It still may be possible for this to be a monovalve problem. The plunger on this is full closed when doing max. cooling, and is full-open when doing max. heat, but is basically pulsing between full open and full closed at all other temperature settings. (Pulse width modulation.)

It is not unheard of for monovalves to be degraded such that they don't respond to the pulses, but still function enough to eventually go to full-open or cull-closed. This might give you the sypmtom of full heat at all but the max. cool setting of your ACC control.

One way to crudely test the monovalve is to pull the electrical connection. I forget now whether no power corresponds to the max. cool or max. heat condition, so try putting the temp. wheel in both states. Then touch the electrical connector to the pins - you should be able to feel the monovalve move as you make and break contact. For that matter, you should also be able to feel the pulses of the monovalve at any temperature setting (by placing your finger on the top of the valve).