When I bought the car a month ago from a guy in Maine he neglected to tell me the heat would not shut off. I drove it back down to NJ feeling like a human raisin. I changed the mono valve and for 2 weeks the heat shut off.
Now it's back on again. The A/c belt is off ; the clutch is fried.
I'm melting, melting, melting. Any Ideas?

If you replaced the monovalve insert only, try replacing the entire monovalve. I had the same symptons and replaced the entire monovalve in my 87 with the one out of my 82 and the constant heat stopped.

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.

Yeah I ran into the same problem,changed out the whole assy and that cured it,Johnny

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85 300sd SOLD ,85 745i,95 740i,1972 Suburban,1938 International

The monovalve body has a check valve in it to keep water from flowing "backwards." If the check valve fails or jams, you could get your symptom. The only way to replace the check valve is to get a whole new monovalve, unfortunately, the "kit" for rebuilding includes only the main valve insert.

In the short term you can pinch or otherwise block off the hot water hose to or from the monovalve. That will cut off the hot water just as if the valve were properly closed. Will not harm the engine as it simply mimics what a good monovalve does.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The purists won't like this ---- but I used to have a '76 Cadillac with the same problem, and a complicated climate control system I didn't want to get into. I went to a hardware store & bought a valve like a hose bib - same circular handle. I can't remember what I used for hose connectors -- probably brass barbs w hose clamps. The whole assy took me about 10 minutes in the store to assemble, and less than that to install in the supply heater hose. When I wanted heat, I opened the valve a turn or so, and the climate control regulated it. On warm days, I closed the valve -- took 10 seconds. Drove the car like that for 3 more years ... It would certainly do until you felt like doing a bigger job.

Thanks for your help.
I pulled the whole monovalve out and put it in my friends ultrasonic cleaner, she uses it to clean her jewelry. I put it back in and I have not had the problem, Yet!

Marshall,

That's very cool! (Pun intended). I never would have thought of doing the ultrasonic cleaning. I'll keep my fingers and toes crossed...

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.

Did she know about it?
Most women would come unglued if you put a car part in something they cleaned their jewelry with.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Ah HA! That explains why the wife was so agitated when I Gunked the 300D head in the bath tub!



Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles