful if it was give it a thumbs up on the screen or give me a few videos the first one will teach you how to check the battery and the second one will show you how to check for parasitic draw remember to leave any questions or comments you might have you guys come up with good tips and tricks and have good questions as well which is great also consider subscribing I publish how-to videos weekly that'll teach you a lot and save your money finally in the description below are my links to my Facebook and Twitter sites so check them out if you use Facebook or Twitter

Channel: ChrisFix

Views: 6,953,993

Rating: 4.9142733 out of 5

Keywords: Diagnosing Alternator Problems, how to test an alternator, how to test alternator voltage output, how to test voltage drop, how to test a failed battery or alternator, how to test and troubleshoot an alternator problem, how to test an alternator with a multimeter, using a multimeter to test an alternator, オルタネータテスト, тест генератор, Alternator (Invention), car battery, car wont start, testing an alternator

Id: LGB6ZEjGm7Q

Length: 9min 6sec (546 seconds)

Published: Wed May 14 2014