[1982_300SD]

Sorry, but I have to ask. (I did do a search though.) In my '82 300SD, hot air continuously flows out of the defroster vents....climate control "wheel" does nothing...push button display does not work....and the fan does not work either. The weather is getting warm here in Northern California, so...how can I at least stop the flow of coolant to the heater core to shut the heat off with out fixing any of the other things I listed? This is the first time I have had to deal with the climate control on my Mercedes, I am mechanically inclined....I own a Land Rover......so what should I start with first?

 

[BenTwombly]

Monovalve? Circulator motor? Blower motor? Climate Control Switches? Acutators (diaphrams)? I'd guess: Fuse.

 

[jdc1244]

Ben’s correct about the monovalve and fuse, check those first. The monovalve’s fail safe default is heat always on.

 

[1982_300SD]

Well, I did not get around to the heater system yet, (but I did replace the highly worn alternator belts this last weekend). Anyway, I was cruising down the freeway today and it started to get hot inside the car (Northern California, weather about high 60's) since the faster you go the hotter it gets because the defroster vent is stuck open and aparently the monovalve too, and so I moved the CC wheel up and down and pushed the display buttons in frustration, and to my surprise, the heat shut off! What is the deal with this? I am affraid it may come back on soon though. So, is it the monovalve or the CC?

 

[1982_300SD]

Heat is back on again.....

 

[1982_300SD]

I finally had time to pull out the monovalve. I see no ripped or ruined rubber. To me it looked fairly new. So, what should I check next?

 

[kyleharvey]

Did you ever figure this out? My '83 300SD has the same problem. I can deal with the broken A/C to a degree, but the heater is on all the time and it is killing me. Feel free to call with any suggestions if you are feeling chatty. 408-259-2795

 

[AzimuthAviation]

Sounds like the controller is toast. I had that problem for a while.

 

[pathogen]

I'm having the same problem my heater seems to be on but only comes out from the feet wells. I don't think the fan turns on either regardless of the buttons I push or the control wheel settings. I did pull the monovalve and couldn't find anything really wrong with it.

I do remember a thread to check the blower motor fuse but I haven't gotten to that yet.