How Do Anti-lock Brakes Work?

How do anti-lock brakes work? This is actually a fairly common question, and one that just about everyone asks at least once. While it's not that difficult to understand how the system works, an in depth explanation is a bit involved. The simple answer to how an anti-lock brake system (ABS) works is that the brakes are engaged and released in a systematic manner which prevents them from locking up. The detailed explanation is listed below.

1. Brakes are applied. The ABS system first engages when you begin pressing on the brake pedal. This forces fluid from the brake master cylinder outlet ports to the Hydraulic Control Unit (HCU) inlet ports. By doing this, pressure is created which is transmitted through four open solenoid valves which are inside the HCU, and onward to each wheel.
2. The main circuit engages. At this point the main circuit (which is usually located at the rear of the vehicle) is engaged. When engaged, this master circuit of the brake master cylinder feeds the front brakes of the vehicle.
3. The secondary circuit engages. This is when the secondary circuit engages as well. Unlike the primary circuit, the secondary circuit is located in the front of the vehicle, and feeds the rear brakes.
4. The ABS module engages. It is at this point when the actual ABS module engages. The ABS module will pay attention to the ABS sensors near the wheels, and look out for when the wheel begins to lock. When this happens, the module will close the normally open solenoid valve for the applicable circuit, which will prevent any additional fluid from entering into that circuit. This is repeated for each of the wheels.
5. The ABS sensor monitors. The ABS system continues to monitors the wheels, and will open and close the solenoids repeatedly. Each time the solenoid opens or closes it does so for only half a second, and then repeats the process. It continues to do this to help prevent any locking of the brakes, which could end up causing the vehicle to get in an accident.
6. ABS disengages at 12 MPH. The ABS system will continue monitoring, opening, and then closing the solenoids that are connected to the brakes until the car slows down. Most ABS systems have a speed setting where it will cut off, and begin acting normally, which is set at about 12 miles per hour.