A/C stops working for unknown reason

89 560SL If the outside temperature is mild the a/c works ok. Not super cold, but ok. If it sits in the sun (Phoenix AZ 110 degrees) the a/c may or may not come on. Yesterday I noted it did come on.
After 10-20 seconds the aux fan started and then after less than a minute the whole system shut down. No a/c, no blower fans, the aux fan stopped and the only way I could get anything was to press defrost. Than makes the fans blow , but the compressor does not run. After a few minutes it tried to run again and shutdown in less than 30 seconds. I have tried shorting the two switches near the sight glass. Nothing changed. This morning the outside temp was mild. The system started up then died after 20-30 seconds. As I was driving to work it suddenly came back on and worked the rest of the time (12-15 minute commute) blowing cold but not really cold even though the temp dial was set to the lowest temperature. This is an R12 original system. Has new mono valve. Belt seems tight. May have some bubbles in sight glass. (hard to tell) Have not had opportunity to put guages on it. But I think the compressor would have to be running to tell anything, correct?
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I suspected that your freon is on low side where it is borderline that would explains why sometime it works or not. Even without compressor running, an experienced AC tech would be able to tell if it is too low or not.
I would also change out your fuses that controls the aux fan... AC, cabin blower to eliminate possibility of poor contacts. Take it out to clean a bit or put new one in.
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