Chevy & GMC Vans

Hi guys, sorry to bother you (again)...
I swapped out the alternator and belt tensioner on my '92 G20 van (350 engine) - and still no joy.
I came to the conclusion that there is a short somewhere in the electrical system: I connected a fully charged (12.5 V) battery and measured the voltage over the poles. I get fluctuating values between 0 and 3 Volts. Disconnected the plus pole and measured again: steady 8.54 Volts.

Any thoughts? I don't have a clue about car wiring - I can wire model railroads but not my van...

Thanks, Erhard

Erhard wrote: ↑Fri Nov 01, 2024 10:33 am measured the voltage over the poles.

You mean the battery posts?

Erhard wrote: ↑Fri Nov 01, 2024 10:33 am I connected a fully charged (12.5 V) battery ... I get fluctuating values between 0 and 3 Volts.

So, to clarify, your battery measures 12.5V outside of the vehicle, and then as soon as you hook up the cable terminals, you measure 3V on the battery posts? Meter is set to DC Volts, not AC?

Erhard wrote: ↑Fri Nov 01, 2024 10:33 am Disconnected the plus pole and measured again: steady 8.54 Volts.

disconnecting a terminal means there is no circuit. This should be exactly the same as measuring the battery outside of the vehicle. Sounds like a dead battery.

Have the battery analyzed with a proper load tester. Most battery dealers or auto part stores will do this as a courtesy.

Measured the battery over the posts? -> Yes
Meter is set to DC -> Yes. I get fluctuating values between 0 and 3 volts over the posts. When wiring a model railroad, this behavior indicates a short...
no circuit -> I know. Just wanted to verify if something is draining the battery which turned out true. The battery is brand new and can be charged without problem. I charge it slow at 2 Amps.

You shouldn't assume the battery is good. Voltage is not a good guarantee, and "charging without problem" isn't either. Neither is being new. You could have been sold a bad battery. If you want to diagnose your problem properly, and eliminate something as a cause, you need to be thorough.

If you had a short , you would see some big sparks when you connect the battery and some components or wires would be getting very hot, including the battery.

How long had it been connected when you measured the 8.5V? A healthy battery doesn't just fully discharge without some signs. Try disconnecting the alternator since that's something recently worked on.

Make sure the battery posts are clean and the terminals are tight. If you still have trouble, try another meter.