No Heat unless on MAX setting '85 300D
 

Not really seeing anything out there that resembles my issue. Only thing that looked intersting were the mention of 2 sensors. One is on top of the dash, but doesn't that only come into play if I were using the AUTO temp feature? Also, there was talk of a sensor behind the glovebox. How often do these fail?

Also, I assume the monovalve would open or close depending on the setting. If on full cold, it is closed, if on full heat, it is open. Somewhere in the middle if the wheel is set between MIN and MAX. Could the valve be the problem if I get no variation of temp between the MIN and MAX setting or is it likely electrical or ?????

Do you get continuous heat at all times with the max setting? How does it function on defrost? I'm thinking it's the pushbutton unit.

If the temp sensor in the center top of the dash has failed or if its wires are disconnected or broken then the PBU won't work unless it is set at the MAX heat click stop. I had that exact problem with my '85 300D-T (W123, OM617).

Note that other problems including the PBU itself can also cause the symptom you describe but the dash sensor is fairly easy to inspect (by pulling the glove box liner) so it's a good place to start.

Jeremy

I looked at that.....wires are in tact, not broken..how can I tell if it has failed?

Yes, shouldn't I? Defrost seems to work OK