1982 300D (Turbo).

Trying to do some initial diagnosis. In past few days I've experienced getting heat at all settings and out of my vents - center vents included.

During my ownership of around 4 years my heat has alway worked well and as designed. My AC has been working (completed a partial system rebuild/refresh in summer 18 - including compressor, drier, sensors, O-rings, evap-temp sensor, expansion valve, etc).

Temperatures outside are in 30s. The other day, I was kinda hot so I fiddle with my ACC and tried to turn on the EC mode just to get some average ambient air - and I felt the strong heat coming out of center vents. Turning to regular heat/ac mode and putting temp dial to min(cold) didn't help. I'm thinking monovalve?

The next day, I investigated things a bit while parked (so not at speed). ACC unit seems to work, all the vac pods do what they should be doing, fan speeds correspond. I checked the fuses, they are good. I checked the wire connection at top of monovalve - looked good. During this test, I was able to get ambient air (not heat) to come out of the center vents when I set the ACC to EC. So then I'm thinking maybe not monovalve?

Next day I'm driving around, and I check out my various ACC settings - it's back to full heat from center vents and all others.

I clearly need to do more investigation, but on the surface does this seem like monovalve related problem? Can monovalves fail intermittently or once they go are they gone? Does this sound more like an ACC unit (bad solder) thing?

Thanks for any initial direction pointing....
-kuene

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Andrew
Montclair, NJ
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1982 300D Turbo
120k
Petrol Blue Green