Long time ago, I purchased a used body+lens: a Canon EOS 500N (entry-level film SLR), with a Sigma Aspherical 28-200mm 1:3.8-5.6 UC. I've used this camera a lot, it works great, except the lens doesn't autofocus (manual works well). It has been like this from the start. The body works well, and other lenses will autofocus on it.
When trying to autofocus, I hear a quiet grinding within the lens, and the motor seems to try to extend/contract the lens, but it barely nudges it, and most of the times there's no actual motion, just the sound.
If I try to turn the lens by hand while it is autofocusing, I sometimes am able to get it to incrementally nudge it in the direction the motor is trying to turn, and thus reach the focus.
Is it possible that the body and the lens are incompatible? Everything else works great, except AF. I was thinking of dissasembling the lens myself, and see what happens. Should I not?
The lens is damaged. Either something is jamming the mechanism or the gears are stripped. Disassembling the lens would help in the first case but less so in the second. And it's not easy to get a lens apart and back together.