As i accelerate the ac starts blowing out defrost,let off and it comes back out vents. I have checked blend door ?

 

Do you mean A/C is blowing out or air is blowing out?
I don't recall if the blend doors are vacuum operated but sounds like it is -a vacuum loss will allow the door to revert to its default position, defrost.

Now if you do mean A/C only, then I don't know what could cause that.

Note: I am considering A/C to mean Air Conditioning by use of the air condition button/selection.

Defrost does engage the A/C compressor but I don't think that should be related to changing the diffused air position.

What year is your truck, and what trim. That may clue in somebody to offer advised related to your exact vehicle.

 

200 f150 crew cab 4x4,yes the air conditioner switches to defrost then when I let off it switches back to panel.

 

What actual year? The typo in your post omits critical digit(s).

If older than 2004, you have a vacuum leak in the supply to the HVAC control head or to the mode control door's vacuum motor.

Blend door has nothing to do with where the air comes out. It only controls the temperature.

 

2001 sorry thick fingers

 

Then the above applies.

 

Pretty sure that there's a check valve in the vacuum system for AC controls. A different type of vacuum leak.

There's Youtube stuff about it.