Thanks for tips. David is right, because the fridge relay runs off the alt D+ circuit it isn't 12V until the alternator starts charging. If the fridge relay is disconnected and one disconnects the blue D+ wire from the alternator then the van end of the wire reads 12V when ignition is on.

After some probing I found that the D+ terminal on the alternator connects to the internal tab on the voltage regulator.

I have two regulators, a Bosch and an unnamed aftermarket. The Bosch ran for 15 years, then brushes wore a bit short, so I replaced it in the last trip with the aftermarket, that I had pulled off another alternator (but given the brush length must be near new). This worked for about 400km while I drove home, but quit in the next trip.

To try if it works with the Bosch, I pulled the brushes out a bit further (they seem to hang by the wires as usual, so wires can be stretched a tad). With the old Bosch regulator it charges again. I guess I'll buy a new Bosch regulator and drive around locally for a while and see if that was the problem.

I expected that if I test continuity between the D+ and ground when the motor off and alt unplugged it would be near 0 Ohm. After all the D+ wire is supposed to energize the field coil. However my continuity tester shows open circuit using either regulators, so there's maybe something less obvious going on, like a min voltage needed to trigger the regulator.

Martin

--- On Sat, 5/14/11, David Beierl <dbeierl@ATTGLOBAL.NET> wrote:

> From: David Beierl <dbeierl@ATTGLOBAL.NET>
> Subject: Re: Voltage at alternator blue D+ lead?
> To: vanagon@GERRY.VANAGON.COM
> Received: Saturday, May 14, 2011, 7:32 PM
> At 07:01 PM 5/14/2011, Poppie
> Jagersand wrote:
> >My alternator is not charging. After various tests I
> found that the
> >voltage on the blue D+ wire to the alternator was 5V. I
> looked in
> >the alternator trouble shooting section in the Bentley
> manual. It
> >doesn't list the D+ voltage. Isn't it supposed to be
> 12V? Could
> >something be loading it down? (I already disconnected
> the fridge
> >relay which I believe also runs on this circuit.)
> 
> It won't be +12 until the alternator starts charging.�
> I can't get to
> mine now to check the voltage but +5 seems not
> unreasonable.� Pending
> someone coming up with an actual reading, if your ALT light
> is the
> same brightness as usual then I think D+ is probably ok
> (when it does
> come up to [charging voltage] the ALT light will go out; in
> the
> meantime there's a resistor parallel to the light which is
> supplying
> maybe 100 ma to that terminal to allow the alternator to
> begin charging).
> 
> Yrs,
> d
>