On my 83 the Left heater knob spins very easily, It has no resistince like it did have and the heat is always on now.The right side one is okay you all know what I mean because The cold setting is when the arrow is points down and the cold is up , 180 Degree turn . Is this something that can be fixed or replaced?

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A couple of Ford pickups F 150,250, 350 missed.
1983 240 D 256,000 miles ,auto, ac.