1980 5 speed electrical issue

I have a 1980 5 speed and I recently got the a/c working. I notice that when the a/c runs it tends to drag the battery down. I drove the car a few miles on the highway with the a/c on; when the a/c is on and the car is at speed the voltage is just below 12 volts. After I got off the highway I shut the a/c off to see how the electrical system would respond. Voltage seemed to recover and would slowly get higher. When I stopped at a light voltage would drop to near 10 volts when the car was idling. When I was at speed voltage would return to near 12 volts. After a while voltage would continue to build to near 13 volts (where it is normally).

I keep the battery well charged; when I do not use the a/c the electrical system seems to perform just fine. I am wondering if I have a weak battery and should replace it or if the voltage regulator isn’t doing its job and cannot hack it when the a/c runs. I assume on this car the voltage regulator is integral to the alternator. Looking for opinions… should I replace the alternator or battery or is there another cause for this situation? Welcome any comments.

Thanks,

Glenn

GlennS;10102 wrote: I have a 1980 5 speed and I recently got the a/c working. I notice that when the a/c runs it tends to drag the battery down. I drove the car a few miles on the highway with the a/c on; when the a/c is on and the car is at speed the voltage is just below 12 volts. After I got off the highway I shut the a/c off to see how the electrical system would respond. Voltage seemed to recover and would slowly get higher. When I stopped at a light voltage would drop to near 10 volts when the car was idling. When I was at speed voltage would return to near 12 volts. After a while voltage would continue to build to near 13 volts (where it is normally).

I keep the battery well charged; when I do not use the a/c the electrical system seems to perform just fine. I am wondering if I have a weak battery and should replace it or if the voltage regulator isn’t doing its job and cannot hack it when the a/c runs. I assume on this car the voltage regulator is integral to the alternator. Looking for opinions… should I replace the alternator or battery or is there another cause for this situation? Welcome any comments.

Thanks,

Glenn

I'd check the ground cable and ground points first... Easy to do, a frequent issues and cheap to fix. Do you have trouble starting the car (i.e. slow cranking)?

Martin,

I don't usually have have problems starting the car. Is there a diagram that shows all the grounding points on the car? Or is there some other reference I can use to make sure I get them all?

Thanks,

Glenn

Glenn, I would not rely on the dash mounted volt meter. Get it checked to be sure what is going on. It just may be time to rebuild the alternator. You could have diodes going bad or something.

OK; so far it seems the best couse of action is to check the grounds if they are ok then go after the alternator.

I would suggest checking voltage at the battery *when* the voltage is showing low.

In my '90 GT, I have seen the voltage drop to zero on the dash gauge, but when I checked voltage at the battery it was still being charged. So apparently I have a dash gauge issue (or wiring related to it).