Alternator Light ON most of the time, How do I determine what's wrong? Options

[ThinAir]

I HATE electrical problems!!!! Okay, with that out of the way... here's the deal.

My car went 800 miles round trip for Red Rock Classic with no problems at all. When I shut the engine off in the driveway I had no glowing alternator light. A couple of hours later I start it up to move the car and the alternator light is on. I ran the car some to try and determine if it was really on or just glowing and discovered that the light went out and seemed to be tied to the engine rpms.

Well I've tried to tighten the belt, but have gotten no different results. Most of the time when I drive the voltmeter is sitting just under 12 volts and the light is glowing dimly, but if I am idling the light is on strong. Once in awhile the light will go out and the voltmeter will show 14 volts, but there is no consistency to it.

The AA Tech Tips book says this:
Condition:
Light off with key off
Light on with key on
Light glows dimly when car is running
Problem:
Bad diode in alternator or bad ground

Since tightening the belt has not made a difference, it seems like it must be one of these things. The question is how do I tell what it is?

Since the car starts consistently I know that the tranny ground strap is well connected and that's been confirmed. Since the alternator grounds to the engine case, where else could there be a bad ground?

[BMartin914]

Ther alternator also grounds through the brushes. If your light is coming on, and all of your connections were good, it would be a possibility that the brushes in the alt. are worn enough that they are no longer grounding - turning on the light. May have been what happened to ernie.