Rear wheel, there is a ring which has many slots in it; the speed sensor reads impulses from that wheel. You might not be able to get to the bolt which holds it on w/o moving the eccentric hub. There are a number of shims that hold it a set distance from the slotted wheel, technically called the "REAR PHONIC WHEEL". My bike had three of them, just make it a point not to loose them.
I'd clean it off, check it for defects, same with the phonic wheel. Check connector and wiring.
Its part 14 in the 1199 parts manual on page 128/148.