I've an '87 300 SDL that I bought recently that appears to have some A/C problems. The shop I bought the car from has been great, but I feel bad about taking the vehicle back several times after they have put several (probably expensive) parts in it to no avail.
The air conditioning works fine until the car has been running 30 minutes or so, then warm (heated) air starts to come from the vents. Turning the thermostat down a little fixes the problem for awhile, put before long the heated air shows up, and you are cooling the car too much to prevent the heat from coming on.
If this isn't odd enough, the warm air only goes to the front driver/passenger. The register in the rear sends out cooled air regardless.
A new pushbutton/thermostat wheen has been installed, I think an A/C relay, still no help. Any ideas out there?