Alternator light stays on despite no keys in ignition.

[patrickt]

Originally Posted by Frentzen

Hi,
When we hook up the battery, the alternator light comes on and stays on despite no keys in the ignition. We had the alternator and its diode tested at a local alternator shop and it checked out fine. We found that the blue wire is hot at the voltage regulator all the time. Any suggestions as to what our next step should be would be greatly appreciated.

The "classic" voltage regulator has four connections to it: I, A, S and F. The charging light is wired between the I terminal and the ignition switch. When the ignition switch is turned on (but the car is not running), the light receives 12v+ from the ignition switch and the path to ground is through the I terminal on the voltage regulator. When the engine is running, and the car is charging properly, the I terminal is no longer a path to ground and becomes energized itself -- so the light goes out. On very rare occasions, the positive output from the I terminal, when the engine is running, can be so high as to light the bulb, but you can forget that for now and instead figure out from which side of the bulb the 12v+ is coming from when the ignition switch is off.

Pull the connector off the bulb and put a VOM lead on each side of the plug, with the ignition switch off, and tell us which side of the plug is getting 12V+. Is it coming from the "ignition switch" side or is it coming from the "voltage regulator" side. Once you know that, you can then explore as to why the circuit is wired up incorrectly. At first blush, I would say the bulb's lead to the ignition switch is on the wrong terminal of the switch and is getting power all the time instead of being switched. But we have to check, we can't just guess.