Need some info please. I have a 1991 Dodege Dakota 4x4 V6 3.9L When I'm driving and step on the accelerator the heater/AC flips over to defrost and also does it when I'm pulling an incline. But once I let up a little or reach speed it flips back to where I had it set befor. Any ideas?

 

The selector doors are vacuum operated supplied by the engine. There is check valve that holds the vacuum when the engine speed is up. That check valve may be back or just a vacuum leak.

 

something i need to check out. i have the exact same year/driveline(unless yours is a 5spd) and mine does that. doesnt bother me but it drives my passengers nuts. lol

 

Either you have a big vac leak under the hood or you have a cracked vacuum reservoir located under the wiper cowl passenger side. The reservoir holds vacuum in reserve to keep things operating when the engine rpms increases. If the plastic tank has a crack in it there's nothing in reserve.