I have a vanagon 1983 air-cool CA.

I would really appreciate some help to diagnose my problem , even my
mechanic does not understand it.

Here is the story:

My battery is new and read 12.6v . I have a new alternator which deliver 14V

When the van idle I read 13.71V at the battery terminal. When I start
lights and fan blower , I read 13.24V.

I tried to check if I had any leak between the battery terminal and the
alternator under load (lights and fan blower):

For terminal + , I have none , well something like -0.1V

For terminal - , not only I have -0.5V at the alternator level but also
to any ground point of the engine,  which is way too much ! ( I have
-0.1V only if I am not under load)

In order to remind how the engine is grounded , here is the pathway ,
from battery to the body using a ground strap , than from body to
transmission front  mounting using a ground strap, than from
transmission front mounting to the transmission via metal contact and/or
3 bolts and stud. than from transmission to the whole engine.

Trying to understand where my 0.5v drops come from , I checked if I had
any voltage drop between the battery ground terminal and the ground
strap terminal bolted to the transmission front mounting . I have none
(-0.04v) .  So voltage drop must happened between the transmission front
mounting and the transmission or the engine.  I plugged my voltmeter
between  the ground strap terminal going to the transmission front
mounting and the engine (any points) and I found -0.45V.

When I plugged my voltmeter  between  the ground strap terminal going to
the transmission front mounting bolt and one of the transmission bolt
(after making it bare metal), I got -0.35V. So the main culprit seems to
be the connection between the transmission front mounting and the
transmission itself. I removed the transmission front mounting , clean
bolts ,studs, washers, any point of contacts, I make them bare metal,
used deoxit D5 and di-electric grease before putting every back. I
retest the ground voltage between the battery and the engine and I got
-0.4V (instead of -0.5V) under load.

 From this point I don't know what to do and would appreciate any
explanation and advices.

I am thinking to reground the engine . If I quickly used a  jump start
cable to reground the engine , I reduce my voltage drop from -0.4V to
-0.2V. I guess I can do better with  a proper dedicated ground cable .

I would rather avoid  that option thought because I would like to fully
understand what's going on first.

Any though.

Thanks for any help.

Chris