If they are like in my ´93 940, then you will find a 2sprocket wheel" behind the rotor. This wheel gives the signal to the sensor which is a small thing sitting just above and almost touching the wheel. As said by previous contributor, don hit the sensor. But try to carefully clean the wheel "wells" with a tiny screwdrive and a toothbrush, this should keep the sensoring system functioning longer. The crud you find there is mostly from the brake pads.