I have been reading about some of the possible problems with the heat. My problem is I can have it in either Auto for fan and Auto for the vent selector or with fan turned up on high and the vent on floor/defrost and it will produce heat and then all of a sudden it will start blowing out cold air out the vents. I can flip it back and fourth and it will switch back. I cannot get heat to come out the vents, only if I turn it on floor, defrost, floor/defrost or in the auto setting. Not sure if it's related, but the vehicle will never get totally warm inside. The air is warm, but it just never seems to get comfortable inside. The outside temp is anywhere between 35-40 and I can run it on high all the way home from work which is about 45-50 minutes and will need to keep my jacket on all the way.

I have a 98 Blazer 4x4 with the auto temp control

 

Just an update from the original problem. I still have the problem, just that I noticed that when it drops down a gear while driving, like out of overdrive up a hill it will start blowing cold air, when it shifts back up it will return to heat. Was wondering if this could be a vacuumm problem or a accuator door problem?

 

The temperature door is electronic, but all of the other doors in the HVAC system are vacuum controlled. You could have a vacuum problem that is causing the vacuum level to drop too low during acceleration (low manifold vacuum at higher throttle angles), but that would just cause the airflow to go to an inappropriate area. The default position without any vacuum applied to any of the actuators is a mix of defrost & floor.