Loss of vacuum supply to the HVAC controller. The vacuum supply comes from the pass side of the engine and goes through the firewall next to the AC lines. It is a small black line.
Cold air from the defrost indicates that the air doors are in the wrong position. The doors are vacuum controlled in most vehicles. check the vacuum system for a leak.
If it blows only out the defroster vents : then check for Vaccume leak Rick ASE L-1 Master TransMichigan.com
I don't know why the vent would not open for the defrost, but it may be related to the fact that your heater core is blown. If your getting nothing but cold air out of the vents when the heat is on, your core is finished and if the core is finished then it is likely related to your vent not opening/switching. Something''s wrong with your heating either way.
Could be a vacuum problem--either broken or disconnected hose or bad vacuum motor under dash or coming in through firewall Defrost is the default position for this system
My guess is that you have a vacuum line that isn't connected or that has a hole. In the cars I've driven, the blower blows air into the heater box, where the heater core lives. Vacuum lines open and close flaps (or doors) on the heater box to direct the air where the control unit says it should go. These lines are usually rubber tubing, and the ends can dry out and crack, causing leaks. I have never worked on a dodge, but I've replaced miles of vacuum line on other cars.
The year, make and model info would help. Also need to know if the blower motor is working, just not getting any heat, or not changing function between defrost, dash or floor vents.
Check for vacuum leak either under hood or under dash Check for bad vacuum actuator under dash Check for bad selector switch in dash
If the climate control on a 2003 Trailblazer will not switch from feet to head vents, try changing it to defroster then back to head. The mode button on the dual climate control model can only be switched from face/feet to feet and then to defrost. The feet and head vents worked at the same time.
Check the floor heat vent and defrost to see if air comes from them. The vents will be vacuum controlled and a vacuum leak will make the vent doors divert air to the wrong place. Check the entire engine and climate control systems for a vacuum leak.
Make sure you have vacuum coming into the cab from the engine. There may be something wrong with the switches.
i had the same problem on my 99 wrangler, it was a leaking vacuum line. it was the one you see under the hood, against the firewall behind the battery.
You either have a bad controller head or still have a vacuum leak or a cracked vacuum line and lastly problems with the a/c heater door operations..........