air conditioner 1994f150 pickup
with air conditioner on and fan operating when motor put under acceleration pressure air shifts from dash vents to defrost port at windshield then when tops a hill and power pressure demand drops the air returns to the dash vents . have replaced valve in dash would there be a vacuum failure causing this ??
So you are loosing vacuum to the heater control head, it is possible that you have a leaking vacuum motor on heater box or a crack in one of the lines, or the selector valve itself, on a Ford defrost is the default setting, you may want to check the vacuum check valve in the heater supply line. If you can't locate the leak you can add another vacuum can to the system, it is not a fix but it will help hold more vacuum while you pull a hill.
There's a check valve in the vacuum line coming from the engine to the climate control panel on the dash. Sounds like it's bad. When you accelerate intake manifold decreases which sends the system into defrost mode. When you let off the gas intake manifold vacuum increases and the system goes back to normal. The check valve prevents this from happening by holding vacuum in the system while you accelerate. HTH. - Jim
I have replaced check valve in vacuum line 2 times (thought first one was bad valve when it didn't fix issue) ... next step would be what ??
Did you find a leak in the lines or control head?
And you can still add in a vacuum can that will help when you pull the hills, not a cure but just a help.
Check for a damaged or disconnected vacuum reserve tank. It usually resembles a coffee can under the hood. Some newer models use a plastic ball or plastic box looking thing. HTH. -Jim
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