Heater Stuck on Hot

I have a 97 B2300, and the heater is stuck on full hot. I can use the Max AC, not the regular AC, but the Max. If it is not on Max AC, the temperature can't be changed. I recently changed the heater control valve, hoping that would solve the problem, but no. I have gone down in the heater core also, thinking some debris has blocked something, but the core is clean too. I have narrowed it down to it possibly being the actual temperature gauge, when I turn the knobs, nothing happens, no noise, nothing. My other thought is that it could be the blend door is broken, but if that is the case, why is the Max AC working? Any help is much appreciated.

Sounds like the cool to hot air duct door is bad does it work off vaccum or electronic if it has climate control its probably a vaccum pump not working but ive not worked on one that model have a nice day.