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On my F150 w. a/c, I have control of fan speed and temperature, but the 'duct selection' is stuck on the dash vents.
If I switch it to anything else, I get the slightest breeze. Example, on defrost: the vents still blow full force, but there is enough of hot air flow on the windshield to clear the center about the size of a baseball. I think I clear more frost by exhaling on it.
I checked vacuum connections, but I have limited experience with the heater assembly. What could be the problem?
Do you have vacuum with the engine running? There is a line that hooks behind the engine on the firewall, and then runs over to the passenger side where it tees into a vacuum storage can, and then goes on through the firewall. I had a vacuum line come loose were it goes by the glove box before.
Perhaps I'm unclear on the older (pre '87) trucks but on mine the duct selection is done with a 'cable' operated arm to the left of the heater box close to the center of the dash.
One time when replacing the heater core, the cable jacket retaining clip slipped and I ended up unable to get the proper ducts as indicated on the heater control bezel.
Maybe the cable has unhooked from the door arm, the plastic arm is broken, or the door itself is jammed in place.