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Ok, brand new battery and brand new from Advance Auto, (lifetime warranty rebuilt) alternator. On first start up, it will show positive charge. When you put a load on it, ie headlights and A/C, voltmeter dips almost to the bottom of the scale. It will move back and forth randomly as you are driving, sometimes showing a charge, sometimes discharge. The old alternator was doing exactly the same thing. I need this fixed by next week, as I'm heading to the beach the week before Carlisle, and hope to have the car fixed by then so I can drive it, since I've already paid my registration.
I would put a meter on the battery and check the voltage with engine off, and then with engine running. See if your meter gets the same readings. Sounds like a voltage regulator, which is probably is the alternator, but I am not 100% sure on that. Might just be a bad new alternator. Cheers
You didn't mention what year C3 - but sounds like a marginal ground. There is an important ground cable from the frame to the engine (near the RH engine mount on my '73). I had a similar problem, and put in a new Frame to Engine graound cable with "star" washers. Also, check the Battery ground cable to the frame connection - make sure of good contact (no rust/paint in the way)
It's on a 79, and I'm trying to fix the situation where it sometimes shows a discharge, so that is always shows a charge. That's what it used to do before I started haveing problems.
Nothing added. In fact I usually have a big subwoofer box with an amp in the cargo area, and have taken that out once I first started having problems, to eliminate that as a possible source of problems.