The pros, cons of ABS

ABS (originally from the German Anti-blockier system, now anglicised to Anti-lock Braking System) is in widespread use on passenger cars, motorcycles and, increasingly, heavy vehicles.

Since it first appeared in 1978, it has gone through several versions, steadily improving all the time.

What are the advantages and limitations of ABS, and how can an owner look after the system to keep it fully functional and reliable?

As the name implies, ABS prevents wheel lock-up under heavy braking. Drivers of older cars will know how easily one or more wheels can lock up during panic braking. When this happens, the wheels in question slide along the road instead of rolling, and this has two undesirable effects.

Firstly, on high-traction surfaces, such as bitumen or concrete, a sliding tyre has less grip than a rolling one. Contrary to what many people think, a car will therefore take longer to stop when wheels are locked.

Secondly, steering control is seriously hampered when tyres are sliding. On a motorcycle, the stability provided by a rotating wheel, which is essentially the thing that keeps the bike upright, is of course also lost, making it inevitable that the rider will fall unless he or she has lightning-fast reactions.

Maximum braking effect without loss of steering control is achieved by applying just enough pressure to the brakes to keep them all on the verge of lock-up. In the days before ABS, expert drivers could do this by modulating the pressure which they exerted on the brake pedal, but the techniques (either threshold braking or cadence braking) required exceptional sensitivity and skill.

ABS provides the same effect without demanding the skill of an experienced rally driver. Now, in an emergency, you can simply plant the brake pedal hard and stay on it. (Never pump ABS brakes !) The electronics will see to it that the braking torque at each wheel is kept at or very near to the onset of lock-up. You will always retain steering control, and, barring some exceptions (notably certain gravel roads), you will stop in the shortest possible distance.

On many cars, the ABS control module does a power-on self-test every time you cycle the ignition. If it detects something amiss, it illuminates the ABS warning light on the dash. But it's always better to forestall problems by keeping an eye on the components of the system. Broken or corroded wires to a wheel speed sensor are a common ABS problem.

Regularly inspect the toothed ring and speed sensor at each wheel for damage from foreign objects. The gap between them is crucial. If there is any evidence of corrosion or dirt at the main connector into the ABS control module (usually somewhere under the bonnet), spray contact cleaner into the connection.

Remember, the best ABS in the world is worthless if you have a conventional brake problem, like worn linings or air in the hydraulic lines. And by the way, when you need to replace your car's brake fluid, try your utmost to avoid getting air into the ABS control module.