Howdy David,

The ALT lamp is wired through the alternator and gets current when the circuit is open - some of this may bleed through to other lamps on the dash mostly due to old grounds on the instrument board.

That is an incorrect assessment of the circuit.

Here is a slightly doctored simplified diagram.



The ALT lamp will not glow at all if the circuit is open. The ALT lamp will glow when current flows in either direction. That is to say, the ALT lamp will glow if the battery voltage is higher than the alternator voltage or if the alternator voltage is higher than the battery voltage. Thus indicating either an overcharge or undercharge condition. The difference in voltage between B+ (battery voltage) and D+ (exciter diodes in the alternator) will determine the brightness of the lamp.

There is no "bleed through" to other lamps. The other lamps are connected to the alternator circuit by design. The reason they are dimmer than the alternator light is because they have diodes in series (dropping .7V across the diode) and the ALT lamp does not. Those diodes will block current flow if the battery is at a lower voltage than the alternator but they will illuminate all those bulbs when D+ is lower than B+. Why? I don't know.

--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 52 year old fat man ;-) -- KD5QBL