HVAC only putting out heat, even when off
 

I posted last year about my HVAC system putting out only heat no matter what the setting was. When I pressed some different buttons after a few minutes it would go start working properly again.

Yesterday my AC was working fine. I stopped the car and a few hours later went back out to drive it and all it would put out was heat no matter what setting it was on. Even in the off position, it puts out heat. In AC mode it puts out heat. In economy vent mode puts out heat. None of the buttons are doing anything to make a difference. Even pressing all the different buttons doesn't make a difference this time.

1991 300d 233k

Start by diagnosing if you have 12V being applied to the monovalve. Make sure it is connected to the coil when you're testing (it's a transistorized circuit). If you do not have 12V being applied, you have a pushbutton unit problem. If you do have 12V being applied, you have a bad monovalve cartridge or a bad coil. The coil can be tested for resistance, it should be somewhere around 14-15Ω if good.

Correct, the system will fail safe to heat mode if the power to the valve fails (either just the valve itself, or the whole climate system is lost).

This sounds like a diagnosis that would also apply to a 1980-something to 1985 300 series W123.
Correct or no?

Thanks-

Similar. 1986+ W126 and all W124 use a common setup that has all of the logic in the pushbutton unit. The pre-1986 W126 and 1981-1985 W123's have a control box and relay box that handle all of the logic.

Ahhh. Okay. Since you mentioned it, I recall skimming over threads here about the automatic climate control system's "control box" and "relay box". I'm curious to learn a little more about those two since I have a 1985 300D...I'll search the archives.

Thanks for the clarification.