Ok, here's the situation. Yes, I've gone blind searching threads, but no one seems to have had exactly the problem I have. 1981 300SD. I have noticed that occasionally my left-center vent will start blowing a little warm air (yes, just that one) and if I cycle the modes on the accu it goes back to cool ec air. Last night I was driving home and the whole system went full hot, enough in fact to noticeably cool the engine temperature. One would think this would indicate monovalve, and maybe that's part of the problem, but I could actually hear the vacuum servos under the dash changing when this occurred. After cycling through the various modes a few times and, listening to all the vacuum servos flapping around, everything returned to normal.... I'm on my way out to check the tube for the temperature probe, but I'm a little skeptical. Any ideas on this one?

Sounds like your push button control unit and/or vacuum actuator on the left vent may be bad - the tip off is that the problem is only from one vent. The going full warm is a classic symptom of monovalve failure, and that's so easy to check I'd start there.

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Tjohn

82 300 SD
77 450 SL (gone)

On W123s, the horn and monovalve run off of the same fuse. So, next time it starts blowing warm, try your horn. If the horn doesn't work, replace the fuse (#13 or #14..can't remember which). With no power to the monovalve, it defaults to the open (hot) position.

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Never a dull moment at Berry Hill Farm.

I think your analysis of a failing mono valve is correct. I am not up on all the story with those, but they are prone to acting just plain wierd at times.
I did a repair on my dad's heater. Similar problems. After a few attempts we finally bought a mono valve rebuild kit. Installed and has worked well ever since.
The push button control unit usually goes south and causes the system to fail to come on at all. Fuses could be the problem, the monovalve is fused, but the fuses on the 123 map differently than the 126.

well, I checked the tube for the temperature probe, and it has been replaced and is in good shape. While in there I started the car and watched the vacuum servos operate, and they did very well. The connector on the monovalve was quite loose, so I tightened up the contacts on that, and we'll see what happens on the commute tomorrow. Thanks for all the replies, I'll let you know the results.