91 Chev Pickup, Manual A/C Control Head and Actuator. Testing these is easy with your Labscope, I plot the feedback signal from the position sensor in the actuator as it is commanded from the control head. Or you could just use a DMM and watch the voltage change.

The First image shows the voltage change from the position sensor as different modes are selected on the control head. The plot shows the response from the actuator as it moves the door to different positions. From this we know the motor is operating, and the motor is moving the position sensor gear. As long as Air is moving from different outlets, all is well. The next image shows the temp door actuator, and as different temps are selected, there is no voltage change. The problem with this was no temp change when moved from cold to hot selection, and now we know why, there is no voltage change on the plot indicating any movement of the position sensor. Next we check output voltage to the actuator from the control head (was good) then check for open motor circuit (was open).

Removing the cover shows the guts of this thing, the red gear is the position sensor drive, the schematic shows the control circuits.

This commutator looks slightly burnt, and was all that looked like a problem, everything else checked ok.