Cadillac Repair: Air Conditioning Problem, ac door motor

Question
I recently purchased a 1998 Cadillac Deville. It was, and is, in excellent condition. I bought it in early spring before the weather got too hot. I discovered that there were problems with the air conditioning and notified the dealer who sold it to me. After wrangling for nearly two months with a local mechanic whom the dealer used, we finally took it to a Cadillac dealer and had an evaporator installed. It blows really cold, but there's still one problem:  a lot of the time (more than it should) the air is blowing out the defrost vents instead of the A/C vents.
I remember that, during one of our local mechanic's sessions with the car, he said he had to replace "the control that switches it from heat to air" (which he said was inside the dash). Is there some sort of sensor that controls this? Could he have installed it wrong?  What can I do? I want to be able to speak intelligently with the dealer when I call it to his attention. Any help you can give me will be greatly appreciated.

Answer
Hi Steve,

      Behind the dash there is an electric motor that controls what mode the climate control is in?where the air comes out. And it is called the up down motor. What can happen is the motor gears strip out and then the motor is heard but doesn't move. The other thing that happens is the motor just fails completely. Less likely is that the climate control panel goes bad and the signal isn't sent to the motor to move.

      So there are a couple ways to go about this. One is to check for trouble codes in the climate control computer and it might store one that points to a defective motor. What I like to do is remove the motor and then by pushing the buttons command the motor to move in both directions. If it doesn't move I try to push it and see if the gears might be binding. The other thing I'll do is to manually turn the shaft that the motor turns and see if the airflow changes. Maybe the door is binding or is stuck somehow and a new motor isn't needed.

      Sensors are typically used to a temperature control. To make sure that the hot cold door that the programmer moves keeps the in-car temp close to the setting on the dash. The mode door motor is usually all the way in one direction or the other whereas the temperature door can be in many different positions and will automatically change are the air temp. changes even though you still have it set say for 75 degrees.

      Hopefully I have answered your question. And sorry it took so long. Bill