My older daughter, a Cornell graduate student will be home for a visit today with her Orange 1982 300D (turbodiesel). The heat will work for about 20 - 30 minutes then shut off. The last time she was home I drove the car on the freeway and experienced the same problem. Around town and on the highway it ran okay for about 20/25 minutes then the fan stopped and the car started cooling down. Apart from that it does some other odd things.

I know this has been discussed ad nausium on this forum but I've never understood the ACC on MBs well enough to systematically trouble shoot problems. Does it sound like the main control unit needs to be replaced? I don't know if the aux pump is working. A year or so ago it stopped heating at highway speed and I replaced the monovalve and that fixed that.