Here are the climate control electrical diagrams for 86 and on...
http://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACCfrom86.pdf

Note in general for all vehicles (not just Mercedes), the refrigerant pressure in the A/C lines needs to be a certain pressure, but not too much pressure before the compressor will be allowed to come on.

So one of the first things I would check is the A/C refrigerant pressure and associated pressure switches.