I have a 2000 with a 2.5 and auto trans. Under acceleration the AC will switch from blowing out of the front vents to blowing out defrost. Letting off the gas will let it switch back. I can find no broken vacuum lines and I'm assuming there is a one way valve somewhere that is bad. The problem is, I can't find it on this truck.

Anyone know where Ford stuck it on this truck? Thanks.

 

Yes, Vacuum Reservoir is not holding vacuum.

In the engine bay passenger side firewall you will see a black plastic vacuum line and a grey plastic vacuum line coming out of the firewall

Grey one goes to heater hose valve

Black one is the one you want, follow it toward the front, make sure it is not cracked or melted, it runs to the Vacuum Reservoir, it will be mounted very low up at the front below Rad support.
A Black ball
It will have another vacuum hose attached, it goes to the intake manifold, might also be "T" along the way and have a Vacuum Check Valve(one way valve), but check valve is often IN the reservoir now, so hoses are NOT reversible

Picture of one here: https://www.ranger-forums.com/attach...dscn0295-1.jpg