The 1947 - Present Chevrolet & GMC Truck Message Board Network

This is my first ever post, but not my first site visit. I have used this forum throughout the rebuild/restore of my 1970 chevy c10. It has been a HUGH help. I wanted to return the favor with some information about a problem I had with my generator light staying on.
The light would come on dimly with the switch turned to on and engine off. When the engine was started the light would become much brighter. I initially thought the field coil of the regulator was bad after running tests from a service manual. But replacing the regulator didn't solve the problem. After two or three days of troubleshooting, I traced the issue to somewhere within the cab. The charging system was working fine as determined by numerous voltage measurements under the hood. I ended up chasing voltages all the way to the instrument panel where I found that the light was not receiving matching voltage to each bulb connector to enable to light to go out. After removing the instrument panel for examination, I found one of the copper tabs on the printed circuit cracked at the connector socket. It looked fine until you bent it into the position it would see when the connector was installed, then it would separate completely and break the circuit. That lead also supplies power to the other idiot lights and fuel gauge.
To be a little more specific, when the switch was turned on, the bulb would light using the initial alternator field excitation voltage supplied through the resistance wire. That was about 3.6 volts. Hence the dim light. After engine starting the regulator field coil contact is supposed to enable battery voltage (actually it seems to be alternator output voltage of about 14.7 volts) to the applied to both sides of the bulb resulting in no net voltage across the bulb so that it can go out. The wiring diagram was not really representative of how the wires were configured in the truck and drove me nuts for a while until I traced them down individually.
Anyway, I ordered a replacement printed circuit and will install and test when it arrives. I fully expect the bulb to work correctly. Another telltale you might have a similar failure is to note whether or not the other idiot lights and/or your fuel gauge aren't working.
So if you have a generator light staying on problem, I hope this might help you resolve it.