Continued electrical problems.
When I hook up the battery, ignition switch off, the alternator light (red light on left of left gauge with "G") lights up. The engine is not started. The electrical is believed to be a 72.
There is also a constant battery drain with the switch on. Thoughts?
Thanks,
L. McC

Hi,
i think the plus diode of the alternator ist shorted.
Manual advice is change the diode.
Don´t know how

The way the alternator light works is one side of the bulb is hooked to switched +12, and the other to the field terminal on the voltage regulator. The alternator has electromagnets in it, not permanent magnets, so it actually needs battery power to provide the field current to make power, until it's spinning fast enough to provide its own field current. That's why the light is on with the switch on (+12 from battery, through switch, through bulb [lights bulb], through VR and field coils, to ground). Once the field current at the alternator is sufficient, the voltage potential between both terminals of the bulb is 0 (charging voltage on both sides), so no current flows through the bulb, and the light goes out. If the voltage on one side OR the other side of the bulb falls, current will flow again and the bulb will light. If, while the engine is running, the fan belt breaks OR the alternator brushes wear out, the field current will drop to 0 from the alternator, so the bulb lights up.

In this case, I'd say you have a short somewhere that's allowing battery power to bypass the switch. I don't think it's in the regulator, since there's no current coming from the alternator to light the bulb, it has to be coming from the battery, which is on the other side of the bulb from the regulator. It's possible the ignition switch itself is internally shorted, so it never really goes "off".

It's a battery light now... you have a battery.





M

I also have read that this might indicate a bad diode.