I'm trying to debug my Climate Control on my 1981 300SD. I have the Model 126 Service Manual on CD-Rom and have found the schematic for the push button control module. The problem is the schematic is practically unusable. This is a major problem with these CDs!!! There is nothing like a book when your trying to read a schematic. Does anyone have a idea where I can get a good schematic of the push button module???

The problem is: The A/C works fine and the temp adjuster is working ok for the cooling. When I need heat is when the problem comes up. When I first start the car in the morning I have heat for a few minutes and that is it. If I unplugged the electrical plug from the monovalve I get full heat and with the plug connected I get full cooling. I have watched the voltage on the monovalve and when I first start the car I get the cycling voltage like your supposed to. After a few minutes the voltage stays on which closes the valve and no more heater. Also when I turn the temp up to MAX I still do not get heat. Also the blower is working properly.

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1981 300SD 164K Miles

Sounds like the mono valve is hydro-locking, water has gotten behind the piston thru the rubber diaphram. Try a new mono valve repair kit for around $25.

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MERCEDES Benz Master Guild Technician (6 TIMES)
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Retired Moderator

I am having the same issue as Corby, by on mine I can't get the air conditioner to work. I only get heat in all temp settings.