A/C - Can't figure out the problem - 92 300D Turbo
 

There is something wrong with my Air Conditioning system that I cannot figure out. Any and all suggestions are welcome.

Problem: When the exterior temperature (displayed by the car sensor) gets past 85F the compressor stops working.

Troubleshooting done:
1. Replaced Relay - did not change anything.
2. I turn the ignition OFF and then back ON. Either one of two things happen: a) If now the exterior temp dropped below 85F the compressor starts working like a charm and continues to work AS LONG as the exterior temp is below 85F.
b) I when I turn the ignition back ON the ext temp continues to be above 85F the compressor stays OFF.

3. It does not matter what control button I select (High blower, low Blower, Automatic Temp Control) the outcome is the same.

Any clues?

Thanks.

Thanks ForcedInduction. For the sake of my education, could you please expand a bit more about each of the "possibles" you mentioned and how the exterior temp reflects the problem?

Super clear !! Thank you very much. Will you happen to have handy what the Optimum pressure for the system is? If not, no problem. Thank you very much again.

Update:
Recovered refrigerant to ensure that any contamination was removed. There were only 0.9 lb of R134 in the system and we charged with 2.2 Lbs (1 Kg) as per spec.
It improved some (less frequent now) but the problem still persists. Sometimes It stops cooling at ext temps above high 80's and sometimes it continues to works fine.
It seems that it is more likely to remain cooling if the blower is continuously set to LOW speed rather than high speed. If when cooling at low fan speed I crank it up, after about about 1 minute the cooling stops.
Any other suggestions?
Thanks.

A/C Pressure Switches
 

Thanks for the Wiring diagram of my 93 300d A/C. The diagram shows two pressure switches: S31 & S32. I have other documentation which shows me the location of all the other A/C components, but I can't find the pressure switches. Can you help again?

A/C pressure switches
 

Musicmaniac, in my 92 300D the switches are next to the receiver dryer bottle glass window where you can check for bubbles in the system. The top one is S31 and the bottom one (red) is S32. Here is a picture. I hope it helps.