How to check if alternator is bad

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How do I test my alternator?
How do you tell if its your alternator or your battery?
Can you test an alternator without removing it?
How do I know if its a bad alternator?

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Not sure if your alternator is in tip top shape? It’s hard to tell whether or not an alternator works if you don’t know what to look for. The easiest way to test it is by using a voltmeter. If you're well in tune with your car, there are a few other methods you may try as well. This test is to give a general health of your alternator and battery. Some newer vehicles may have computer controlled charging systems and would require additional testing.

Things You Should Know

Use a voltmeter to check your alternator; rev your engine to 2000 RPM, keep the engine running, and test the battery with the voltmeter.
If your voltage goes from ~12.2 V to ~13-14.5 V, your alternator is doing well. If your V stays the same or decreases, that's a bad sign.
Alternatively, take your car into an autobody shop that's willing to test your alternator for you, free of charge.

1
Buy a voltmeter. You can find one at any auto parts store for less than $20.[1] Don't worry about getting an expensive one; a cheap voltmeter will do for this purpose.
- If you have a multimeter, you can use that instead. A multimeter measures voltage as well as other electrical properties such as current and resistance. You want to measure voltage when you're checking your alternator.[2]
2
Check the battery first. The battery is needed to start the vehicle, which, in turn spins the alternator at sufficient speed to keep the battery charged. This means that if your battery is too low, you will be unable to start the vehicle and thus you will also be unable to test the alternator, rendering any additional voltmeter-testing totally useless. If the weather is cold or your battery is old, your battery could be the issue and your alternator might be just fine. That's why you need to check the battery before checking the alternator.[3] Here's how to do it:
- Turn off the car. You'll want to make sure your engine is off before attaching the voltmeter.[4]
- Open the hood.
- Connect the voltmeter to the battery. Place the red lead of the voltmeter to the positive battery terminal and the black lead to the negative terminal. Avoid touching your skin to the battery.
- Read the voltmeter. If the battery reads above 12.2 volts, it has enough juice to start the alternator, which can then be tested with the voltmeter.
- If the battery doesn't have enough voltage, either get the battery charged and re-test or try a different method for checking the alternator.
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3
Start the vehicle and rev the engine to 2,000 RPM. This will draw power on your battery, which should cause your voltage regulator to kick the alternator into high gear.
4
Keep the engine running and retest the battery with the voltmeter. When you read the voltmeter this time, the voltage should go up to at least 13. If varying the RPMs causes the voltage to fluctuate between 13 and 14.5 volts, your alternator is in good shape; if, on the other hand, the voltage remains the same or decreases, your alternator isn’t working properly.[5][6]
- Repeat the process with the lights, radio, and AC on. The alternator is charging if the battery voltage stays above 13 volts with the engine at 2,000 rpm and all accessories on.

1
Check the alternator gauge. If you have a volt/amp gauge, it will read the alternator output for you. Run engine at 2,000 rpms for tests and turn on the blower fan for the AC or Heater, the headlamps, and any other accessories that put a strain on the alternator, and watch the gauge to see if it decreases voltage or amperage. As a rule, if the voltmeter is higher when the engine is running than when the engine is not, you can confidently assume that the alternator is charging.
2
Listen to the alternator while the engine is running. If there is a problem with the bearings you may hear a squealing sound coming from the front of the car, which becomes louder with more electrical accessories using power at the same time.[7]
3

Turn the radio on and rev the engine. Tune your radio to a low number on the AM band with no music. If the radio whines or goes fuzzy whenever you hit the gas, the alternator is quite likely the culprit.[8]
4
Find an auto-parts store that tests alternators for free. Since every store would love for you to buy your new alternator from them, many will try to get the edge-up on their competitors by offering free testing. Dismount your alternator and bring it in to be sure.[9]

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When it is very cold outside, before you start your car, Turn on your headlights for a min or two, and turn it off again. A warm battery will always start a cold car.

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Even if you conclude the alternator is not working, the problem may originate elsewhere. You may have a blown fuse, a bad relay, wiring, or a defective voltage regulator, for example.
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Some people recommend testing the alternator by starting the car, loosening the negative battery cable, and waiting to see whether or not the engine dies. Do not attempt this method; it can fry the voltage regulator, the alternator, and/or the electrical components.
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Keep hands, loose clothing, long hair and jewelry away from moving parts when checking under the hood of the car with the engine running.
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Article SummaryX

The alternator in your car is responsible for recharging your car’s battery and powering the electrical system. If your headlights aren’t as bright as they usually are, your car’s electrical system isn’t working properly, or you’re having trouble starting your car, those could all be signs that your alternator isn’t working properly. Fortunately, you can easily test your car’s alternator to see if it's working using a voltmeter, which is an instrument that measures voltage. Before you test the alternator, you’ll need to make sure your car’s battery is fully charged. Turn your car off, pop the hood, and connect the voltmeter’s positive probe to the positive terminal on the battery and the negative probe to the negative terminal on the battery. Then, check the reading on the voltmeter. If it’s less than 12.2, you’ll need to charge your battery before you can test the alternator. Otherwise, have a helper start your car and rev the engine to 1500 RPM. Check the reading on the voltmeter again. The voltage should increase by between 0.5 and 2 volts to around 13.6 to 15.8. If the reading stays the same or drops, that means the alternator isn’t charging and you may need a new one. If the reading is more than 2 volts higher, that could mean that your alternator is overcharging. Either way, take your car to an auto repair shop to find out what’s wrong. To learn how to diagnose an alternator problem by checking the gauge or by sound, keep reading!

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How do I test my alternator?

How do you tell if its your alternator or your battery?

Jumpstart your car. If you can turn the engine on, but it dies shortly thereafter, your car alternator likely isn't able to charge the battery. On the other hand, if you jumpstart the car and later on it fails to start on its own, it's likely a dead battery.

Can you test an alternator without removing it?

“With the car running, you can turn on all the heavy loads; turn on the heater blower motor, turn the lights on, etc.,” he tells us. “Then check the voltage at the battery terminals. That'll give you a general idea of the health of the alternator.”

How do I know if its a bad alternator?

Signs of a Bad Alternator Some of the things to look for are no-starting and trouble starting, dimming lights and problems with stereo system output. If your car starts but stalls when you're underway, your battery is probably not being recharged due to a faulty alternator.