I just got my 300SDL home last week and the main problem is no heat. When I push any of the buttons other than the "O" there is a buzzing noise and I tracked it down to what I think is the monovalve near the battery. It makes the buzzing even if the engine is not running with the switch in the run position. The noise continues if I disconnect the electical plug on the top of the valve and sounds as if it is being cause by coolant flow or leakage by the valve. Also, I don't seem to be getting much airflow from any of my vents. At this point I am thinking there is some type of control problem where the auxiliary pump is running but the monovalve is closed. The monovalve is used to cut off the heat, right? So it should be normally open without power?

The monovalve is basically an electrical coil that when energized and current is varied by the temperature control on the heater control panel the valve opens or closes based upon the setting of the wheel and the input from the air temp sensor on the top of the dash. Its a flawed system at best and you really need the troubleshooting guide (either cd or shop manuel) to diagnose properly. BTW, it should be normally closed without power.

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DDC/MTU....Setting the pace
1985 300cd
1983 300d...aka "The Yummymummymobile"....Wifes ride
2006 Wesmac 38' Sportfisherman, Single DDC/MTU 12V-2000 1500 hp

Well it turned out the noise was the pump motor bearing. Replaced the pump and repaired the monovalve diaphragm with super glue and silicone gasket and now I have heat. My vents are not working right and I just found that one of the vacuum valves is not working. Its the diverter flap. Not sure what it does. I had a spare one of these from the junk yard and it had two bad valves so I opened it up to determine the failure. Found that the problem is only bad solder joints. Repaired this one and glued it back together. I will install it today if I can find the time and hopefully everything will work as it should. Just put in new motor mounts on Friday and wow what a difference. The problem is mainly the right side one. It carries more weight and also the torque reaction pushes down on it.

Sure about that?

The monovalve is definitely spring loaded open. Also, there is a small orifice through the brass part in the middle of the diaphragm. Apparently, the action of the monovalve is supposed to be highly damped by this small orifice and the diaphragm. I was supprised that I was not able to get good heat until I repaired the diaphragm on this normally open valve. Without the damping action, it seems to stay closed once the controller starts to activate it.

The truth of this statement depends upon exactly what you mean by it. Without power, the monovalve will allow full coolant flow thru the heater core.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13