My battery shows 12.74 V before starting the engine on my 1.7. One running it shows around 12.90 V.... Obviously something wrong.... Bad earth...? It seems there is not a total failure of the alternator....Thoughts? Any suggestions of what and how to check??

Check resistance from your ground and positive from the battery to the alternator. They should be very low. I would look at the voltage regulator though if this issue occured suddenly.

If you have the old metal style voltage regulator I would suggest replacing it with the new solid state ones. They are pretty cheap and much more reliable.

When my voltage regulator failed I suspected it because all of my ground and positive resistances were appropriate and it was a sudden failure which I was hoping wasn't the alternator.

Plus if I was wrong, an extra voltage regulator in the glovebox isn't a bad idea.

Odds are it's a bad VR. There is a test for it that involves pulling the VR and measuring charging voltage, but that is a classic symptom of a bad VR.

-Aaron

What is that method of test??

I have 00.4 ohms when earth tested.

Got any part numbers for newer parts superceded etc?

Not sure if this is a common symptom, but when my VR went out, the red alternator charge light would never illuminate. As in, its supposed to when the key is switched to run, then go out when the car starts and stays running. Mine never came on at all.

I have 'G' light on as normal, and off as normal...

It's on pelican parts under 914>Electrical>Voltage Regulators , part 043-903-803-B-M44, here's a link also (914 voltage regulator on pelican)

Well worth it for $30, I'd get a few filters, valve cover gaskets and miscellaneous consumables while you're ordering.

How can you test the VR?

I *think* if you remove the VR and hook up a volt meter to the battery with the motor above some nominal RPM (2000 RPM is probably enough) you should see a high voltage. Something like 18V. I would do some searches to verify that and if you attempt it be sure to shut it off pretty quickly so you don't cook your battery.

Ohm readings are unreliable. You need to measure if there is a voltage difference (drop). An ohm meter will measure the resistance of a connection but not it's ability to carry current. The tiniest of wires will measure zero ohms.

I tried to test it from the procedure but I wasn't able to determine whether the VR or the alternator was bad based on the test. I forget where I found that procedure, try googling for it, but I think it might have been in the 914 book.

Are you reading voltage at idle or above 2000 rpm? he alternator does not charge much at idle. You should add an Earth (ground for us colonists). Additional Earths are always helpful on a 40 year old car.

DO NOT run the engine and disconnect the battery. This WILL damage the regulator. You can only do that crap on early Amerikan Kars...