I have 2003 f-150 whenever I turn the a/c on it shoots out the vents then straight to the defrost.when I turn the a/c off air comes through the vents fine.
Sounds like a blend door motor/actuator problem.
Sounds like a vacuum hose issue. If there is a crack/leak in the system, all air is diverted to the defrost vent. Get under the hood along the firewall and check your vacuum hoses.
Look for the small vacuum line routed near the battery that goes from the intake through the firewall.
Now that you said that, I do agree that your response sounds correct. Kudos to you Tom.
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