When selecting defrost, floor, or dash vents, the vents doors do not shift. Selecting say defrost, pulling the fuse and re-installing it causes the vents to properly shift. Temperature control is acting the same way.
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In your HVAC unit there are at least two 'doors' that are moved by servos. (1) regulates your temperature - the blend door, which adjusts/blends the amount of air coming through your heater core and your air conditioning evaporator. (2) regulates where the air goes - the mode door (there may be multiples of this depending on your model, etc), that selects floor, dash vents, and defroster vents. One of your servos may be bad, or your mode door may be jammed.
This is a common problem on this site. In your case I would look for a vac leak or a sticky blend door. Since the direction of air flow changes when you are accelerating, I'm going to guess the blend door or doors are controlled by engine vacuum. There may be a vac leak between the dash and the engine, or the blend door may not be able to move back and forth correctly. Defrost is the default position when a problem occurs, to make sure the windshield is clear of fog or frost.
it sounds like the switch lever for the vents, floor and defrost is not switching the vent door that goes to the defroster hose. You need to pull the switch consol out loose from the dash and follow the cable for the defrost that goes to the hose for the defrost. You might also have a broke switch that won't pull the cable enough to open the air for the defrost hose.
Have you checked the diverter doors under the dash??
It sounds like a vaccum hose may have fallen off your vent controller.
I have had similar problems and found that the air WAS moving, but it was all going up through the defrost vents at the windshield.
A quick check of the vac. lines showed that one under the glove box was pulled off.
with the engine running, try setting the air settings to all the options ( vent/floor/defrost..etc..)
See it you can feel or hear the diverters moving under the dash..