RE: a 1985 300TDT. As of recently, The heat only works 50% of the time. When it does not work, there is no fan/blowing activity, and no heat coming out of the vents, even at highway speeds. The rest of the time it works fine. I replaced the mono-valve (using the MB supplied repair kit) with no luck (although that could be defective). There is no fan belt on the A/C compressor though. The A/C hasn't worked in years. Only the p/s and alternator belts have been changed. I doubt that has anything to do with it but...