1990 500SL Climate Control
 

Working on a friends car. After several trips to the shop I think I fixed the A/C leak they couldn't find.....the high pressure shrader valve wasn't screwed in tight!! Now that it is charged and holding pressure he is able to confirm that the A/C is intermittant. Owner is not that mechanically inclined, he thinks it is an electrical problem, had it to the dealer several times when the car was new for similiar problems. I used to have a '91 126, does this car have similiar problems with solder joints breaking in the control panel? Any other known problems areas?

No, and no.

I suppose the first thing to consider is whether or not the compressor clutch is engaged during those times when the A/C is not functioning.

AC problem
 

On other cars that I have owned (non-Mercedes) the intermitent air conditioning (cycling) is caused by low FREON or whatever product is used. Usually you need to evacuate the system to where the system has a vacuum and then introduce the freon. It will hold the full quanity at that stage. Just a thought.