Hello everyone.

My "new" (to me) '79 450SL was a bit chilly on my drive to work this morning. We had a warm weekend in SC and I actually used the AC yesterday for a bit, mostly to check it out. All vacuum switching works well, the compressor runs, etc. But no heat. Blower works fine but I get cool air.

I am thinking monovalve, as the heater lines to the unit are hot and the outbound/return lines to the core do not get hot. Rotating the temp dial does not seem to make anything click, but there is another (potential?) issue here. My temp gauge does not work. Moves maybe a quarter inch at startup but never registers the true temp. I was planning to ohm out the sensor (once I figure out what it should read) and replace it if bad. Is it possible the climate control system needs to see a baseline temp before it will open the valve?

Just a thought. Anyone?