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?

Just because it's so easy... Check the contacts on the voltage regulator. When my G light was intermittently coming on, wiggling the VR would make it go out. I cleaned the connector contacts and it's been perfect ever since.

Here's hoping you get so lucky.

check fuse #9. you may have inadvertently kicked it getting in the car. i had this EXACT problem.

check the easy stuff first bro

Well I've checked fuse #9. It's good and besides, I'm running one of those nice JWest fuse boxes with the nice screw-on covers. I've also cleaned the contacts on the voltage regulator and tried a known-good VR. It's looking like the only thing left is the alternator.

Is there a way to test the raw output of the alternator before I remove it from the car? With removal being such a bear, I'd really like to make sure first.

check the contacts on the alternator harness first. McMark had the same thing happen on the way back from RRC. turned out all he needed was to clean the contacts ...

Andy

Clean contacts for the alternator wiring harness didn't do anything either. I'm running a new Bowlsby harness so I'm sure it's good.

I looked in the Haynes & VW manuals and both describe a procedure for testing the alternator that involves a Sun battery switch and looks really complicated. Isn't it possible to put a multimeter from the brown lead to the red or green and measure the direct output of the alternator? If it is, which do I pick? I'm assuming I'd be looking for 14V. I just don't want to ruin an alternator by hooking up something wrong.

Looking at the Haynes wiring diagram, I can't really tell where the stuff from the alternator goes. I see where it all goes INTO the VR, but I don't see how anything comes OUT to get back to the battery! Someone posted diagrams of the major circuits a long time ago, but I haven't managed to find them back yet.

I'm not sure how accurate it is but you can run the car over to your FLAPS and have them test it for you.

They have a unit specifically for testing in the car.

Duh - I just called my FLAPS and they say they can do it, but the lines are long today so I think I'll put it off until tomorrow. Thanks!

So... along the way I found this topic by Brad Anders in which he states that his alternator had a bad ground. Since this is one of the possible causes cited by AA in the 700 tips book, I'm wondering what this is. The alternator is grounded to the engine, which is grounded through the tranny ground strap - if this is all connected properly, what else is there?

BTW the Richard Atwell page that Brad references is an incredible resource. I'd forgotten about his site.

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.

The problem here is corrosion. Many of the mechanical connections for the alternators ground path are dis-similar metals. Corrosion leads to high resistance so you have poor electrical connections.

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.

I fixed your link Ernie.

Thanks, Mike. Obviously I didn't realize it had a problem!