I have a 92 F-150 with a heater (or might be a vacuum problem). When I drive faster than a certain MPH (about 55) then the air that is coming from the heater switches from blowing on the floow (where I want it) to blowing on the windshield (the defrost setting). I have no control over this other than lowering my speed. No combimation of settings seems to make a change (other than off).
I am pretty sure this also happens when the air is cold, but haven't tested with the a/c as I don't use the a/c.
Anyone have any idea what I can fix? I would really like the air to blow here I want it to blow.
Thanks,
Joe.