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Look on the intake manifold of the engine for a cracked hose. If a hose behind the control levers is leaking, you may be able to hear it with the engine idling. There is a possibility that the defrost door is operated by an electric motor too. Check the linkage on the door to see if it slipped off.
When the gas petal is pushed down the engine needs a amount of vacum to run.If there is not enough vacum it will pull vacum off heater/ac box inside which controls where the ai flow is controled.Check intake for vaccum leaks,hoses,connections.look for vaccum lost.
vacum line off or cracked from intake manifold to vacum controller under dash. to test under dash buy cheap vacum test from parts store conect tovacume port underdash pump up and with ac running you will gain control of vents. take your time and researh location of vacum line. good luck
There is a vacume hose going from the intake manifold to the passenger compartment. This hose has a check valve and a vacume resivore. When you steap on the gas the manifold vacume drops and the a/c goes to the default mode (defrost). The hose is small like a ball point pen's ink tube. sometimes the hose rots and leaks.
The problem is that the control is vacum operated,so either the rotary vacum switch is bad or you have a vacum leak under the hood or under the dash area.
If this happen only after you replace the battery, then you need to go look around where the battery is at, maybe a vacum line came off, vents work off of vacum.
Check over on the passenger side under the hood by the fire wall, there will be some elec. connection, just below that there will be a small vacum line that should go into your fire wall, check that it has vacum, it should also have a one way valve, small round looking some time that get clog, needs to have vacum that is what power the blend door's.
your blend doors work off of vacum, check vacum line under hood close to firer wall ( there should be 1 vacum line that goes into the firer wall ) check it for vacum.
prbly have a leak in your vacume lines, have to pull the heater controlls and make sure every things conected and not leakin if its good youll have to trace your vacume lines farther
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