There is a common oversight in the maintenance of today's anti-lock braking systems (ABS). Speed sensors, located on each rotor or hub of four-wheel disc applications and the differential housing on rear-drum applications, supply information regarding wheel speed to the vehicle's on-board ABS computer. The sensors, being magnets, attract metallic dust. The reference rings, having ridges and slots, become clogged with road grime and brake dust. This will affect the system and cause the ABS malfunction indicator lamp (MIL) to light. The solution: Clean them! A clean rag and some degreasing cleaner is all you need.
Jack up the car and place it on jack stands. Remove the tire and wheel, with a lug wrench.
Determine whether your vehicle is equipped with an exposed or captive sensor. If the reference ring is visible behind the brake rotor, then you have the exposed type, most common with disc brakes. Concealed sensors are mounted inside the differential housing and do not need cleaning on a regular basis, since they are not exposed to the elements.
Using a wrench, remove the sensor from its mounting bracket, taking care to not break the wires.
Wipe the tip of the sensor softly, with a clean rag sprayed with degreaser.
Clean the reference ring with a stiff, non-metallic brush and degreaser spray. Be sure to rotate the hub and clean the entire ring.
Carefully re-install the sensor into its bracket and replace the tire and wheel assembly.
Repeat this process for each wheel as appropriate.
