Antilock braking systems (ABSs) are electronic systems that monitor and control wheel slip during vehicle braking. Modern antilock braking systems all feature the following major components: Electronic Control Unit (ECU), Modulator Valves, Wheel Speed Sensors (pickup and exciter), ABS Malfunction Indicator Lamps, and Diagnostics. An ABS adjusts brake pressure much faster and more accurately than can drivers. It’s faster because: electronic controls are very fast and ABS modulator valves are physically closer to the brakes than is the driver’s foot brake valve. 
The following schematic shows the Typical Wiring Diagram of a Tractor ABS (Anti-lock Brake System). The Tractor ABS System consists of: Engine Electronic Control Module, Modulator Valve, ABS Electronic Control Unit (ECU), Wheel Speed Sensor, Rear Axle Service/Spring Brake Actuator, Battery, Diagnostic Computer, Antilock Lamp, Traction Lamp, and Traction Enable Switch.