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?

[ThinAir]

Replaced the alternator today. What a pain, but it fixed the problem. Fortunately I had a spare alternator from my 72 1.7L so it didn't cost me anything.

I found the FLAP in-car test to be interesting. They have a rig that connects to the battery terminals and has a magnetic pick-up that goes on the negative cable. After running for a minute or so at 2,000 rpm it tells you if the alternator is putting out. In my case the answer was no, but at that point the tech can't tell if it's wiring or alternator. A bench test is needed for that.

Given what I've learned so far from reading Richard Atwell's stuff, I would not have trusted this test without first knowing the status of my battery and voltage regulator. In my case I know the battery is good and I had swapped the VR for a known good unit with no change in behavior so the only thing left was wiring or alternator.

Since everyone needs to do this job eventually, I took pictures today and am going to try to write up a tech article to guide the uninitiated so that hopefully the pain is minimized.

Thanks again for all the help, gang. I did not realize that my FLAP could do an in-car alternator test and I didn't want to go through all this without some assurance that the alternator was the likely cause.