# Testing whether alternator working.



## zhong (Aug 1, 2010)

Can one test whether your car alternator is working or not if you don't have an alternator tester? Can test using voltmeter?

If so, how can you test?

Thanks


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## kendallt (Feb 21, 2007)

Remove the belt, spin the alternator, it should spin easily. Turn key on, spin it again, if it feels 'notchy' it's working. 
If it feels notchy with the key off, the diodes are bad and it will drain your battery while parked. 
The notchy feeling means that the field is energized and it's in charge mode.

Traditional method is to pull the battery cable while vehicle is running, if it dies, alternator is not charging, if it keeps running it is charging. Modern cars have so many electronics in them that a surge from doing that could cause damage though. 

There is also the screwdriver test, take a screwdriver and place it near the back shaft of the alternator, Some parts of the alternator become magnetized when it is working that aren't magnetized when it's not working.


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## Basementgeek (Feb 7, 2005)

Hook up your voltmeter to the + and - of the car battery, read the voltage. Should be 12
volts DC or more. Now start the car, with it running read the voltmeter again. It should
about 14.4 Volts. Now, with running turn on your headlights, read the meter it still should remain about 14 volts DC.

What ever you do, do NOT disconnect the battery with the car running.

BG


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## zhong (Aug 1, 2010)

Basementgeek said:


> Hook up your voltmeter to the + and - of the car battery, read the voltage. Should be 12
> volts DC or more. Now start the car, with it running read the voltmeter again. It should
> about 14.4 Volts. Now, with running turn on your headlights, read the meter it still should remain about 14 volts DC.
> 
> ...


So if lots below 14 volts reading means alternator not working?

Thanks


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## Wrench97 (May 10, 2008)

kendallt said:


> Traditional method is to pull the battery cable while vehicle is running, if it dies, alternator is not charging, if it keeps running it is charging. Modern cars have so many electronics in them that a surge from doing that could cause damage though.


In modern vehicles this can cause voltage spikes and damage electronic controls I would not recommend trying it.

Test the voltage at the battery car running, head lights on heater fan on high, at a idle you should see between 13 to 14.6 volts.


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## kendallt (Feb 21, 2007)

Wrench97 said:


> In modern vehicles this can cause voltage spikes and damage electronic controls I would not recommend trying it.
> 
> Test the voltage at the battery car running, head lights on heater fan on high, at a idle you should see between 13 to 14.6 volts.


 That's why I said "Modern cars have so many electronics in them that a surge from doing that could cause damage though."
You can have an alternator that charges perfectly,but it can still be the reason you have a dead battery if diodes are bad or if it's stuck in 'charge' mode. 

Using a voltmeter will tell you if it's charging, but the other methods I mentioned will tell you if the alternator is bad without pulling it for testing.

Edit:
Screwdriver test works because the field coils become magnetized during charge. If it's stuck in charge mode, the coils are always energized so when the key is on, with the belt off it feels notchy when rotated by hand. Often, with belt off and someone turns the key on, you will see the alternator pulley drop into the nearest notch.


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## wolfen1086 (Oct 4, 2007)

zhong said:


> So if lots below 14 volts reading means alternator not working?
> 
> Thanks


Yes, or at least only partially working


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