It's probably a similar system to the one used by Etak with their early car navigation systems. Basically, they used a special adhesive tape that contained a small bar magnet every inch along its length. This was applied to the circumference of the inside of each front wheel rim, and a Hall effect sensor was attached to a point on the front axle (usually to a part of the brake caliper assembly) where it was within an inch of this tape as it passed by. Each magnet passage would produce a countable pulse, and the Etak computer could determine the wheel speed and direction of rotation for each front wheel. *Supposedly* it could also detect a turn by the differential speed of the front wheels, but it also had a flux gate compass to help determine direction and turns. A pretty clever system that worked quite well, considering it was all done without reference to external navigation data sources like GPS, LORAN, etc! Since your system apparently produces only a single pulse per revolution, I would guess there's one magnet somewhere on each wheel rim. The speed resolution with only a single pulse per revolution is probably not good enough for ABS use. Hope this helps, Stan