Ac vacuum under heavy acceleration ac starts to blow out of defroster. When acceleration eases ac starts to blow out on the floor then when acceleration ceases it starts to blow out of the vents.

recheck install either incorrectly done or something got disconnected

Hi,
Most Ford products have vacuum actuated control valves in the heating and a/c system. Ford designed a fail safe system into their cars and trucks that if all vacuum is lost that the system would default to defrost so at least your windsheild would stay clear. Typically there is a canister under the hood mounted to the fender well that looks like a metal tomato juice container from the 60's. That cannister is the vacuum reservoir that is supposed to hold the vacuum under high load (heavy acceleration) conditions. If that "tomato juice can" is either disconnected or if the check valve in the vacuum line is bad, then whether you are heating or running the a/c, under hard acceleration air will come out of the defroster vent. Check all your vacuum lines running from the intake manifold to that canister and back to your heater and a/c system for cracks or leaks, Also check that the check valves in the vacuum lines are in place the right way (they should only flow one way- toward the engine.) Listen under the hood when the engine is running for the hiss of a vacuum leak. Fix any leaks that you find. I hope that starts you off in the right direction.

Check your vacuum lines, check valve and vacuum canister for leaks. these doors are operated by vacuum and defrost is the default position with no vacuum. get a vacuum gauge and pull vacuum on different parts of the system like pull vacuum on the line going through the firewall, make sure it holds vacuum if it doesn't you have a vacuum leak under the dash. Next, check the line going to the vacuum canister it takes a little longer to pull vacuum on since it is a larger canister and the system uses the vacuum stored in there when the engine is not pulling enough vacuum like while under acceleration. also check the one way check valve on the line going through the firewall they occasionally go bad and would cause this. Check all other vacuum lines for deterioration and cracks and replace all or as necessary the stuff is pretty cheap at the auto parts store so you might as well do it all while you are already there. Harbor freight has a vacuum gauge pretty cheap and advance auto and auto zone have loner tool programs for free and might have one for you.

You have lost vacuum to the AC control head from the engine vacuum reservoir, go under the hood and check for damage hoses ans pinched lines going through the firewall on the passenger side.

Hi, this actually sounds like your heating system may lack vacuum. The system is designed to fail in the defrost mode. Some vehicles shut down ac compressor under heavy acceleration, but you did not mention heavy throttle.

So, look for vacuum leaks, you may need to install a vacuum reservoir. You may have engine performance issues, causing low manifold vacuum, you may have a check valve that failed, so under acceleration, the vacuum is not shut off, but rather evacuated into the engine...
Hope it helps....

Does it only change when going uphill, or under heavy acceleration? Or is it just when ever it feels like it? Generally, it's vacuum controlled and on a Ford, loss of vacuum when accelerating or pulling a grade under heavy loads can be normal, causing the air to come out of the defrost. To answer your question, the blower door is located in the dash, generally behind the radio area.

Good luck!

may possibly be a vacuum leak... look underneath to make sure all the vacuum hoses for the vents are intact. there is a small resevoir (usually mounted on the firewall near the brake booster) it looks like a film cannister or similiar. that may have failed. you should be able to similate that same action by flooring it on a level road. you will hear the vents change as it loses vacuum.

u have a loss of vacuum to the ac mode control switch, this switch applies vacuum to diff vacuum motors to move air to diff outlets, dash, floor, defrost etc. either the vacuum line or vacuum resivior at the engine is leaking or the mode control is leaking vacuum.

I have this same problem with my Cherokee, it seems to have something to do with engine vacuum. Could be a faulty EGR valve.

you have a vacum under the hood pluged into the wrong spot