clt1wjs

Need help! When I start the car I get readings of 11.94 volts at the battery, 11.94 at the alternator. The voltmeter reads 12 volts and doesn't move. I have 3 different new batteries that I have been trying and I have 3 alternators that are new and all test good at O'Reilly's with loads on them. I have the same result regardless of which battery or alternator I use. I have check all grounds and used wire wheels to clean surface and re connect. I have pulled the center dash and check the bulbs and found the Warning lamp/battery bulb was blown. I replaced it. I have a new ECM, new wiring harness to the starter, new battery cables, I have pulled every fuse, cleaned the terminals in the fuse box. I have pulled the wiring harness connection on the firewall and cleaned all connections. Any ideas where to look next?

TimAT

Check the terminals at the alternator with a test light. The output stud and the large wire in the connector should be hot all the time. The small wire should be hot with the key on. IIRC, 81 used a relay fed by the choke heater to feed that small wire. If that relay dies or loses it's signal, then no charge. YMMV, my memory ain't what it used to be.

With the engine running, momentarily jumper the small wire to the output stud on the back of the alternator. If that starts it charging, then you found the fault.

Jason 1979

Does the GEN light come on when you try to start it?

My 79 did the exact same thing. The GEN light was missing, it is on a wire that goes to the alternator. Took me forever to find that problem. I don’t know if a burned out light would cause the problem, but a missing one will do exactly what you describe.

just noticed that you did check the dash. Do you have continuity between the dash and the alternator?

clt1wjs

Well.... the bulb on the dash was blown. Replaced it, Battery light on dash is bright, battery is brand new and tests good. Alternator is brand new and I had it tested under load and it is good. Car starts, runs, does not charge, The battery with the engine off reads 12.56.The battery when running reads 12.32. The red wire into #2 terminal reads 12.32 while running. The Black wire reads 14.8 volts. The BATT terminal reads 24 volts. Any ideas?

Jason 1979

See if the brown wire on the alternator is getting voltage.

This is seems so much like the trouble I had with mine. I spent days tracking it down.

clt1wjs

#1 brown wire while running shows 14.8 volts, #2 Red wire shows 12.32 but continues down as the car runs.longer

Jason 1979

This seems so much like the issue I had, I can’t stop thinking it is similar.

with the ignition on, do you have 12v on the brown wire to the alternator?

assuming the wiring is correct, you would have to have voltage on the brown wire for the Gen light come on. But if your car is anything like mine, the wiring has been so hacked up over the last 40 years you never know.

the brown wire, from my limited knowledge “excites” the alternator. Without voltage there it does not come on in the car, but will check fine at the store.

clt1wjs

I decided to replace the 6 awg wire running from the alternator to the battery. When I pulled it through I found that at sometime someone had spliced it three times. All three splices had issues. Put the new wire on and it starts right up but the battery light remains on. the battery with the engine off is 12.46, volts. The battery running has 14.28 volts. The gauge shos it charging and it is. All connections are tight, belt is tight. Don't understand why the light remains on when all indication the charging system is working correctly. Checked fuses and all are good as well. Any ideas???

Jason 1979

Pretty sure the wires from the positive side of the battery are red. This may help.

found https://www.corvetteforum.com/forums...ar-is-off.html seems similar.

And this https://www.corvetteforum.com/forums...-stuck-on.html

Antz81

Do you have the service manual for your car? There's a chart to follow to diagnose the problem.

Redvette2

My 2 cents worth: Looking at the schematic provided it seems that the issue is between the light and the alternator terminal 1. Assuming that the red wire at the top connection is coming from the battery + and thru the fusable link and is a positive voltage around 12 -14 volts, then something might be miswired in the area highlighted in red below. For the light to be on the voltage at the lower connection needs to be low (assuming the top connection is positive) and not at 12-14 volts as no current would flow. Yet you mentioned that the alternator terminal 1 is at a high voltage. I would suspect somehow the light's lower connection is connected low or to ground somehow. Perhaps when the light bulb was blown the PO (previous owner) wired terminal 1 to a location to bypass the light and get things charging again.

Redvette2

TimAT

Look at the printed circuit on the back of the gauge cluster for a broken trace. After all the things you've checked or changed, that's about all that's left.

Jason 1979

I think Tim and Red are correct, the gen light is getting a ground somewhere that it should not. One thing I would point out is that the wiring schematics I have are for a 79, they are probably the same, but if checking the circuit board fails it is probably worth finding schematics for the 81.