Suddenly yesterday, my AC was blowing hot air on every setting except for Max AC. When on Max AC, then heater control valve closes but with just AC, it opens back up. The **** for floor, defrost, etc. seems to work just fine, but it blows engine hot air at those places except for Max AC.

It seems as though the temperature control **** is stuck in hot or quit working all together, AND (I'm guessing here) that Max AC overrides the temperature control ****? That's the only thing that make sense to me.

Thoughts?

 

Turn on the heater and check to see if the heat can be regulated from hottest to a cooler temp by turning the temp ****. If it doesn't change then maybe the blend door is broken. Had a 97 had this problem.

 

I don't think it changes the temperature at all by moving the **** cool to warm. But even if the blend door is broken, why would the heater control valve open to send hot coolant to the heater core while on AC but close when on Max AC?

 

All air goes through the A/C evaporator core, whether A/C is ON or not. The blend door allows none to some to all of the air next to either go through the heater core, or bypass the heater core. Without a hot heater core, it would not be possible to adjust the inside temp to anything but whatever the A/C system was capable of cooling at that time.

MAX A/C on vehicles with a heater control valve, shuts off coolant flow through the heater core, so the core cools down, and the A/C is a bit more efficient as the heat source has been removed. But no temperature adjustment is possible, as there is no hot heater core.

The overall A/C-heater system concept is called a "reheat" system, as air is cooled by the evaporator core, then heat is added back as needed to the air stream to adjust temperature to one's liking.

Since you get cooling at MAX (heater valve OFF), but get hot air at non-Max A/C position (heater valve ON), it sounds like either your blend door is hard over to HEAT, or is broken and allowing most/all air to always go through the heater core.

I assume by 2006, the Superduties would have the temperature blend door controlled by a electronic actuator, like the F-150 starting in 1997 did. So could have a blend door actuator problem, or problem with temp control k-n-o-b potentiometer, or wiring in between the two.
Knock on wood, I have never had a problem with a blend door actuator (yet). But many people over many different Ford vehicle models have.

 

Electrical control of the blend door was the only way it was ever implemented since the Super Duty was introduced in '99.

You are quite correct, the problem is a failure of the blend door to operate.