I am not an expert on this but here are some things the fixed similar problems in my '86 300E.

-hot air even with ac compresor on; replaced monovalve which controls wether coolant is going to your heater or not.

-air comming from every vent; vacum servos replaced, big job, dash has to come off (replaced evaporator while at it).

-A/C not responding, hot air; replaced climate control box.

Like someone before said, I would check to see if you have a freon leak first.

I don't know how things are up north, but here in florida the vacum servos rot after 10 yrs or so.

hope this helps...