What could be the problem Actually only blow out of defrost vents no matter which position I turn the knob...
Sorry typo A.C
If a manual heater system, then there is a vacuum loss to the control head or the vacuum servo failed. If it is an HVAC unit then there is either an electric servo motor failure or a broken blend door.
How do I find out if it manual. or hvac
HVAC will have a digital readout, a manual will just have dials and switches.
It's manual. So how do I fix it
You'll have to check the vacuum hoses running from the vacuum tree in the engine bay through the firewall to up under the dash. That hose will be cracked or split. Also under the dash that same hose and or hoses might be compromised or popped off one of the vacuum control motors. You might have to remove some panels to get your head under there and or dash removal.
Servo motor has cables attached to it to switch the flow of air! In my Toyota the path that the cable took placed too much stress on the servo motor gearing which was made out of plastic! Stripping the teeth from the gear, the motor would rotate to the point where the damaged teeth were located, then the unit would freeze, not allowing my heat go from my feet up to the defroster vents! Have someone turn the knob while you are under the dash look for cable movement. This cable usually directs air flow to the proper vent. I disconnected mine from the motor and moved it manually to the defrost position ( it was winter). If the cable moves freely. Then check the motor that moves the cable! I would recommend finding the pages from the factory service manual. If its vacuum operated then you have a leak in that system. Mine were electrical motors no vacuum lines!
Have you had any work done on the car? If so they could have knocked the main vacuum source line off the manifold tee, that would be the first place to start. These lines are a hard vinyl type, and they do tend to just up and break, so look them over carefully, fortunately your car only has 1 line that goes through the firewall and to the heater control valve, from there it gets a little messy, but if your careful you can trace them down and find your culprit.
After reading the info above I checked the engine bay on my 2000 Taurus. The larger vacuum lines were all fine. Then I looked at the small ones. On the fire wall in front of the passenger side there were 2 very small vacuum lines connected to a T. One was cracked right were it goes into the rubber end. As I pulled, it broke completely off. I removed the broken piece from the rubber end and reinserted the vacuum line, then plugged it back on the T. My ac went from blowing thru the defroster vents back to blowing out of the dash panel vents immediately! Thanks for the info.
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