How to test an alternator?

VeloceMitch · #1 (**permalink**) 03-21-2008, 06:12 AM

I am pretty sure my alternator is afaulty, although it could be the voltage regulator.
Basically it's stopped charging the battery, I loose power after driving with electrics (Lights etc) running after half an hour or so. The car misfires and the light go very dim, eventually the battery will go flat.

The alternator warning light doesn't come on, but I suppose this might be a blown bulb or loose wire.

I have just done a fair amont of welding on the car, but the battery earth was removed, I know alternators can be dmaged by the arc current, but surely not with no earth lead on?.

How can i tell if it's the alternaor itself or something else?.

kjp · #2 (**permalink**) 03-21-2008, 07:19 AM

I had a similar issue a couple years back. One of the three wires going to the alternator had shorted due to what I believe was heat from the exhaust manifold which was close by but never touched. Check these for continuity back to your regulator, and replace if needed. Might not be what you are experiencing here, but may help.

5yearplan · #3 (**permalink**) 03-27-2008, 07:00 AM

You can test the voltage with a volt meter, test the battery voltage while the car is running. If you have a volt meter with an inductive pickup, you can test how many amps it is charging. If it is charging less than 13.5 volts, but the correct amperage (its like 50-75 amps above 2k RPM I believe), then it is the voltage regulator, if the amperage is wrong than it is the alternator, and usually they can be rebuilt as all it would need were new brushes. Or call your nearest auto parts store, here in the states most any parts store will test it for free.