You didn't say it was unplugged when you measured the voltage. Was the ignition switch on or off when you had no voltage there? With it plugged in, if the battery light turns on with the ignition switch on, engine off, that circuit is working. There is nothing left but a bad generator. With the battery light on, you can expect to find less than 2 volts on that wire. If you find 0 volts and the light is on, that wire has to be grounded someplace. That 0 volts will tell the generator to turn off. If that is what's happening, you can prove the generator is ok by running a wire to a small 12 volt bulb, then to that terminal in the generator. That bulb will act as the dash light, minus the short. From your description, it almost sounds as if the short could be in the voltage regulator itself. The clue would be finding 0 volts on that terminal instead of 1 - 2 volts, and even that test bulb would stay on. Your observation that the dash light never turns on when the connector is unplugged from the generator indicates the wire itself is not grounded, and if a short does exist, it is likely in the built-in voltage regulator.
When the system is working properly, the voltage regulator will apply 12 volts back onto that small wire so you will measure over 12 volts on it with the engine running. That's what turns the dash light off.
This is a real terrible design and it is common to go through four to six in the life of the car. What more and more professionals are finding out is to reduce repeat failures, you must replace the perfectly good battery at the same time as you replace the generator. It will work fine in pre-1987 vehicles, but these newer generators develop a lot of voltage spikes. As batteries age, they lose their ability to dampen or absorb those spikes. I don't mean to imply you have to change the battery RIGHT now. I mean you should consider installing a new battery once you get the charging system repaired.
Sunday, December 5th, 2010 AT 4:43 AM