James---This valve and its quirks seems to be a popular subject once
again. First, be sure that the cable and its sheath have not separated
under the dash, a few inches behind the knob. If it has, it's repairable
but most owners would find that drilling and tapping for set screws
isn't worth the bother, and simply replace the whole works.
If the cable is attached as it should be behind the dash, it could be
that an adjustment is in order where the cable is secured to the control
valve slide.
Try loosening the clip that holds the sheath to the valve body, moving
it fore or aft to see if this helps open or shut the valve. The cable
inside the sheath can also be moved in or out where it connects to the
slide.
If this whole package isn't broken behind the dash, there should be some
adjustment at the heater valve itself that permits the valve to be
closed and opened at the dash control. The action here is similar to the
hood-opening cable, but without the return spring. I don't think
lubrication to the valve is possible or beneficial.
DT
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