1982 300d turbo with auto climate control, 89,000 miles on odometer. Wife reported today that there was not heat driving back from the store. Worked fine yesterday. Went out and test drove and had good heat after warmed up. Got on highway and the heat diminished and cool air started blowing out the vents. I had the heat on max and also tried several other heat settings but no change. I know the auto climate control adjusts the amount of heat output, but it was starting to get cold even with the temp dial set on max. Came home and restarted an hour later and heat was fine after it warmed up a bit.

Does this sound like the water valve is in need of a rebuild or is it more of a climate control unit problem.

Thank You,
mh

 

Classic symptom of torn diaphram inside the heater valve (MONO VALVE). Get a repair kit for a mono valve. After replacement, verify the aux. water pump mounted down low by the A/C receiver drier works. Is turned on with auto button depressed and car at warm idle. Use a test light to verify voltage to it and feel the pump for turning on. On my 81 after it had warmed up for 10 min or so, I simply turned the engine off, turned the key to the run position and after about 30 seconds I could hear the pump turn on and run. Do the diaphram with the car cold, radiator cap removed. Less chance of loosing alot of coolant that way. You simply loosen the valve on its bracket pull forward to access all 4 screws and gently lift the cover off. Pay attention to the order of the wave washer and spacers in relation to each other. Its mounted to the right of the battery facing the engine very close to the large rubber cowl drain hose. Hope this helps. They car tear overnight so one day its great, next day wife is cold!

 

Thank you very much carnut. I was driving it yesterday and heat was normal, then it diminished to cold. I punched the off button on the climate control, then punched EC and the heat returned. Will try your suggestion for replacing the mono valve.

 

Dont buy the whole valve, just the repair kit

 

Yes, sorry I meant the repair kit.

 

I believe that my '78 300cd does not have a mono valve. Instead it uses some kind of servo motor controlled valve unit (located at front left of engine compartment if looking from front of car). I have been having a similar problem with intermittent heat, does anybody know what typically goes wrong with these units? My climate control is the style with a wheel for temp. selection and a bank of vertical buttons to control the air.

 

I'm getting intermittant heat. I've verified the aux pump is working. I have power to the control valve, but it cycles on and off about every 2 seconds. I have read that the control valve does cycle, and that the valve is open when de-energized.

I should be able to disconnect the control valve and have water circulation, correct? Does this mean that my control valve diaphram is torn?

Any help would be appreciated!

 

I'm getting intermittant heat. I've verified the aux pump is working. I have power to the control valve, but it cycles on and off about every 2 seconds. I have read that the control valve does cycle, and that the valve is open when de-energized.

I should be able to disconnect the control valve and have water circulation, correct? Does this mean that my control valve diaphram is torn?

Any help would be appreciated!

 

Do you have a number for the kit and where it can be bought and is it the same for the 88 420 SEL
I would appreciate it.
Thanks.

 

I have the same problem with heat but if I'm just idleing below 1000rpms, everything works perfect. The heat regulates from cold to hot but as soon as I go past 1000 to 1200 rpms it gets cool. Weird!