Just thought I'd share in case anyone else has the same problem. My 300SD
heater got stuck on. I knew the Mono valve caused no heat problems so perhaps it stuck full open. I took it apart, rebuilt the diaphragm. I checked
the solenoid and it didn't "fire" with application of voltage. Checked the continuity of the coil. Oh no, it was open. So I took it out and sweat soldered
the posts. Still nothing. Since I had no easily obtainable replacement coil I used
a "solder sucker" to suck out the old solder from the center of the posts then
carefully cleaned the little wire inside and re-soldered. Bingo. Mono Valve works
again. I find a lot of little electrical things on this car are due to cold soldered
joints.

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85 300TD FED-Daily
84 300SD-Wife's
86 XJS-Sunday
66 GMC-Work- Given to my stepson
83 BMW Airhead- Given to my stepson

Absolutely correct. Another example is the early cruise control electronics (W123 and W124 and others), which commonly fail due to cold solder joints. Never heard of this kind of monovalve repair, you are to be congratulated.

Jeremy

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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

Or, i have a working monovalve for sale $10

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Cheers!
Scott McPhee

1987 300D

Do you still have this valve?

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1984 300D, 1972 LT1 Corvette, 2014 Artic White 3LT/Z51 C7, 2013 650i Convertible BMW, 1994 Kawasaki Vulcan, 1992 Yamaha Seca II and 1996 Dodge Ram 2500.