An anti-lock braking system or anti-skid braking system (ABS) is common nowadays, but it used to be that only the most expensive flagship cars had it. It was special enough that the cars equipped with an ABS had the legend “ABS” adorned on their flanks. Now, it is commonplace enough that it is hardly mentioned, except on the catalogue. The only time you may think about it is when the idiot light on the dashboard does not go out…
In general, the ABS is a four-wheel system that prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions. A pulsing sensation can be felt in the brake pedal, accompanied by a fall and then a rise in brake pedal height and/or a clicking sound. With recent advances, however, these sensation can sometimes be absent and only the ABS idiot light flickering or lighting up is the only giveaway that the ABS is functioning.
HOW IT WORKS
The ABS consists of the Hydraulic Control Unit (HCU), an Anti-Lock Brake Control Module and front and rear Anti-Lock Brake Sensors. When the brakes are applied, fluid is forced from the brake master cylinder to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of the HCU to each wheel.
Sensors on each wheel count or sense the rotation of the wheel (wheel speed) via a toothed or slotted disc. Vehicle speed is sensed through these sensors as well and all four should be the same speed, either in normal cruising speed or under braking.
If the anti-lock brake control module senses any wheel is about to lock (via the brake sensor data), it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit. The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel again to cross-check and if that wheel is now decelerating normally, it opens the solenoid valve for that circuit. This “cycling” happens many times per second. Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake.
SOME ABS DETAILS
Malfunction of the anti-lock braking system will cause the anti-lock brake control module to shut off. Normal power-assisted braking remains, of course. Loss of hydraulic fluid in the brake master cylinder will also disable the anti-lock braking system since the anti-lock brake control module monitors all electrical anti-lock functions and hydraulic fluid levels.
Each time you drive the car, at approximately 20kph (12mph), the anti-lock brake control module turns on the pump motor for approximately a half second. This is a normal function of the self-check by the anti-lock brake control module. When the vehicle speed goes below 20kph (12mph), the ABS turns off. Most malfunctions of the anti-lock braking system and sensors will cause the ABS warning indicator to be illuminated. If the vehicle is also equipped with traction control, this will also be disabled.
Most light trucks and SUVs use a form of ABS known as Rear Wheel ABS. The Rear Wheel Anti Lock (RWAL) system reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. The system monitors the speed of the rear wheels during braking. The Electronic Brake Control Module (EBCM) processes these values to produce command controls to prevent the rear wheels from locking.
Contrary to popular belief, ABS brakes will not stop your car faster. A good driver (read: racecar driver) may be able to defeat ABS brake stopping distances. ABS brakes are there so that you maintain control of your vehicle by avoiding wheel lockup, especially if you are caught out by conditions or not such a good driver anyway. The fact is that you have no steering control to avoid disaster if your wheels are locked up. Driving on slippery or wet roads you still need to allow for increased braking distances since the wheels will lock up much easier and the ABS will cycle much faster.
If your ABS light is on despite driving over 20kph or over the preset distance, it means your ABS is malfunctioning. This may be dangerous because you may have already been used to driving with ABS and still drive and brake normally without compensating for the ABS failure. Do drop by the shop and we can troubleshoot the problem, which may be as simple as a dirty wheel speed sensor or a low brake fluid level. See you soon.