There are no user serviceable parts for most good lenses. Does this mean one should avoid taking their lenses apart if they break down? Short answer is YES. The longer answer is it depends upon your background, the tools you have on hand, your comfort zone when it comes to working with parts with very small springs, switches and screws, and the area in which you can work. That said, I do not recommend you take your lenses apart unless they are completely out of warranty, and you have already committed to buying a replacement.
That said, I just opened up a Sigma 28-105 with a Canon mount (it is not my lens, but the owner had nothing to lose by anything I did to it) that had stopped working. It would focus and zoom, but the iris was stuck somewhere around f/11. The problem manifested itself after prolonged exposure to the Arizona torture chamber known as a car trunk. There are only a few reasons an iris that is electronically controlled will stop working.
- The contacts to the camera are too dirty, not this time.
- The gears that couple the motor to the iris have lost teeth, not in this case.
- The iris is jammed, it works just fine manually.
- The ribbon cable that provides power to the stepper motor, BINGO!
Now I can only hope to remember where all the parts go by the time I find a replacement for the ribbon cable.