weird das ac problem

[kwftlaud]

i have a 2001 itasca class c with a e450 van

I am having a weird dash AC problem

At idle and low city speeds the ac works great.
Before I left on my trip it worked great, but, the AC would blow how air if I gave it a lot of gas ( think going up a hill )
Now, it won't work on the highway at all where as 2 days ago it would.

If Im going 60 MPH the as doesn't cool. If I accelerate from 2000 RRPM's the blower will stop coming out of the ac vents and will come uot the floor vent.
When the AC is not blowing cold air it feels like its just the vent.
I don't think its a compressor problem because when its on at idle its really cold, and when it goes warm it is immeadeate , nothing like its not pumping freon.
To me, it seems like a vacume issue.
So questions:
when does the dash vent aperatus get the vacuum from? where does it go thru the firewall?
2) where is the fresh air intake for the system? Maybe tge flap is opening at high speems and flooding the AC with warm air.

Has aany one else dealth with this problem.
I really don't think its a compressor probems as it will start working properly when I am stopped at a red light.
THanks
Kip

[twinboat]

Vacuum line starts at the engine to just behind the hood seem, heading under the AC coil in the engine compartment.

Under it is a vacuum chamber and check valve with a line coming in to the cabin near the passengers floor board/firewall.

I ran a new line on my 99 E450 from under the hood following the radio antenna wire up to the small pop out panel on the top center of the dash. I also installed a check valve in the line out by the AC coil.

Fords fix is to remove the right inner fender to fix the bad line under the AC box.

YouTube has some videos of the fix.

[Skip426]

Most of your description relates to a vacuum supply problem ; but usual failure under load is for the dash controls to go to full heat , defrost mode .

You have to start with supply from the engine to a vacuum reservoir ( usually a softball size plastic unit under the hood ) there should be a check valve between the intake manifold : gas engine : and the reservoir to prevent the problem , beyond the reservoir any vacuum leak can cause your issue as supply from the engine stops as throttle increases .