I have a 2000 Cherokee and I have a problem with the heat and AC. A month or so ago, I replaced the evap and the heater core. I got the AC system vacuumed and recharged to specs. The AC works fine. When I switch the heat/cold temperature **** to heat, the heat works great as well. The problem comes when I go back to cold air. Usually it takes a time or two of turning the car off and on to get it to blow cold air again. When going from cold to hot air, the switch is almost immediate, and it always works. When going from hot to cold air is when the problem occurs. It always, eventually goes back to cold air, it's just how long it takes that is the problem. I thought of a blend air door or door motor issue, but it seems odd that it will work fine going one way, but takes a while, but eventually does work the other way. Any ideas?

 

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I believe those doors are vacuum servo operated and vacuum servos have a default position (open or closed) when no vacuum is applied. My guess is u have a sticky/slow acting servo or vacuum leak when vacuum is being applied to move the servo from one position to the next.

 

The doors are vacuum for the different vents, but there is a motor for the door that controls the temperature