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.

 

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?

 

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.

 

I looked at my 93 and it looks nothing like your picture. A few days ago I took it on a long drive on the interstate (about 1 hr) it practically froze me out (45 degree air coming in always - with 80 degree ambient). Before I started, I noticed that neither of my AC fans were on. Do you know what would prevent BOTH from coming on. Obviously the compressor clutch was pulled in because the air was working. I think it must be something in the fan circuit.
I'm getting ready to give this car to my grandson, but I don't want it to be in too bad a shape when I give it to him.

Thanks for your help.

 

Thanks to your picture, I finally located S31 and S32 (a little different location from your 92). In your picture you labeled a temporary bypass of S32. What is this used for (anything labeled temporary makes me think it would only be for testing)?

Thanks

 

Ok, I'm glad you located them. When I said temporary, I meant that I was going to leave it bridged until the time the refrigerant leaks out of the system and at that point replace the S32. I just did not want to waste the charge. So far, the system seems to be holding the charge very well so my "temporary" may stay there for a while :-). Another thing you need to be alert is that sometimes the overload relay (the one with a red fuse on the top) may get loose or the connections that come to it may be loose and that will shut the compressor off. Fortunately, the main symptom is that you will loose RPM indication. The needle will just stay at 0 RPMs or sometimes erratic. I hope these tips help your son stay cool. Good luck.

 

Thanks for your response.

I have two more questions:

1) What kind of material did you use to bypass S31?
2) If you read my earlier post you see that the A/C works fine at Highway speeds. That indicates to me that the fans are not working. Do you think a bypass would fix this problem? The MBZ shop told me that the RPM indication is the problem (and they wanted $1200 to replace the compressor).