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What is ABS (Anti-lock Braking System)? A Complete Guide

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In the past, suddenly hitting the brakes could make the wheels lock up and skid. Once this happens, it can be challenging to regain control of the car. This is dangerous not only to the people inside the vehicle, but also to everyone else on the road.

Luckily, modern vehicles are equipped with a standard safety feature that prevents wheel lock-up. It is known as the antilock brake system (ABS).

abs brake system
The ABS system prevents the wheels from locking and skidding during abrupt stops.

What is ABS?

The ABS system prevents the wheels from locking during abrupt stops. It lessens the danger of skidding, allowing the car to stop without going sideways. The system also allows drivers to retain enough steering control to avoid crashing into other cars during sudden and heavy braking.

The ABS system can also improve braking on slippery roads. It does this by monitoring the relative speed of the wheels against each other. The gathered data is used to modulate brake pressure, which helps control slippage and maintain traction when braking.

Here’s a fun fact about the ABS: it was first used in the 1950s on an aircraft. It became a part of a vehicle in the 1970s, specifically on a 1970 Lincoln Continental. Drivers were given the option to get “Sure-Track,” a rear-wheel braking system for their vehicle.

ABS: A Mandatory Safety Feature

ABS became a mandatory feature on all cars starting September 2013. The National Highway Traffic Safety Administration (NHTSA) made it a requirement together with electronic stability control (ESC) systems under the provisions of the March 2007 Federal Motor Vehicle Safety Standard (FMVSS) No. 126.

Although this safety feature was mandated fairly recently, antilock brake systems were already commonplace among car manufacturers. Some vehicles built in the 1990s and 2000s (and even a few in the 1980s) already had them.

what is abs system in car
ABS became a mandatory feature on all cars starting September 2013.

What is an ABS Sensor?

An ABS sensor, also known as a wheel speed sensor, is one of the four primary components of an ABS system. It monitors the speed of the wheels and gathers data that is used by the ABS controller to prevent tire slip.

What are the Components of the ABS?

There are four primary parts involved in an antilock brake system: the pump, controller, wheel speed sensors, and solenoid valves. All these components work together to ensure the car can stop at the shortest distance possible without its wheels locking when braking.

  • Pump – Most antilock brake systems use a pump to restore brake fluid pressure to the brake lines after the valves have been released.
  • Controller – The electronic brake control module (EBCM) is the brain of the ABS. Also known as an electronic brake module (EBM) and controller antilock brakes (CAB), it uses data from the sensors to determine whether or not to pump the brakes.
  • Wheel speed sensors – These sensors monitor how fast the wheel or wheels are rotating.
  • Solenoid valves (inside the modulator assembly) – Valves in the brake line allow, block, and release pressure on the brakes by assuming three different positions
abs brake system parts
There are four primary parts involved in an antilock brake system: the pump, controller, wheel speed sensors, and solenoid valves.

How Does the ABS Work?

The ABS controls tire slip by keeping track of the relative deceleration rates of the wheels during braking. If one wheel slows faster than the other wheels or the threshold set by the ABS, the system perceives this as the wheel starting to slip. If the tire is on the verge of slipping, it might be in danger of locking and losing traction.

The ABS responds by decreasing hydraulic pressure to the brake on the affected tire. This lets the wheel momentarily speed up to gain traction. Once the traction is regained, brake pressure is reapplied to slow it. This cycle repeats until the driver eases pressure on the brake pedal or the vehicle stops.

abs system diagram
The ABS controls tire slip by keeping track of the relative deceleration rates of the wheels during braking.
Play the video to see how the ABS system prevents the wheels from locking during abrupt stops.

ABS Limitations

There are two instances where the capabilities of an ABS is limited. It may not be able to provide the shortest stopping distances given the following circumstances:

Straight stops made on smooth, dry pavement by a skilled driver

On a smooth, dry road, an expert driver can hold the wheels consistently closer to the ideal slip rate than the ABS. This is because current antilock braking systems allow tire slips as low as 5%, a rate lower than the maximum achievable tire traction.

However, for the average driver, the ABS will stop the vehicle in the shortest distance possible.

Braking on loose gravel or in deep snow

When driving under these two circumstances, the ABS won’t be able to provide the shortest stops. This is because a locked wheel would be able to stop the car faster than the ABS can. Loose debris can build up on the wheel’s front and form a wedge that could stop the vehicle.

However, with an active ABS, this wedge won’t form. Some vehicles have an option to turn the ABS off when driving on these roads to allow shorter stopping distances.

abs system
An illuminated ABS warning light means the antilock brake system has been deactivated because of an error.

What Does the ABS Light on the Dashboard Mean?

An illuminated ABS warning light means the antilock brake system has been deactivated because of an error. If the car also has stability control and traction control systems, these systems will be shut off because they depend on the same wheel speed sensors.

The ABS light should briefly activate every time the car is started as part of a system check. If the light stays on, this means there is a potential problem, and the system has been shut down.

Driving with an illuminated ABS warning light is dangerous, especially when traveling on slippery roads. It typically indicates that you have diminished steering control and increases the risk of your wheels locking during sudden brakes.

To switch off this warning light, refer to a repair manual for the appropriate steps. If you don’t want to DIY, you can always bring your car to an auto repair shop and let a mechanic fix the problem.

what are anti lock brakes
The ABS system monitors the relative speed of the wheels against each other and modulates the brake pressure, which helps control slippage and maintain traction when braking. Click the infographic to view full size.
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