Originally posted by stevebrot Assuming that other screw-drive AF lenses work on your camera, the problem is most likely with the lens.
IIRC, this lens has a AF/MF clutch (push-pull?) on the lens barrel. Make sure this is in the AF position and the switch on the camera is also in the AF position. It should not say MF in the viewfinder of on the rear LCD. If MF is displayed, try cleaning the contacts on the lens mount and see if that helps.
Pentax Forums user @pepperberry_farm wrote a recent review on this lens and is likely still an owner. With any luck, they will drop in on this thread with advice.
Steve
I do still own this lens - it's one of my favorites..
it does not have an AF/MF switch, but has a push/pull clutch as described by Steve....
mine does not click at all in AF mode - it makes the typical screwdrive sound, but nothing unusual...
the focus ring spins freely with the lens is in AF mode... as Adam stated, this may be an issue with the focus pawl in the camera body itself...