I've had a strange charging problem come up in the last couple days, and I just can't figure it out.

What the problem is, is that I'll run the car for awhile and it will be fine, around 14v or so, and then out of nowhere, it will just tank... down to like 11V, as if the alternator is doing nothing at all, then a minute or two later, it will quickly jump back up to 14v, where it was before.

I know its not the guage, because all the lights dim when this happens. The 'choke' light does not come on when this happens, and the alternator tested fine. The battery is also now dead. I checked all the wiring on the alternator and battery, and its all fine.

What could possibly cause this to happen?? I need to get this fixed ASAP!

 

Checked the brushes and holder (and slip rings) lately?

 

Its a new alternator... less than 1 year old. Would they really be bad already?

 

Just checked the brushes and holder, they seem just fine to me. The brushes slide freely and they have barely any wear on them. The alternator itself tested OK. I'm sure it must be something else.

 

Something intermittently putting a BIG draw on the system?

Maybe there's something wrong with the alternator field coil wiring (car's wiring harness to the alt.)?

 

How would I check something like that?

 

Can anyone help with this?! How can I check the wiring outside the alternator? I'm pretty sure thats the problem.

 

 

Ok, I just tried giving it a boost, and nothing... cranks just as slowly as ever. I have this feeling my battery is now frozen. Does this sound right?

Thanks for the diagram Vader, but how do I trace the problem down to a specific part?

 

If the battery is frozen, do not charge and do not attempt to jump start it. Thaw it, then charge it to full state if it is not already damaged.

As for diagnosis, you should have a constant 12.6V or more at the BAT stud and 'A' terminal for the regulator, and a nearly 12V supply to the 'B' terminal whenever the ignition is on. If the ground connections, battery connections, and regulator supply to the alternator are good and consistent, there isn't much left but the alternator/regulator assembly itself, or the belt drive system for it.

 

Once the generator has started charging it will not quit even if it loses the connection at terminal B.
To check for integrity of the gage 2 wire you could temporalily hook up a lamp from inside your car to the BAT terminal. If this lamp follows the rest when the fault occurs then the wiring is ok. If it doesn't dim along with the rest then there's an intermittent break in the gage 2 wiring to the battery.
Another test is to disconnect the resistance wire, and connect a small lamp (inside car) to terminal B. You may have to rev the engine to get the generator going with this connection. Then the test lamp will be bright and the Volts indicator lamp will extinguish. If there then is a (thermal) intermittent break in the field wire, the Volts indicator lamp and the test lamp will be in series and they will both glow with half brightness.
I have a feeling that there is a fault in the field that only shows itself when the rotor gets up to temperature. It gets most hot at low rpm's.