Jean-Bernard Fischer , Mar 17, 2011; 12:27 p.m.
Have you tested the lens in Speed priority? You need to mount the motor or the winder.
If the aperture closes past 2.8 in this setting, you have a problem with the aperture ring mechanism, not with the cams or the aperture itself.
Anyway, you will end up opening the lens, but it helps to know where the problem is located. If it is the aperture ring, opening on the mount side will give you access to the problem area; if the aperture itself has a problem, I think you should access through the front (not absolutely sure here, but from experience on other lenses, it is usually the case).