I had this same problem in my 1982 vanagon, I solved it by adding an 8 guage ground wire from the case of the alternator directly to the unibody in the engine bay.  Be sure to clean off the paint first on the unibody and I dabed a bit of antisieze under the wire on both ends to prevent future corosion.   The old grounds were just not working well enough.  Problem solved.  I just dont think the VW engineers designed the grounds to work so well after 20 years of corosion.  
David 
1982 Westy

ate:    Sat, 17 Jan 2004 00:13:10 -0500
From:    Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: voltage over battery: 12.5-13.1

Not too good. Proper charging voltage is 13.8 volts. Does it go higher
when the engine is revved?

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Wesley Alden Pegden
Sent: Friday, January 16, 2004 11:46 PM
To: vanagon@GERRY.VANAGON.COM
Subject: voltage over battery: 12.5-13.1

Hey guys,
I've been having some dying battery issues lately.  Anyways, I replaced
my
battery under my autozone warranty.  At an idle, with no headlights or
heaters or anything on, I'm reading something like 13.1V over the
termainals.  Turn on some headlights, the heater fan, and maybe the rear
windshield defroster, and its closer to 12.5V or 12.6V.  Are these
numbers
about right?  The alternator's new (rebuilt) as well, as of just before
christmas.

thanks,
wes
'84 vanagon

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