4000 A/C Question, Low speed fan

["Scott M." <scottmo@teleport.com>]

Andrew,

>All other Audis (and VWs) I've seen had the fan on low
speed all the
>time when the A/C was switched on. I assume yours should,
too.
>This will not affect the A/C output, but will make the
engine
>run a bunch hotter with the A/C on.

The cooling fan must run when the AC is on otherwise the
pressure in the high pressure side of the AC system (from
the output of the compressor through the condenser and over
to the expansion valve at the inlet of the evaporator) will
skyrocket.(300-500psi) 

 The cooling ability of the AC system will be compromised
as you are unable to remove the heat from the high pressure
gas and convert it to a liquid. Some aftermarket systems
have a "plug" in the high side that is designed to blow out
under extreme high pressure and I have seen this occur when
the radiator fan was kaput and the AC was running on a hot
day. The 5000 systems have a high pressure switch that
turns off  the compressor clutch if the high side pressure
goes above ~400 psi. They also have a high pressure switch
that turns on the 2nd speed fan if the pressure rise above
~200psi.

When servicing air conditioning systems in a closed shop we
often had to place a large electric fan in front of the
radiator/condenser on vehicles that used an engine driven
fan to keep the A/C high side pressure from getting out of
control.

HTH
Scott M.