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Heat/AC/Fresh Air Flap

   > 2. Using the A/C while running heat at defrost works almost like
   >    magic, it is really effective. I've had it on three previous 
   >    ( yes Japanese ) cars, and really miss it on cold damp days.
   My '86 5000s does this (A/C on defrost in the winter). Why doesn't
   yours do it? I have the 'newer' control box meaning it has fan speed
   override controls for 'low' and 'high' rather than 'Auto low' and
   'Auto high'.

   Bob Kunz

'Cuz Audi knew better back then...

Seriously, the whole thing is [or appears to be] a simple mechanical
system (but who knows what nefarious relays are lurking in inaccessible
nooks and crannies...). Left (temp control lever) turns on AC, right
turns on heat (wire/cable to mechanical valve in engine compartment).
Two levers to control air up (windshield) and down (footwell). Four-
speed fan (with relay control) such that:

   ignition (key switch) power activates load-reduction relay;
   which activates AC power relay;
     passes power to heat/AC four-speed fan switch;
     passes power to AC microswitch which:
       activates AC dual relay which:
         passes power to AC clutch relay/thermostat/switch/etc/soforth;
         passes power to heat/AC fan speed "low" irrespective of switch;
         passes power to high-speed-radiator-fan (*)

(*) I just rewired this to instead control the high-speed-fan RELAY, saving
    about 8 electrical feet of voltage-dropping wire plus several voltage-
    dropping connections, all told winning me back about a half-volt to the
    fan...(and having the interesting -- and, to me, desireable -- side ef-
    fect of having the radiator fan current *ONLY* flow through the radia-
    tor fan fuse, and not sometimes timeshare with the heat/AC fan fuse!).

and somewhere amongst that morass of "logic" exists a vacuum servo
which controls the fresh air flap. NOWHERE in any manual/diagram/etc
can I find *ANY* mention of what controls this bloody vacuum servo!
I would presume that one of the above mentioned relays eventually Tees
off to control a vacuum switch to the servo, BUT WHERE???