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5000TQ Heater control flap/bowden cable/motor problem


One problem that I ran across with my previous 86 5000T AC system was the 
servo motor controllling
the bowden cable that adjusts the air flow would sometimes stick in one 
position. If I gently
touched the lever all of a sudden the motor would respond and begin to move 
the cable
to adjust the air flap and correct the overly warm air temp. I had  seen 
similar symptoms 
as you reported where the inside temp would  continue to climb even when 
set to only 
70 or 72 degrees. Of course you should check and verify that the bowden 
is not binding before looking anywhere else for problems.

As you may know these AC systems were used in the GM Cadillacs for awhile. 
checking the cable, I  suspected the motor was bad and  went to check  on 
a new one from a friend  who was working for GM as a training instructor. 
reported that in some cases the motors internal plastic gears that drive 
the lever 
are stripped or the  motor brushes can also be worn out. 

He also pointed out that GM had a service bulletin that indicated that at 
times the variable resistor
(potentiometer) that is mounted on the servo motor to indicate lever 
position would go open
 circuit due to corrosion  on the internal resistive wiper contact and that 
the controller box 
(amplifier) would then "think" the lever was in the correct position even 
though it was not. 
By touching the lever I was reinitiating the resistive wiper contact which 
then caused the controller
to see the correct position voltage and cause the motor to start up and 
move the lever.

GM recommended soldering a 680K resistor on the position potentiometer 
across the center (resistive wiper) terminal to one of the other terminals 
(either the ground or +12). This resistor would create a voltage drop 
across the two 
terminals in the event the center wiper terminal ever went open circuit 
this voltage would start the motor turning until the wiper would again make 

If your motor is bad you can sometimes find a replacement from a GM
Cadillac system. You can just remove the Audi lever from the existing motor 
and then install it on the replacement GM motor. You may also want to add
 the above mentioned resistor to prevent future problems.

Scott M.