Just a quick note on diagnosing the monovalve:

If your monovalve is still basically working, then it will go to "full-open" when the electrical connector is pulled off of the top, resulting in full-heat. If you have no heat, but then you get heat by removing the elecrical connector to the monovalve, it tends to implicate the control unit (or maybe something else, like a sensor of some kind) rather than the monovalve.

If, on the other hand, you are still getting cold air, even after the monovalve electrical connector is pulled, that tends to implicate a stuck monovalve (fixable with the repair kit.

Bob Schleicher, 1991 420SEL, 174,000 miles.
bullswin5@aol.com