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You either have a problem with your climate control vacuum system, or your blend door is broken.

A leak in the vacuum control circuit may occur during acceleration (slow leak), at all times (large leak), or only when specific functions are selected (indicating a leak in that portion of the circuit). If it is a vacuum leak you suspect, consider these causes:

- No vacuum to the function selector switch.
- Function selector switch leaks vacuum.
- Damaged/kinked/pinched vacuum hose.
- Leaking/damaged vacuum control motor.
- Leaking/damaged A/C vacuum check valve.
- Leaking/damaged A/C vacuum reservoir tank.
- Vacuum actuator arm not connected to the door crank.

If the door is busted, there is a "work around" to pulling the whole dash:

https://www.ford-trucks.com/forums/s...threadid=24962

If the workaround doesn't work, then you'll have to replace the whole assembly:

https://www.ford-trucks.com/forums/s...hreadid=152937

But definitely check for any broken or disconnected vacuum lines.