ok guys, i normally have a/c problems figured out but this one has me wondering. im working on a 99 f150. heres the scenario start the truck,drive off and the a/c is ice cold. punch the gas and it goes strait to hot air.(as in the heat is on) i get out with it still blowing hot, check and see that the compressor is spinning and the low side line is ice cold like it should be. (the clutch is engaged) this has me wondering if the blend door isn't somehow switching between the evaporator and the heater core. im guessing its vacuum related due to throttle position when it quits. any ideas guys?

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I concur with the blend door and vacuum ideas. I'm not sure exactly how your system is supposed to work, but I would think there would be a vacuum reservoir and maybe a check valve here and there to maintain vacuum in the system when manifold vacuum goes to near nothing, as when WOT occurs.

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
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yeah, sounds like a classic case of a failed check valve.

that about what i was thinking. time to start digging. thanks guys

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Try www.ford-trucks.com