The car in question is a 1988 760 turbo. The heater/AC controls are completely different than on the 740s that I am familiar with. I have posted this on the 760 forum with no response, so I am cross posting it here. Although the fan motor sounds like it working hard, the airflow coming out of the vents is weak. If the selector is set to the dash vents, the fan does not run at all and if the selector is set to dash and floor vents, air only comes out of the floor vents.

I pulled the heater control unit and measured voltages on the selector switch. When the dash vent was selected, the voltages on about 3 different contacts were 0.24 - to 0.3 volts. Selecting anything else gives readings of over 5 volts at various contacts depending on what is selected. The low voltage readings occurring and the the fan not running when the dash vents are selected leads me to believe that this is an electrical problem rather than a vacuum motor problem.

Any suggestions?

How difficult would it be to replace the whole thing with the vent fan and temperature controls from a 1985 740 turbo? If I remember correctly the vent selector "switch" on the 1985 turbos is actually a vacuum valve setup instead of the electrical switch used on the 1988 760 turbo.

I noticed wires going to what I assume are the vacuum motors that operate the flaps which control the airflow. Are these sensors that detect the position of these flaps or are they solenoid valves that control the vacuum motors?