Another alternator not alternating

[S-Car-Go]

Yesterday the stock charging system quit working. Admittedly the previous owner hacked the wiring harness, but it was working. I have all new under hood harnesses ready to go when I pull the motor in a month or so. Anyway, the local parts store says the alternator is good. I swapped out the regulator as a crap shoot, but that didn't help.

With the motor running at 2000 rpms on battery I read the same battery voltage on the back of the alternator (Batt term to Alt Grnd). Same voltage (11.5v) at Batt Term to Field. With the car off, I read the same 11.5 v at Alt Batt to the negitive post on the battery and at Alt Grnd to positive post of the battery. The car instantly dies if I pull the positive battery cable while running.

This sounds like the alternator isn't charging to me but the store says no. What am I missing?

 

[mustangdave]

Loose belt or more likely, a short in the wirng.

 

[A67StangMaster]

Check your ground make sure all the wireing is connected tight... it just takes one wire not connected solid to not to work....

 

[rbohm]

check the wiring at the regulator itself. when my merc had the very problem you have now, the wiring there was shot. also check your grounds, and have the battery checked as well. a battery that is shorting out internally will have the same effect as a bad alternator.

 

[2nd Mustang]

Including all of the above advice, I once had a bad battery that would kill the charge from the alternator. We could jump start the engine, but it would die as soon as we removed the jumper cables.

 

[SoCalCruising]

I've had:
poor ground (powder coated parts),
short to ground,
shorted battery,
and bad regulator
all causing charging problems.

 

[S-Car-Go]

Thanks for the replies gents. Unfortunatly I need a little extra help since I'm not dealing with the stock wiring harness (or even color coded wires).

So... I have battery voltage across alternator ground to pos on the battery. That tells me the ground is good.

I have voltage from the + terminal on the altnerator to the Neg battery post, so the "hot" side of the alternator circuit is okay.

The problem must be in the wiring going to the "field" terminal going from the Alternator to the regulator.
Can anyone tell me what voltage I should see if I lift the "field" wire and measure field wire to pos or neg on the battery or what the field terminal on the alternator should read, referenced to the pos or neg battery posts(wire disconnected, car running)? Or am I missing something with this logic?

Remember I have a home made harness w/o the normal plugs or splices and all the wires are black... Can't wait to put the new harness in