Second generation climate control
 

The car is a 1983 300CD but this applies to any 2nd generation Climate Control (horizontal pushbutton unit) with a monovalve heat control.

For those of you with the following symptom; AC works when first turned on, then gradually blows warmer and warmer air until full hot.

Before you start throwing expensive parts at your problem, check the seating of your monovalve cable. If power to the monovalve is lost, it defaults to fully open and max heat. This condition will gradually overwhelm the AC and result in hot air being blown from your vents. Of course, the same thing will happen if the diaphragm of your monovalve is shot.

I was ready to buy a rebuilt pb unit before I decided to get methodical and check everything. Cost of this repair = $0.00.

Third generation climate control ACCIII