How to Check an Alternator
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.
Using a Voltmeter
Buy a voltmeter.You can find one at any auto parts store for less than .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.
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.Here's how to do it:
- Turn off the car. You'll want to make sure your engine is off before attaching the voltmeter.
- 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.
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.
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.
- 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.
Monitoring Your Alternator
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.
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.
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.
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.
QuestionI have replaced my alternator three times, and each one has failed soon after installing it. What could be causing this?wikiHow ContributorCommunity AnswerAftermarket alternators on some vehicles are known to have this type of problem. The vehicle computer box might be controlling your charge rate to the battery and blowing alternators up due to excessive resistance. Rectifier diodes in aftermarket alternators are known to pop like this. Have the computer box checked and replace the alternator with a genuine manufacturer alternator.Thanks!
QuestionMy car keeps blowing up alternators. I have a Mustang V6, 2004. The garage guy showed me that the alternator still gives current although the car is switched off. Is that why my alternators are blowing up?wikiHow ContributorCommunity AnswerAn alternator generates current when rotating. The alternator is rotated by the engine via a belt connected to the crankshaft. There is no possible way that it is generating a current when stationary. Take it to a different garage/mechanic.Thanks!
QuestionWhat if I'm driving and my voltage gauge drops to zero, but then goes back up again?wikiHow ContributorCommunity AnswerSounds like a ground losing connection. Check all grounds around your wheel wells -- they are liable to rust, which can cause a ground to come loose.Thanks!
QuestionHow can I tell if my diodes are bad in a voltage regulator?wikiHow ContributorCommunity AnswerA quick way to check is by using a multimeter. Touch the big post on the alternator (the positive terminal post on the alternator) with the red lead of the multimeter, and touch the other lead on the alternator case. With the multimeter set to measure resistance, it should measure more than 550 ohms if the alternator diode is any good. Make sure when you do it that the battery is disconnected to prevent any electrical shorts or shocks.Thanks!
QuestionHow can I tell if I have a bad alternator?wikiHow ContributorCommunity AnswerTest alternator by connecting voltmeter on DC 20, red on positive battery terminal and black on negative battery terminal. Start vehicle, and voltage on voltmeter should increase to around 13 volts and stabilize there. If it decreases and starts to drop until vehicle eventually dies, you have a defective alternator. Check output pins from alternator too for accurate voltage readings and connections.Thanks!
QuestionToday I replaced my car alternator with a new one. I then checked the voltage when the car was idle and it was 13.5 volts and when I turned on the AC and radio it was 13.4 volts. I read on the internet that the volts should be 14.2 when idle. I am just wondering what's wrong?wikiHow ContributorCommunity Answer13.5 sounds about right actually it should only be 14 the difference between 13.5 and 14.2 is not that much especially if you're running everything. The battery should set at 12.6 without the car running. If your voltage regulator is turning down your voltage then that's a setting in your PCM.Thanks!
QuestionI've taken off the alternator and changed the brushes but still have the same problem. I charge the battery; car starts; when I switch off the car, everything is just dead. I don't know if it's my alternator or battery that is faulty. What can I do?wikiHow ContributorCommunity AnswerIt might be a parasite drain. Aside from your normal lights and other standard electronics, have you installed additional lights like LED bulbs or subwoofers? If yes, your alternator may be doing charge and use, in which case you need to change your battery to a higher amps battery in order for it to retain current after the engine is off.Thanks!
QuestionThe electrical lights and air works, but my car won't turn on. Could it be an alternator problem?wikiHow ContributorCommunity AnswerYou need to have your alternator tested. If your alternator is good, it might be the starter. If it's not your alternator or starter, you have another problem. It could be a timing belt or timing chain.Thanks!
QuestionMy alternator is working at 100 percent, but the battery indicator is on when the engine is running. What could be causing this?wikiHow ContributorCommunity AnswerCheck your alternator input into the battery with a gauge. If it's good, then likely the battery isn't holding a charge properly.Thanks!
QuestionWill my headlight not work if my alternator is faulty?wikiHow ContributorCommunity AnswerThe headlights draw power from the battery, just as ignition, audio, starter motor and everything else. If the alternator fails or doesn't deliver enough power all of these will continue to draw from the battery until it is empty. The battery won't recharge (properly) anymore, however. When the battery is discharged completely all electric equipment including the headlights (and most painfully the starter motor) will stop working.Thanks!
If my voltage reading from a meter keeps dropping and fluctuating back up, is that a sign of a faulty alternator?
Why might the alternator not be charging on a 1998 Infinity 130?
My new battery losses charge over a few days, the resistance across the alternator (off the car) is 12k Ohm from live to earth, this ok or too low?
What would cause my car to stop running after I added a new battery and starter? Is it the alternator?
Why does my car idle at 1200 rpm then drop to 750 rpm when AC is on with only 12.5v reading?
- 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.
- 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.
- 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.
- 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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Video: How to Test an Alternator ( Testing the Voltage Regulator, Diode rectifier and Stator)
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