Question about 1999 Ford F150 Regular Cab

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Switch on dashboard does not change air direction to flow & vents

I have a 1999 F150, 4.2L, V-6. The switch on the dashboard that has the A/C, Floor, Defrost, Vent/Floor, etc. directions on it will turn on and off, however, it will not change the airflow directions when switched to the different locations/settings. Everything goes to the defrost. The other dashboard switches work fine for hot/cold and speed control. Can you help?

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  • Ken Mifflin Feb 26, 2009

    Your advice was very helpful. I had a check engine light come on recently and was trouble-shooting that on my own. I checked my vacuum hoses and found a hole in the main line going into the engine compartment firewall to the dash. I fixed the hose leak, but not my original problem. I still have only defrost mode. The hot and cold work, but not the direction. I am going to try the door actuator motor.

  • Ken Mifflin Mar 08, 2009

    Sorry it took so long to respond back. You were 100% right on with the vacuum leak and which hose it was that was causing the leak. I replaced the hose and everything works fine. Thank you again.



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Defrost is the default mode so if the system fais, you alays have defrost. The mode doors are controlled by vacuum motors. I would suspect a vacuum leak in the engine compartment on the hose going from the engine through the firewall and into the heater controls. Some systems have a vacuum storage canister that can also leak. The canister stores vacuum when vacuum drops going up a hill.
Or, you have a bad heater control head that is not passing vacuum to the vacuum motors on the mode doors.

Here's a diagram of the system for reference.

switch on dashboard does not change air direction  - 826b4f9.gif

Posted on Feb 16, 2009

  • 2 more comments 
  • gerry bissi Feb 16, 2009

    • Improper/erratic direction of airflow from outlet

    • Function selector switch.
    • A/C vacuum check valve.
    • Vacuum hose.
    • A/C vacuum reservoir tank and bracket.
    • Vacuum control motor.
    • Vacuum actuator arm.

  • gerry bissi Feb 16, 2009

    also, did someone work in the engine compartment recently? If yes, they may have disturbed the vacuum hose and melted it against the engine. Sometmes you can hear a hiss under the hood when this happens.

  • gerry bissi Feb 16, 2009

    more info. # 7 is the door actuator motor taht would be bad if no vacuum leak is found. Not a hard job for an average skilled person.


    Part Number


    Manual A/C control assembly

    A/C pressure cut-off switch

    A/C cycling switch

    A/C compressor

    Blower motor

    Heater blower motor switch resistor

    A/C electronic door actuator motor

  • gerry bissi Feb 26, 2009

    I would check the vacuum line to see if it's connected to the heater/a/c control assembly or leaking inside the vehicle. I still suspect it's the vacuum hose.



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