The FD/FL mount was mechanical only. There's no electronic communication between the lens and the body as you'd have with a current Nikon lens. You can't autofocus. You have to set the aperture with the lens's aperture ring. Your D3100 cannot meter accurately with this lens. You can't shoot in any modes other than M because the body can't tell the lens what aperture to use.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well. If the "do it yourself" approach doesn't work out for you and you need conventional repair, please let me know.
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