Anti-lock braking system

An anti-lock braking system, or ABS (from the German, Antiblockiersystem) is a safety system which prevents the wheels on a motor vehicle from locking while braking.

A rotating road wheel allows the driver to maintain steering control under heavy braking by preventing a skid and allowing the wheel to continue interacting tractively with the road surface as directed by driver steering inputs. While ABS offers improved vehicle control in some circumstances, it can also present disadvantages including increased braking distance on slippery surfaces such as ice, packed snow, gravel, steel plates and bridges, or anything other than dry pavement. ABS has also been demonstrated to create a false sense of security in drivers, who may drive more aggressively as a result.

Since initial widespread use in production cars, anti-lock braking systems have evolved considerably. Recent versions not only prevent wheel lock under braking, but also electronically control the front-to-rear brake bias. This function, depending on its specific capabilities and implementation, is known as electronic brakeforce distribution (EBD), traction control system (TCS or ASR), emergency brake assist (BA, EBA or HBA), or electronic stability control (ESP, ESC or DSC).

How Antilock Brake Systems Work

Since most cars on the road today have some form of Antilock Brakes (ABS) I think we should take a look at how they work and clear up some mis-information about them.

As always, what I describe here is how most systems work in general. Since different manufactures have their own versions of ABS their values, specifications and part names will differ. If you are having a problem with the ABS on your vehicle you should always refer to the specific service and repair manuals for your vehicle.

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.

During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound.

Vehicles with ABS are equipped with a pedal-actuated, dual-brake system. The hydraulic system consists of the following:

  • ABS hydraulic control valves and electronic control unit
  • Power brake booster
  • Brake master cylinder
  • Necessary brake tubes and hoses

The anti-lock brake system consists of the following components:

  • Hydraulic Control Unit (HCU).
  • Anti-lock brake control module.
  • Front anti-lock brake sensors / rear anti-lock brake sensors.

Anti-lock Brake System (ABS) operates as follows:

  • When the brakes are applied, fluid is forced from the brake master cylinder outlet ports 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.
  • The primary (rear) circuit of the brake master cylinder feeds the front brakes.
  • The secondary (front) circuit of the brake master cylinder feeds the rear brakes.
  • If the anti-lock brake control module senses a wheel is about to lock, based on anti-lock 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.
  • If that wheel is still decelerating, it opens the solenoid valve for that circuit.
  • 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.
  • The anti-lock brake control module monitors the electromechanical components of the system.
  • Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains.
  • Loss of hydraulic fluid in the brake master cylinder will disable the anti-lock system.
  • The 4-wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a three second illumination of the yellow ABS wanting indicator.
  • During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.
  • Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.
  • When the vehicle speed goes below 20 km/h (12 mph), the ABS turns off.
  • Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated.

Early ABS

Anti-lock braking systems were first developed for aircraft in 1929, by the French automobile and aircraft pioneer, Gabriel Voisin, as threshold braking an airplane is nearly impossible. An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models.

A fully mechanical system saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and, in automobile use, somewhat unreliable. However, a limited form of anti-lock braking, utilizing a valve which could adjust front to rear brake force distribution when a wheel locked, was fitted to the 1964 Austin 1800.

Chrysler, together with the Bendix Corporation, introduced a true computerized three-channel all-wheel antilock brake system called "Sure Brake" on the 1971 Imperial. It was available for several years thereafter, functioned as intended, and proved reliable. General Motors introduced the "Trackmaster" rear-wheel (only) ABS as an option on their Rear-wheel drive Cadillac models in 1971. Ford also offered a system called "Sure Trak" on the Lincoln Continental Mark III and the Ford LTD station wagon.

Modern ABS

In 1975, Robert Bosch took over a European company called Teldix (contraction of Telefunken and Bendix) and all the patents registered by this joint-venture and took advantage out of this acquisition to build the base of the system introduced on the market some years later. The German firms Bosch and Mercedes-Benz had been co-developing anti-lock braking technology since the 1970s, and introduced the first completely electronic 4-wheel multi-channel ABS system in trucks and the Mercedes-Benz S-Class in 1978. ABS Systems based on this more modern Mercedes design were later introduced on other cars and on motorcycles.

ABS brakes on a BMW motorcycle

In 1988 BMW became the world's first motorcycle manufacturer to introduce an electronic/hydraulic ABS system, this on their BMW K100. In 1992 Honda launched its first ABS system, this on the ST1100 Pan European. In 1997 Suzuki launched its GSF1200SA (Bandit) with ABS.


A typical ABS is composed of a central electronic control unit (ECU), four wheel speed sensors — one for each wheel — and two or more hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel, and when it detects a wheel rotating significantly slower than the others — a condition indicative of impending wheel lock — it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel. The wheel then turns faster; when the ECU detects it is turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied and the wheel slows. This process is repeated continuously, and can be detected by the driver via brake pedal pulsation. A typical anti-lock system can apply and release braking pressure up to 20 times a second.

The ECU is programmed to disregard differences in wheel rotative speed below a critical threshold, because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two. For this same reason, a differential is used in virtually all roadgoing vehicles.

If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.