"I don't understand how the light could glow without you seeing the problem on your voltmeter, whether or not you have the failure I'm describing."

I think Charlie142 is into something with his diode idea. Because the Batt bulb is "comparing" voltage from 2 sources:

1) the output from the 3 "Exciter Diodes", which feed the D+ or 61 terminal.
These exciter diodes are in parallel with
2) the 3 main positive diodes feeding the main B+ output (Gary's metered voltage)

All 6 diodes should be at the same potential. That's what puts the light out—the same voltage on each side of the bulb.

My hunch is that something in the alternator is causing the exciter diode output to be "stuck" at a level lower than the main diodes—and
the difference increases with revs, making the light brighter.

I would compare the D+/61 output to the B+ output, looking for a difference that increases with rpms.


--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.