The Anti-Lock Braking System (ABS) works alongside manual control of the brakes, to ensure that braking is carried out in the most efficient way. This reduces locking of the brakes, skidding and helps deliver optimal traction when braking. ABS has been mandatory on all new cars in the US since 2013, and in the EU since 2004. Statistical analyses of accident databases suggest that ABS significantly reduces involvement in multi-vehicle crashes on wet roads.

 An Anti-Lock Braking System (ABS) in the car

Prior to ABS, only very experienced and skilled drivers were able to confidently execute the braking techniques required to arrest a skid or brake optimally. This is because only racing regularly, or driving frequently in icy conditions, would provide a driver with enough practice to master these techniques.

Cadence and Threshold Braking

To understand your ABS, it is useful to understand these braking techniques, as it is these that your ABS essentially executes on behalf of the driver. Firstly, Cadence braking (or stutter braking) is a driving technique that allows the driver to both steer and brake on a slippery surface. It can also be used to effect an emergency stop where traction is limited.

Often referred to as ‘pumping the brakes’, cadence braking involves pressing and releasing the brake pedal quickly and repeatedly. A human driver can perform one lock-release cycle per second with practice, while the Anti-lock braking system performs these operations automatically many times per second in rapid succession.

While cadence braking is effective on most surfaces, it is less effective at slowing the vehicle than keeping the tyres continually at the optimum braking point which is called Threshold Braking.

Braking beyond the slipping point causes the tyre to slide. At this point the frictional adhesion between the tyre and driving surface is reduced. The aim of threshold braking is to keep the amount of tyre slip at the optimal amount, the value that produces the maximum braking force. This is a difficult technique to master, and requires a lot of practice. A modern ABS system will kick in as soon as tyre slip passes this optimal point.

How does ABS work?

ABS functions by measuring how quickly each wheel on the vehicle is rotating, and the rate of acceleration or deceleration. All cars with an Electronic Stability Control (ESC) system will have ABS included as part of this system, though not all cars with ABS will have a full ESC. When the driver applies the brakes sharply, the ABS uses its sensors to detect when the wheels are approaching lock. The ABS then rapidly increases and reduces braking pressure, multiple times per second. This is the way in which ABS replicates the Cadence braking technique, but far more rapidly than a human driver can.

an Anti-Lock Braking system seen on the wheel

If the ABS kicks in when you are braking, the tyre slip has exceeded the optimal amount. This means that braking has occurred after the point the Threshold Braking technique aims for. In this sense the ABS is acting as insurance against the driver’s ability to execute a difficult technique. 

We can consider that the ABS works by;

  1. Executing Cadence Braking faster and more rapidly than a human driver
  2. Activating as soon as we miss the Threshold Braking point

What makes up an ABS?

An ABS is comprised of four main components the sensors, valves, Electronic Control Unit (ECU), and the Hydraulic Control Unit.

  1. Speed Sensor - This sensor monitors the speed of each wheel and determines the necessary acceleration and deceleration of the wheels. It consists of an exciter and a wire coil/magnet assembly, which generates pulses of electricity as the teeth of the exciter pass in front of it. Typically, an ABS will have four speed sensors, one for each wheel. Many vehicles have two or three instead.
  2. Valves - The valves regulate the air pressure to the brakes during the ABS action. There is a valve in the brake line of each brake that is controlled by the ABS. The resistance that you feel when braking suddenly at high speeds is actually the brake valves controlling the pressure that is being transferred to the brakes from the master cylinder.
  3. Electronic Control Unit - The ECU is an electronic control unit that receives, amplifies, and filters the sensor signals for calculating the wheel rotational speed and acceleration. The ECU receives a signal from the sensors in the circuit and controls the brake pressure.
  4.  Hydraulic Control Unit - The Hydraulic Control Unit receives signals from the ECU to apply or release the brakes under the anti-lock conditions. The Hydraulic Control Unit controls the brakes by increasing the hydraulic pressure or bypassing the pedal force to reduce the braking power.

Problems with your ABS?

Your ABS control module will notify you with a warning light if there is a malfunction in the system. Most frequently it is not a fault with the actual ABS itself but with one or more sensors, or the wiring to the sensors. The most common ABS problems occur when sensors become contaminated with debris or metal shavings.

Malfunctions also occur when sensor wiring becomes damaged, resulting in intermittent or no continuity. If you have a malfunction in the ABS, physically check all wiring and the brake sensors first. When checking the brake sensors, look for metal shavings and other debris that could cause false feedback to the electronic ABS controller. False feedback causes the ABS to trigger when it shouldn’t, or not function when it should. If the problem is with the system itself, as opposed to sensor readings being impacted by debris, then you should have the brake system looked at by a professional.