On my fathers 79 300SD the little wheel that is used to select the temperature is not changing the temp. Instead the car is blowing cold air. Any and all help would be appreciated.

Tough system without a roadmap. You probably have an inactive system. There is an inline fuse involved that is in the fuse box on 116 chassis. Check the fuse.

To see what's happening listen closely to the servo unit under the hood. It has a clockwork mechanism inside that rotates one way or the other depending whether it wants heat or A/C. It also "parks" after the car is shut-off. This activity can be heard with the engine off or with a stethoscope.

Usually what happens is the watervalve part of the servo leaks internally and disables the mechanism through corrosion or short circuiting. Unfortunately once immobile the amplifier (behind glovebox) burns itself out trying to direct the servos internal motor drive.

This system is hard to diagnose. We built a tool to analyze it many years ago. It has the ability to separately run the servo motor (to see if it can be controlled - amplifier failure), monitor the feedback potentiometer (to evaluate the current position of the servo) and has red and green LEDs to indicate the polarity of the motor signal (allows verification of amplifiers response to temps and temp setting). A good wiring diagram and some ingenuity can duplicate these functions; sort-of.

Our usual test sequence is to evaluate whether the servo can be driven. If so we try an amplifier. If not a new servo and then probably an amplifier. Expensive!

The Servo is essentially brand new. It was changed last year.