How ABS works
ABS systems use a combination of electronic and hydraulic systems to modulate the brakes individually to prevent them from locking up. Antilock systems basically consist of wheel speed sensors, a hydraulic control unit (HCU), and an electronic control unit (ECU).
During braking the wheel speed sensors measure the rotational speed of the wheels and forward this information to the ECU. If the ECU determines that one wheel is decelerating at a much greater rate than another (indicating that the brake is about to lock), the ECU directs solenoid valves in the HCU to open to release hydraulic pressure to the brake until the rotational speed of the errant wheel matches the others.
Because of the complexity of the system, when the engine is started, a self-test sequence occurs. If the check is successful, the ANTILOCK and BRAKE lights will extinguish. If some component fails to check out, one or both lights will remain lit. One or both warning lights may also come on while driving, also indicating a problem. If just the ANTILOCK warning light comes on, some ABS component has failed, but the car still has power-assisted brakes. If both the ANTILOCK and BRAKE lights come on, the car is without power-assisted brakes, which is very dangerous. If just the BRAKE warning light comes on, this may be caused by low brake fluid or worn brake pads.
3-channel ABS v/s 4-channel ABS
In a 3-channel ABS system hydraulic pressure is supplied to the front brakes individually, and is supplied to both rear brakes as if there were only one, although wheel speed is measured at all four wheels individually. This system is less complicated and cheaper to build but is does not provide as much safety and control as a 4-channel ABS.
In a 4-channel ABS system hydraulic pressure is supplied to to all four brakes individually, wheel speed is measured at all four wheels individually. Wheel lockup can be controlled and prevented on all four wheels indiviadually. Improved safety and control over 3-channel ABS.
why ABS can sometimes be bad for you
ABS does not work with part time 4WD
Taking ABS a step further: BAS
Taking steering a step further: ESP
Using ABS sensors for traction control: 4ETS