How to Check A Car Alternator

By Syed Ahmer Imam

This blog will demonstrate to you how to check a car alternator and diagnose its problems whether it's working right or not.

Symptoms With a Bad Alternator

Some symptoms that you might have with a bad battery or a bad alternator are that your battery light is ON on the dash, the engine not even cranking at all, or cranking slowly or there's clicking when you start the engine. But the engine doesn't start so just clicking the battery seems weak or is it dead, the interior lights or headlights seem weak. These symptoms might also be the signs of a bad alternator or a bad battery. The battery seems weak, interior lights or headlights seem weak, and rumbling or squeaking noises come out from the alternator which indicates that there could be a bad bearing these are the symptoms of a bad alternator or battery.? And stuff like that.

This blog is specifically for the alternator. This blog will demonstrate to you everything that you need to know about how to diagnose and check the alternator.

How to Check Your Alternator?

To check and diagnose a car alternator, you need a multimeter and you need to set up your multimeter to DC voltage, you need to set up the multimeter to 20 volts DC or something above 15 volts as car batteries' voltage range is under 15 volts DC.

First, you need to check your battery's potential difference or volts, your battery has a negative terminal and the positive terminal. You need to make sure these terminals are clean so that you'll evaluate a correct and precise reading when you will be going to attempt this test.

In case of clean terminals put your multimeter negative(black) probe to the negative(black) terminal of the battery and the positive(red) probe of the multimeter to the positive(red) terminal of the battery and let's assume you have a good battery and make sure you are getting 12.6 volts or something around it. It could be a little bit lower or a little bit higher, it is not a big deal but 12.6 volts is the average volts or potential difference you might get with a good battery. 

To check if the alternator is working properly, we're going to start the engine with the multimeter's probes attached to the battery and ideally what should happen after starting the engine is the multimeter's display voltage reading should go between 14.2 to 14.7 volts or around 14.4 volts but less than 14.7 volts and if you are getting such readings on your multimeter that means your alternator is running correctly and charging up the battery correctly. If you're getting over 14.7 volts then it implies that your alternator is overcharging the battery and it may make harm the battery, so you ought not to get such readings. If you are getting under 14.2 volts depending on how much under 14.2 volts you are getting, let's just say you are getting 13.2 volts that mean your alternator is not strongly or properly charging the battery but the alternator will be still charging it because it's still above 12.6 volts, when you turn ON the accessories such as your headlamps, etc but to charge the battery it is not going to be enough. 

You need to start the engine so you could examine it by yourself and you will see if you have a good battery that your battery is running about 14.2 volts and that's exactly where we want to be. There's a voltage regulator in the alternator that regulates the voltage and it should be at 14.2 to 14.7 which is exactly where we want to be. Let's say you have a bad battery it'll be either below 14.2 volts or above  14.7 volts then we are done with the battery, now you need to put a load on the battery and alternator so we would be able to diagnose whether it's an issue with a bad battery or a bad alternator. So we'll be gonna turn the headlights ON and we will be going to turn the radio ON and turn the AC ON and all that stuff together is going to put a load on alternator and battery. After that what we should see is this shouldn't drop too much we want to make sure the voltage doesn't go below 13 volts, with all that load if the voltage still remains 14 volts then that means you have a very good battery as we don't want this to drop below 12.7 volts with all that stuff ON. If you are getting such results then your alternator is running great.

Now you need to do the second part of the same test, you need to shut off everything which you turned on you'll see the voltage that was dropdown will spike back up. If you were getting 12.6 volts initially when you didn't start the engine, now when I shut the car off it should be higher than 12.6 volts and it means that the alternator's charging the battery. 

So let's say you shut the engine off after a quick ride and if you will get around 12.9 volts then it is the surface charge eventually this will drop back down to 12.6 volts so no need to worry. 

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 will need to gonna check out a couple of things first 

off, you want to check out the connections of the battery, so take the leads off the terminals of the battery and sand it all around, and tighten it down again to get a really good connection because sometimes lose connections might cause the alternator to have a hard time 

charging the battery.

