'86 coupe digital dash problem

Same thing almost

Mine is doing almost the same thing. The radio had an electrical whine that went up and down with the engine RPM. I had the car (86 vette) in for a front alignment. When I left the shop I noticed the radio wasn't making the noise anymore. About 10 minutes later I noticed the voltage display was reading 10.5 volts and the switch was set on coolant temp or whatever the other side of the toggle is. When I switched back and forth it would always revert to the low voltage readout. Being a hot day I had the A/C cranked on max, by the time I got home the volts were down around 8 volts. Checked the battery with a meter and was 8 volts, checked the alternator and was also reading 8 volts. Alternator should be reading 14+ volts as far as I know when the car is running. Picked up a rebuilt alternator, put in a fully charged battery, hooked everything up and.........I get 12.3 volts at the battery and alternator when running, my digital display reads 11.7 volts. It should be reading 14+ volts at the alternator. Bad alternator rebuild?? or did something kill the rebuilt alternator instantly? Makes no sense to me, Any ideas anyone.

Well, I got mine resolved. Took the rebuilt alternator back to the rebuild shop, they bench tested it and it was producing a charge, even though my voltmeter didn't show it. The rebuild guy suggested a wiring problem was most likely the issue. Since my hood prop is gone and the alignment shop had used something to prop the hood, I started looking at wires etc. What it ended up being was the small brown or black wire in the plug at the alternator(my plug has a big red and the brown or black one) The wire had been pulled enough out of the plug that the metal contact in the plug wasn't touching the pin on the alternator. I plugged in the plug and carefully pushed the wire back in a little at a time using needle nose pliers. Started car up, volts reading 14.1, dash lit up normal, no more BS. Good luck with your problem, hope you get it resolved.