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The e-150 heat/ac system uses vacuum to control the air direction through a series of doors. There is most likely a vacuum leak causing the defrost door to keep air flowing away from the dash vents.
check your vacuum line's!
A vacuum hose routing diagram for a 1988 E 150 van can be found in the vans service manual. The manual can be bought at most auto parts.
The door motor is replaceable but not repairable. It is probably a vacuum controlled type so make sure that there are no vacuum leaks.
Remove motor mounts and raise motor approximately 4 inches.
It came from the factory with the ( 5.0 liter / 302 cubic inch V8 engine ) 1988 Ford F-150 ( VIN " N " )
If the windshield wipers are not working in a 1998 Ford F150, the cause could be a blown fuse. There could also be a problem with the vacuum hoses connected to the wiper motor or the wiper motor itself.
blown fuse for interior fan motor, bad fan motor, blending doors not operating correctly, bad switch that is not turning on motor, motor jammed with organic debris, mice nests blocking ducting, or wiring/connector probs, bad power transistor if equipt with one. does heat work?
vacuum pipe diagram for honda ballade 150 carburator
The 1988 F-150 doesn't have a master air flow sensor.
In vacuum it is about 150 meters.
Check for a vacuum leak.