O.K. Here is my question. On a vehicle with an alternator light (GM),power from the accesory side of the ignition switch goes through the alternator light bulb to the field terminal of the alternator. On my 1936 Pontiac with OHC6 and a standard internal regulator Delco alternator, I have to rev the engine past 2000 RPM to get the light to go off. Why?
I had this very problem on a car of mine and it had an easy fix.
The alternator uses the dash idiot light to tell it when the car has been turned on and the alternator should produce current. It seems to be somewhat sensitive to the amount of current through the dash light circuit. I substituted an LED lamp replacement for the light bulb, and the super low current consumption of the LED wasn't enough to "start" the alternator! I too had to rev it to 2000 or so. It was very repeatable and predictable. Once I'd revved it and the lamp went out, it would charge correctly, even down at 700 rpm idle.
Switching to a higher-current dash lamp ought to fix it. Sounds silly, but easy to do!
