Something else is going on here...
Set at Max, cool air?
One notch before max, air is warmer...
Unplugged monovalve, hot air...
Set at Max is the same as pulling the plug from the monovalve, essentailly. When current is removed from the monovalve, the valvel defaults to OPEN, thus, heat. You have observed this...
What does not pan out is that you did not get heat when set to max. Repairing the monovalve is a good idea for a number of reasons, if the system is old, but I do not think it is your problem. Sounds like, to me, you have a problem in the feedback circuit, in either your dash sensor, the evaporator sensor, or the climate controller itself.
Similar issue has been discussed ad nauseum, on the "other" forum,
http:///www.bewnzworld.org/forums.
Good luck sorting it out...