air conditioning

i have a 2000 5.4 triton v8. When i have the a/c on and i give it gas the air comes out of the defrost vents instead of the dash vents. any ideas why and what i could do to fix it?

I haven't worked on one of these trucks, but if they're similar to most vehicles, the duct system is vacuum operated off the engine intake manifold. Usually, when problems such as yours happen, there is broken or disconnected vacuum hose. Listen for a hiss, under the hood and under the dash. Check the vacuum hoses for disconnects or breaks. This sometimes happens just after the engine has been serviced when the mechanic accidentally disconnects one.

There may also be a vacuum storage cannister, a round ball type thing or other shape that a vacuum line goes to, in the engine compartment. It's purpose is to provide stored (backup) vacuum. I suspect that this or it's feeder vacuum line is the culprit as it appears you are losing vacuum upon acceleration,which is normal, and this storage unit is supposed to provide that backup vacuum to keep the duct controls on the as-was position.

Also, and hopefully not, there maybe a problem with the main vacuum control module. This is where all of the vacuum lines connect behind the dash, usually behind the a/c control panel. These lines are then regulated by the position of the a/c, vent, panel, etc. knob.