Were you able to check on this? I have exactly the same issue - I was getting no heat after I started rolling this winter, changed the monovalve and everything worked great. Now, during the summer, I get heat constantly. I'm not even really sure how to check everything that might go wrong. I saw the wiring diagram with the resistance on the heater core temp sensor expected at given temps, so that should be pretty straightforward. But I'm not sure how to check the heater control unit (I believe that's what it's called). The silver box that sits behind the glove box.
Anyway, I'd appreciate any clues on how to diagnose and your results. Thanks!
1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.