All I can say is that I am surprized too, but I have totally dismantled these
early systems right down to the last wire and there just isn't any switches
in the system to turn the compressor on and off due to system temp or
pressure.  I have seen '82 Rabbits with the ambient air switch (none on the
van system in '85).

I think the main reason that the van system is so simple/primitive is that it
was done as a dealer install after the vans were shipped over to the US.
This is the only reason I can think of for making the system this way.  After
'85 all the newer vans had a more advanced A/C system, like the ones on all
other cars (ambient air switches, high and low pressure switches, etc).
Yet another VW well thought out design :)  But these systems seems to work
rather well in spite of being primitive and I have never had a problem with
one of them icing up the evaporator.

Just FYI,
Ken Wilford
http://www.vanagain.com
John 3:16

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