Just got this news today
All, I was getting my AC serviced this morning thinking that I had lost enough R-12 since my last recharge 3 years ago so that the compressor would not engage unless the defrost setting was used. With the full charge and push button in AC the center vents give good air flow when in AC mode with nothing from the defrost vents but the compressor still does not engage. When we switch back to defrost, cold as a meat locker (rare as it may seem from a 87 190D). As I have come to understand this the defrost settings bypass the sensors and control of the compressor is always on usually a good supporting clue of needing more R-12 with a supporting visual of bubbles seen in the top of the reciever/dryer. The good news was I was only 12 oz short of a full charge, bad news is I am trying to determine, vacum block problem, potentiometer, or push button unit. Since I now live in the central florida I like the sounds of the resistor bridge that makes the right value for the output leg thus forcing the mode door to be in the correct position. Most of this system has been replaced with the last AC service with the exception of the vacum block, potentiometer, and push button unit. Where do we start?