I have a 1999 Ford Taurus in excellent condition which has recently developed a problem with the AC air flow direction. Normal flow direction with the adjustable nozzles directed on the driver and passenger operates fine until i increase speed or go up a hill. At this point the flow transitions to the defroster nozzles. The air is still cold but directed upward.
When the engine returns to an even RPM, the air flow will eventually transition back to the Driver/Passenger nozzles.
The air flow is controlled by doors in the ducts that are operated by vacuum motors. The vacuum comes from the intake manifold of the engine. There is a check valve between the intake manifold and the vacuum reservoir for the HVAC. The check valve maintains vacuum when engine vacuum is low (throttle open). You have a bad vacuum leak or a bad check valve.
I suspected vacuum but know little about AC. Thank you very much for your prompt response. I will investigate.