First you must see if the compressor is turning. Next you must see if the PBC (pushbutton controller) is telling the compressor controller whether to turn on. This is very easy, go to the low pressure switch and check that both sides measure the same voltage to ground. They should both be about 0.1 - 0.2v. Now turn the EC position. The voltage should go to battery voltage when the A/C is switched off.
If you have the signal at the low pressure switch (which is easy to get to on the drier I forgot to mention). Then you must go to the Klima or MAS relay (I forget which year we are talking) and unplug it. One should make a jumper with an inline fuse and jump 30 to 87 for the A/C and this will make the compressor come on to verify the refrigerantion system.
If all this goes well you must decide why the Klima or MAS didn't work. If it were my car and it had a Klima I would give it a lobodomy. The technique has been discussed more than once. The up side is no more intermittants. the down side is that one must be smart enough to turn the A/C off when the compressor locks up. Or one has to have the sense to quit driving it once one was so stupid as to not turn off the A/C and now the Alt light and everything else is lit up now that the belt is on the road.
Bosch Master, ASE Master, L1
33 years MB technician