Another spot to look at is the back of the alternator and make sure all the wires are plugged in and secured in your alternator. Also, make sure 

that the wires should not shot from anywhere and that it should go all the way to the battery and isn't disconnected at all then 

check for any corrosion, any harm to the wires, any frayed wires, any crimped wires, all that stuff could cause an issue. Likewise, your alternator may have an external voltage regulator so check the wires to the external voltage regulator to ensure that it's going there and ensure

 it's clipped in, and check whether the external voltage regulator is working or not. Finally, when the car is running or the engine is ON, you need to check 

that the alternator is spinning fine means you need to make sure that the belt is tight on the alternator because that's how you get your power through the alternator. 

Now the last test you need to do 

is a voltage drop test for the negative and positive sides 

of the battery fo that 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 AC ON, your radio on, and then after a while and you need to check while connecting the black lead multimeter to the negative side of the battery, and then we're going to touch the red lead of the multimeter to the alternator case, after turning ON the engine. You need to turn ON everything on your car while your black lead of the multimeter attach it to the grounds and then take your red lead of the multimeter 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 will see reading then what you want to do is you want to go and raise your RPMs to about 1500 and if you will see the reading is around 0.05 volts that means you're good to go, you shouldn't want to see a reading that is 0.1 volts or 0.2 volts in that range or greater, if it's 0.1 or 0.2 or greater then you have a problem, if your reading will come out to be around 0.05 volts so your  good to go, now let's just say that you get a reading that's 0.1 volts right so what you want to do next remove your red lead of multimeter from the alternator case and touch it to the bracket of alternator (Bracket is the place from where alternator is attached to the battery) and if you see a substantial voltage drop that means the connection between the alternator and alternator bracket needs to be clean through sanding 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 need to

connect the red lead of the multimeter to just like a bare metal part on the engine block and if you will see 0.04 volts so again you don't have a problem it's almost the same as what you read you get on the alternator case and it shouldn't be 0.1 volts, but if you have a problem 

and you will see a substantial decrease in voltage from your alternator case to the battery ground then that would be where your 

problem is, at that point, you would need to clean the connection between your mounting brackets and your engine block, and now if you don't find your problem here then the least significant point you would check is your chassis ground (Chassis ground is the place from where your engine is ground), maybe it connects to the firewall or any place inside your engine room to any bold connected to the body of the car, what you'll do next 

you'll take your red lead of multimeter and connect it to the ground and if you will see around 0.05 volts still the same and remember to bring the RPMs up to about 1500 when you will do all these tests the idea is that keep 

testing the ground connections until you will need to find where is a bad connection, 

sand down the bad connection and get it clean once you find the bad connection, so this way you'll achieve a good connection, and then ideally that will fix your concern.

Now you need to test the voltage drop across the alternator and 

the positive side of the battery. To do this test, again you need to ON all accessories on the 

car running, you'll need to bring RPMs up to about 1500 and connect the red lead of the multimeter to the red positive of the battery, and then we're going to connect our black lead of multimeter to the B+ post on the alternator (remember the B+ post has a nut). So you need to make a good connection right on the B+ post, remember to rev up the engine to about 1500 RPMs during this test, and during this test, you need to continuously 

look at the multimeter you don't want the reading to be above 0.3 volts, so you will need to get less than 0.3 volts.

 If you are getting below 0.3 volts, you are good to go, now let's just say you're above 0.3 volts then the next place you want to check is the nut of the B+ post of the alternator, and if you see a substantial voltage drop you know the the the 

connection between the nut and the stud needs to be cleaned.

 The next place you'll need to check is the lead that's coming from the alternator of the B+ post 

into that wire there's a metal piece sandwich between the B+ post and its bold. You will see it's in between the nut and the B+ post on the alternator, so just touch it with a black lead of the multimeter then we'll need to check the multimeter reading and if there's no significant drop below 0.3 volts so that means you are good to go.

The idea of doing all these tests is to keep testing all the connections until you will find where the 

connection is bad and then once you find that connection just sand it so it'll clean and you're good to go hopefully.