Hey guys time for another check light I got! 1990 F350 460, 5 speed, dana 60, and sterling 10.25? i think? big rear end.

The rear anti lock brake light is always on. Brakes are fine, full of fluid, pads good.

I read somewhere that there is a speed sensor or something on the axle, and there is a little magnet that get shavings on it? so it fools the truck into thinking the wheel is stopped? Help please Thanks guys! Matt