The electronic control unit receives signals from different sources. A switch at the brake pedal provides a brake-operating signal. Another in the ignition system signals the engine is operating. This sets off the automatic check the ABS conducts every time the engine starts.
Another input is from the wheel speed sensors. These signals are used to control the hydraulic control unit and anticipate wheel lock. If a wheel starts to lock, the electronic control unit operates the solenoid valves to reduce hydraulic pressure appropriately.
Source: CDX Global
