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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
to adjust the air flap and correct the overly warm air temp. I had seen
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
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.