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So here's what happened to me-

Had to go into work yesterday, was driving home and all of a sudden my GEN light (It's called 'C' on mine because it's 72) comes on SOLID. No flickering, not dim, but solid. I drove home just fine, but of course I was worried. Popped the hood, and see that the cable from the battery to the alternator is disconnected. It must have been loose? Anyway this morning I reconnected everything, and the gen light is still on solid. Battery voltage is at about 12.5 with the key off, 14.5 when it's running, so I feel the alternator is in fact charging. I had an extra alt that I got from Karl off a 72 he parted out, so I put that one on. Still the GEN light is on. Again, 14.8 V when it's running, and I verified that when I start it with the electrical plug on the alt (going to voltage reg) disconnected battery remains at 12.5 V or so. Last thing I've done is R&R the ground on the engine, body, and frame. No dice. Any ideas?

I'm going to get out my magnifying glass and look at the wiring diagram now, and maybe look into getting a new voltage regulator. Unfortunately this doesn't sound too similar to what I've seen elsewhere with people and GEN light problems.

Good luck. Does the same wire that controls the Gen light on a 72 power the relay coil for the fuel pump? My 77 the fuel pump quits if the Gen light comes on. I recently replaced the voltage regulator because the Gen light was coming on, but would go off if I pulled on the one relay switch. Worked fine for about 2 weeks, but now it is coming on again.

72 should have a mechanical pump on the engine no electric. Check all the small wires from the alternator to the voltage regulator. Also check for a blown 15A fuse in the fuse box.

Fuses tested good, yeah it's a mechanical fuel pump. The battery is definitely charging, I drove to work and back just fine and the battery voltage is good. At 14.7 V when running, that's good. Still chasing down the problem, obviously. Need to spend some more time with my wiring schematic and the volt meter. Trouble is the regulator I have on there is a sealed unit, I can't take it apart to look at it so if something went wrong, I'll not be able to see it anyway. I might wait on getting a new voltage regulator in favor of just doing the conversion to the internally regulated alternator. Lastly, I also might open up the wiring harness and inspect.

I suspect the regulator is blown. If the alternator output wire to the battery came loose it allows the alternator internal voltage to spike up temporarily, and this will fry the regulator especially if it is a sealed electronic unit. The fact that it is charging 14.7V tells me that it is not regulating but putting out full voltage. After the engine runs long enough to replace the current needed for starting (a few minutes) it should drop back to 13-13.5 volts. Just replace it, or like you said replace the alternator with a built in unit. In the mean time keep an eye on the battery as it may boil out it's water from overcharging.

I concur with egg, it's the same symptom as when you disconnect the regulator altogether. I did it once while cleaning connectors and forgot to reconnect it, same deal. It would run/charge but had the "C" light.

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1976 2x4 LUV: 4-speed, 32/36 Weber, desmogged, 3.42 gears

My '76 LUV and Impulse 5-speed for sale:
viewtopic.php?f=8&t=24576

Yup, it's the sealed unit. I be gettin a new regulator, thanks guys.

I guess I could just edit my last post, but for the sake of accuracy I won't.

Didn't get a new regulator, went down to the Poop Boys and got an alternator. Asked for one off a 1983 Nissan Pulsar 1.6L, it's the internally regulated, 50A Hitachi. It bolted on (to my surprise, as it's a little bigger than the stock unit). Just had to loosen the bolt on the upper alternator bracket and pivot it up a little bit.

For the wiring, the battery wire and ground wire go in the usual spots, then I had to do something tricky. The harness on the truck for the voltage regulator had 5 wires: white, black, orange/white, green/white, and black/white. I made jumpers to jump the wire that comes from the I terminal on the alternator (formerly called F for field--now it's I for indicator light) to the white/orange on the harness, which goes to the dash indicator light. I jumped the wire on the other alternator terminal to the white wire in the truck harness, which is the voltage sensing wire and connects to the hot side of the fuse box. The black wire needs to be wired to ground although I haven't done that yet (alternator case is grounded through the frame and mounting bolts, although this isn't the most reliable).

Everything behaves normally again! Here were my tests to verify this.

1. Key on, engine not running, all indicator lamps lit. Turn key on, oil pressure and charge turn off. Get 15 V in the system. Voltage does not vary with RPM, stays rock steady at 15 V indicating regulation. Normal condition.

2. Reverse jumpers for Indicator and sense on the truck harness, turn key on (not starting engine). Charge light not on, something's wrong. The other jumper configuration was correct. Change jumpers back to original.

3. Disconnect battery wire from alternator. Key on, all indicators lit. Start engine, charge light comes on indicating no charging. Normal condition.

The 50A unit is nice! I was monitoring voltage and at nearly idle (1200 RPM or so) was still getting 15.0 V with hi beams, radio, and heater fan on.

As far as what was said earlier re: overcharging, it's not the voltage applied so much as it is the current flowing in the circuit. I still need to connect an ammeter but I would suspect since there's no voltage fluctuation with RPM that it is regulating just fine. I had one go bad once on my other truck and it output like 19V trying to keep up with no voltage regulator. In a good system with good grounds (just re-did mine) you get 14.8-15.0V when it's running.

Sorry for the long post, but I might as well document this. Pictures to follow.

I have a 76 and have the same issue. I have found there must be a ground in the dash. if I reach up under the dash a wiggle the wires ( not very professional I know ) it will go out. Has been going on for several years and just havent bothered to remove dash and fix.
Jay

You may have more problems. TAt least on my 77, the only Gen light connections under the dash are the multi pin plug to the cluster and the socket of the bulb. One wire runs to the ignition fuse and one to the voltage regulator.

I took a good look at my generator tonight. The brushes and commutator are badly worn. That leads to an intermittent problem I blamed on the voltage regulator. So I am out $35 and a couple of weeks of problems. I may be able to stick AAP on warranty.

A new alternator fixed the problem. since I had my 8 year old receipt, no charge at Advance Auto Parts. So far, i have paid for one battery for my 77 truck. It was a Firestone Forever.

Guess you didn't put in an internally regulated one...but glad you got it fixed