Thread: testing alternator

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how do you test an alternator to check if it's charging properly?

I just picked up a multi-meter, I figured that is what I would need, but what setting would I even use?

I have an electric fan and if it's hot the thing turns on as soon as you turn the key to "on" and it draws all the current and then it won't crank well. This happened at a gas station today and then of course once it didn't start the first time it was that much harder to start the next crank. When all was said and done I had to get a jumper cable hooked up cause it wouldn't even crank.

I heard a good battery is closer to 13 volts than 12, according to my volt meter mine is 12 at best.

go to walmart, get a cigerette plug in battery tester, there by the battery's and cost 14.99, they show true voltage, and weather it is charging or not. I used mine to diagnose a bad regulator ( wasn't stopping charge ) because it was charging to 14.9 volts, and then the belt would squeal constantly on the alternator, changed the regulator and it charges to 14.5 volts and no problems.

A small piece of steel should stick to the back of the alt. where the shaft is.If not its not chargeing.

What make of alternator do you have?

try to push the cooling fan blades with your fingers
if they move then the belt is too loose.

If it is a Delco alternator there is a little D shaped window in the back( with built in regulator)

while it is running stick a small metal rod (nail ) to ground out the regulator
( Full field bypassing the built in regulator)
if it reads 14.7V when you are full fielding it and less than 13V when you are just running the motor then the built in reg is shot

with your meter hooked across the two terminals on the battery should read 14.7 V or so
if not
the alternator brushes or something else bad in the alternator.

when I'm full fielding it would I put the test probes on a ground and the other on the B+ post?

I think mine is a delco, it has two little tabs right next to each other on the back that I plugged something into when I was wiring it up.

Hook the meter probes red positive and black neg. or a good ground.

If it is a Delco then the 2 plugs turn the alternator on with the key
one on with the keyand the other to the idiot light and power

I tested for voltage at idle and got about 11.5 volts, my battery is not charging properly even when I drive for a while.
I couldn't really rev it up and hold the test probes at the same time because I was alone, but is this correct for idle or should it be higher?

My daily driver reads 14.5 at idle.

Sounds like the alternator isn't putting out.
If it is a Delco they are dirt cheap.

did you try to full field it?

The built in reg is cheap and pretty straight forward to install
most good repair manuals have a step by step for testing and replacement of the parts.

what do you have the alternator in - what engine - what body

Those in the know use Delco----

It is a 3 wire alternator. The prongs are on top, but I didn't mess with those.
It is a delco though.
It's on a 402 bb chevy in a 50 pickup.

How many AMPS
Did you find the little D shaped openning near the center of the back of the alternator?
hank

this is where you full field the alt. the reg. is in there.

Iunno if someone said this or not previously, but my favorite way to check the alternator is lossening a battery cable, starting the car, then disconecting the cable. If the car dies then the alternator is probly bad...

I've always heard that removing the battery cable while the motor was running is a good way to destroy the diodes in the alternator.
Anybody else have thoughts on that

I dunno what a diode is but my dad told me it would be alright. He worked for Chevrolet in the 70's so I usually trust him lol

You CAN blow the diodes by disconnecting the lead. Even in an inboard or inboard/outboard boat, you can blow the alternator just by switching the selector switch from battery one to battery two. There is that split second when there is no load on the alternator, and it spikes.

This is very simple, just go to any Auto Zone, or similar, and they will put your car on a machine that gives you a whole lot of info, and it is a free service. Why guess? Plus they will be able to tell you what your starter is drawing, how good your battery is, etc.

Done this many times, takes 5 minutes.