The ABS light comes on whenever there is a fault in the anti-lock braking system in a vehicle. The fault can be a low amount of fluid in the ABS reservoir to an electrical malfunction in the wheel speed sensor, control unit or the pump, but regular braking function is unaffected. Repair involves testing or replacing control unit and pump, the wheel speed sensors or power cycling the entire system.
Testing the control unit involves checking that the fuse is working properly and attaching a scan tool that reads stored troubleshoot codes to tell the technician the source of the problem. The scan tool can also cycle the pump on and off to check its operation. The wiring harness to the controller is also checked for damage as well as unplugging and plugging the control unit back in to check for loose connections. The wheel speed sensors tell the ABS controller how fast the wheel is spinning by reading a magnetic, toothed wheel on the end of the wheel axle. If, during braking, the controller sees that the wheel decelerates more than possible, the controller opens a valve to reduce brake line pressure until it senses an acceleration in the wheels It then uses the pump to reapply braking pressure, until it senses deceleration in the wheels. The controller cycles like faster than the wheel can change speed, keeping a maximum amount of deceleration without locking and skidding the wheel.