HVAC system output goes to defrost and floor upon acceleration

[David Michael]

Ed,

You almost certainly have a fresh air flap servo that has broken off the
mount and become disconnected from the vacuum line. I had this exact problem
a few months ago and had to install the Audi fixit part and a new vacuum
line.

Remove the passenger side center console side panel and the pull apart the
vacuum line connector from the controller to the servos. See if the red (I
think) line holds vacuum using a mity-vac. I bet it does not.

See
  http://members.aol.com/c1j1miller/climate.html#Fresh%20air%20flap
and
http://www.sjmautotechnik.com/trouble_shooting/ac.html#recirc

Whats happening is that the "default" position for the flap is recirc - so
when you 1st start the AC, it's in that mode. When it decides to switch over
to "fresh air" the controller switches the vacuum a disconnect hose, big
vacuum leak, dash vents close and you can't get any air out of the dash
vents unless you are coasting downhill creating lots of vacuum.

Installing the Audi repair part is not trivial if you have not done it
before. It took me a while to figure out how to get the screws that hold the
repair bracket in.  I ended up using a pin-vise to pre-drill a pilot hole to
get the screw to grab.

Good Luck

Dave
90 200QA 220k (resurrected)