Hey all, my fiance's 2000 gmc jimmy is having some heating problems.

First off its a climate control system. It blows hot air out of all points(floor, vent, defrost when selected) when the temp selector is on the highest possible setting(and clicked into position).

But once you take it off the highest setting things change. When set to vent only, with the temp set just below highest setting its blowing out cooler than normal air. When you put it on floor only setting it blows hot air and everything works correctly(temp is controlled correctly by selector). And when you put the selector on mix of vent/floor you get two different temps coming out. At your feet its the correct hot air coming out but coming through the vents is still the cool air.

Could it be the temperature blend door actuator? Or any other ideas out there?

 

Sounds like it could be a combination of problems. Blend door and perhaps another actuator/valve, such as the mode valve.

Either way, it looks like you're going to have to remove the dash board & heater/hvac box.