A/C servo Motor, was Dave the air lock is open

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

Mike,

As others have mentioned, the A/C system in the 5000/200TQ
is a system derived from the Harrison CCOT system used in
General Motors cars
(Cadillac?,  a little help here Graydon?).

I replaced the servo motor/potentiometer on my 86 5000T
using a  motor/pot assembly from a GM Cadillac system. I
removed the lever arm from my old motor and pressed it on
the Cadillac motor/pot assembly which was identical to the
one used in my Audi (except for the lever of course).
Another problem that shows up besides the motor/gear
malfunctions you mentioned is the position sensing
potentiometer can go open circuit because the wiper contact
inside the potentiometer gets a buildup of oxide from lack
of movement. This causes the servo motor lever arm to not
move and does not move the cable that adjusts the inside
air flow/temp. Sometimes gently pushing on the lever arm
will get the motor to start moving. 

When the potentiometer goes open circuit, the op
amp/comparator used to monitor this pot voltage "thinks"
the lever arm is in the correct position and does not try
to run the motor to move the servo arm (cable adjusting air
flow). According to my GM contact, GM had a service
bulletin that recommended soldering a 680K ohm resistor
across two legs (connections) on this position sensing
potentiometer. This 680K resistor will ensure there is a
voltage difference at the input to the op amp/comparator if
this pot goes open circuit which will get the motor turning
until the pot wiper contact is re-established.  A little
contact cleaning spray could help clean up the pot wiper as
well.
My A/C system temp control finally worked as expected for
several years after installing the motor and this resistor
mod.

HTH
Scott M.
CSJGSJAMATMTSEAMCCSCPSUCSBSEEHSMENPPMMPMMOEAM yada yada
yada.....

> Flap which redirects air from default mode
>(defroster only) to dashboard outlets still not working -
something else to
>look at - also, little motor which moves heater control
valves has a slight
>case of stripped gears ($%*#@^&^!)
>Anybody have: bad motor with good gears?
>how to get gears and potentiometer on bottom of motor into
>correct positions? (Methinks this could be a "balanced
bridge" system - the
>motor moves until the variable resistor on the end of the
shaft matches the
>resistance of the inside air temp sensor thermister,
whereupon the "wein
>bridge" is balanced and the controller logic shuts off the
motor. As the
>inside air temp changes, the logic moves the motor in the
correct direction
>and distance until it balances again?