jmoore_98

When I get under load such as going up hill my air conditioner will switch vents. Checked all the hoses and tubes. Need help. It's hot in Texas

351w

well it sounds like a leak in the vacuum hoses somewhere because when you open the throttle under load that is what would have the most influence on engine vacuum.
but without more info on the situation it is hard to say where to start looking.

351w

maybe if your E-250 has C-6 auto trans pull your vacuum line off the vacuum modulator and see if it has blown out forcing trans fluid into the vacuum lines, which will affect the heater controls and etc.
another sign to look for is if the trans fluid is a little low or needs added to regularly, which indicates the modulator has gone bad.

big1

The heater/AC switch has a vacuum leak or the vacuum canister is cracked. Add this to what was stated above.

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