Anti-lock Brake System

Construction

The anti-lock braking system utilises exsisting components of a vehicle. Such as the brake calipers and wheel cylinders, brake rotors, brake lines, brake booster and master cylinder. It also however incorporates a few components of its own. Such as an electronic control module (ECM), wheel speed sensors, intergraded teeth on the rotors and/or axles and hydraulic control unit and motor pump (HCU).

ABS works very quickly in order to prevent the wheels from slipping. Yet still having enough pressure at the brakes to ensure stopping of the car when intended by the driver. So within milliseconds the system will release brake pressure when a brake is locked up, to prevent slipping of the tyre on the road. Then without delay, increase the pressure right back up to ensure stopping power. It will do this continuously until no wheels are locked up.

Under normal braking however, the ABS does not interfer at all. The only time it takes over is under heavy braking and any wheel becomes locked up and the driver is no longer in full control. It ensures that the vehicle maintains steerability (you can still steer) and stability (you won't spin out). It will also ensure that your vehicle has enough control through the wheels to manouvre around obstacles.

The three common ABS setups are single channel which the rear wheels are on the same channel (strictly stops the rear wheels only). 3 channel where again the rear wheels are on the same channel but the front wheels get their own individual channels. And 4 channel where each wheel has its own individual channel.

Inside the hydraulic control unit there are solenoids which are the muscle of the system. They work in pairs for each channel. They have three positions; normal braking position and pressure increasing position where the inlet valve from the master cylinder is open and the outlet valve to the reservoir is closed. Pressure holding position where both the inlet and the outlet valve are closed, so pressure will not increase nor will it decrease. And pressure releasing where the inlet valve is closed so pressure from the master cylinder will not increase and the outlet valve to the reservoir is open in order to release pressure to somewhere remote.

http://www.youtube.com/watch?v=Vy0p3OnF_18

This link shows the ABS operating with a scan tool displaying the variety of signals that are being processed. As you can see the wheel speed sensors take in all the information rather quickly and communicate it to the ABS ECM. Also as the brakes are applied the brake light swtich is active (on). It shows that it understands the engine is running as well as reading the throttle position. The ABS actuator will obveasly be on too as the ABS warning light would be alluminated if any so called problems were to occur.

Above is the ABS relay being signalled against the wheel speed sensor signalling that it is 'locked up'.