Hot Hot Heat

I had the same issue with my 82 300SD when I got it with the heat being stuck on when ever I turned on the climate control. The first thing to check is that the Monovalve is getting voltage which is very simple. Just start the car, set the climate control to minimum temp setting (set climate control to either AC on or EC, shouldn't matter for this test) and take the 2 pin electrical connector off of the Monovalve assembly under the hood (it is right up next to the firewall towards the passenger side of the car) and measure voltage. You should measure 12V at that connector if I remember correctly, if you are measuring nothing or only 1-2 volts you have an issue with the climate control brain in the dash. If you do have the proper voltage you just need to buy the mono-valve repair kit and this is a very commonly replaced part to fix heat related issues. Once you get the heat issue fixed, if your center vents are not blowing like they should or not at all the vacuum actuator for the vents is failing and unfortunately the only way to fix this is to take the entire dash out of the car and repair the vacuum actuators. There are repair kits out there for this as well as instructional videos because I have done this to my 83 300D. As far as the fan being erratic, I have never dealt with that but sounds like the brushes need to be replaced in the fan motor and I know there is a kit available for that too.

Thanks to my 8-month-old "social planner" (ha ha), I haven't had time to start off with the climate system repairs yet, but I'm planning on starting off at the monovalve and working from there. I wanted to add for the community's consideration, there's one other significant quirk that may help with this diagnosis in the meantime. When I click on any of the fan settings, the fan is slow to start, if it starts at all... The control unit likes to be clicked to defrost for a moment and then back to EC for the fans to start blowing! Does this raise any red flags for any climate control wizards out there? If anyone has any other input, please jump in! And thanks to kentbelt for the very helpful info thus far!!!

I rebuilt the monovalve with a used inner element pulled from my 1982 300D which turned out to be worn but NOT torn, just as a test. (Also put in a new thermostat and expansion tank, then a flush and fill with MB coolant and distilled water, as long as I was messing around with opening the coolant system.) The monovalve repair has fixed the problem of the vents blowing nothing but hot air. The auto temp function now seems to work nicely! Whoo-hoo! However, now the center vents don't function at all. Hmmm... Back to the drawing board...