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.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

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

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

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