No heat above certain speeds on 82 300D
 

Ok. Here's the deal. On the 1982 300D, the heat works great at low speeds. It gets real warm and toasty inside. But today, I noticed at highway speeds, more specifically, about 70 mph, the air starts cooling through the vents. I had the thermostat control all the way to MAX, but it just blew cool air, but when I kept the speed below 55, it was nice and hot again. Am I dealing with a torn monovalve diaphragm or something else?
-Joe

Never had it happen, but sounds like monovalve.. There was a fellow just a week or so ago with the exact same deal I believe..

MonoValve is bad.
 

Replace it and the problem should be gone. :)

Does anyone have part numbers or prices for a monovalve?
-Joe

Price should be around $38.00 - $40.00
About 30min of time to R&R

FastLane has it
 

1982 Mercedes Benz 300D Turbo
Part: Mono Valve Repair Kit
R3023-15174 Mono Valve Repair Kit Bosch IN STOCK $41.14
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1FE046QDU1FH03PI4E&year=1982&make=MB&model=300-DT-001&category=R&part=Mono+Valve+Repair+Kit