Stopping In Time: What Are Anti-Lock Braking Systems?


How ABS works to keep you safe.

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What does ABS stand for? It's an abbreviation for 'anti-lock braking system' and is one of the major active-safety advances first introduced in the '70s. The anti-lock brake system is now common on most cars and trucks, and is considered a prerequisite these days. Modern ABS works in conjunction with the standard brake system, electronic stability control, and traction control in cars. But what is ABS actually? It's a system that dramatically reduces slippage and loss of control during hard braking, caused by the locking up of one or more of your vehicle's wheels. This is especially hazardous in inclement weather where you need to be able to rely on skid detection tech and subsequent effective braking.

ABS Test Mercedes-Benz

Background on the Anti-Lock Brake System

Concepts of the modern ABS system were developed since the early 20th century, mainly for the aircraft and rail industries, and it became commonplace in aircraft in the '50s. Several variations - some of which were rear-wheel anti-lock brakes only - appeared in the '70s on US cars. The 1978 Mercedes W116 is regarded as the first production car with fully electronic, multi-channel, all-wheel ABS; the automaker fitted all its cars with standard ABS since 1987.

Braking System

How ABS Works

ABS determines when a wheel is about to lock under braking and prevents it - and the uncontrollable skid that would otherwise follow. How ABS works is by using wheel-speed sensors to monitor the individual wheels' speeds and rapidly releasing and reapplying brake pressure to the wheels to keep them rotating. This brake 'pumping' multiple times per second keeps the wheels on the verge of locking up. This type of electronically controlled braking dramatically reduces stopping distances. Many semis and some cars use momentary applications of the ABS in strong winds to stabilize the vehicle. ABS works on either a drum or disk brake.

Braking Distance

The Types of ABS Systems

There are three types of ABS systems available:

  • Four Channel ABS: Widely considered to be the best of the available systems, 4-wheel ABS individually monitors each wheel by means of separate valves and sensors at each corner. This helps to make sure the greatest braking force is applied when necessary.
  • Three Channel ABS: Here, each front wheel has its own separate valve and sensor, while both rear wheels share one, located in the rear axle. Although still efficient, this system requires both rear wheels to lock up at the same time in order for ABS to kick in.
  • One/Two Channel ABS: Although quite outdated now, these systems make use of either a pair of sensors (one for the front wheels, one for the rear) or only one, located on the rear axle and more common to pickups and vans with rear-wheel ABS.
Car ABS Block

The Components of ABS

As you can see on this diagram of what ABS systems look like, there are four main components of ABS:

  • Speed sensors. These monitor the speed of each wheel to sense an imminent lockup.
  • Valves. These allow, block, or release the pressure to the wheel about to lock up to keep it rotating. It can do this multiple times per second.
  • Pump. Generates the brake pressure and restores it once any of the valves released some of it. Enough brake fluid is stored in an accumulator for this purpose.
  • Controller. The electronic module that receives all the data from the sensors and acts as the control unit to determine when pumping the brakes is required.
Car ABS control module Mercedes-Benz

The ABS System in Conjunction With ESC and TC

ABS and electronic stability control (ESC) have been compulsory fitments on all cars sold in the US since 2013, from a humble sedan such as the Chevrolet Sonic to the largest SUV, such as the Lincoln Navigator. ESC uses the ABS's wheel sensors to monitor cornering behavior and pre-empt a loss of control, whether the brakes are being applied or not. By using the ABS system to pump each wheel's brake as necessary, it allows you to guide an out-of-control car that has started to skid back into line. Traction control is an additional system that only prevents wheelspin, also using the ABS to stop spinning wheels and restore traction.

Driving Without an Anti-Lock Brake System

It goes without saying that driving without ABS or ESC could result in uncontrollable skids, especially in adverse road conditions and under sudden deceleration from braking. Under maximum braking and without the benefit of ABS on a slippery surface, the car will lock its wheels and skid on in a straight line, regardless of steering input, leaving the driver unable to avoid a potential collision. ABS offers significant safety benefits, and is, thankfully, generally standard on modern cars.


Does my car’s ABS system require maintenance?

ABS systems are maintenance-free unless a component fails, in which case you will be notified. This can often be a result of a module malfunction, damaged sensor wiring, brake system neglect, or contaminated brake fluid. It's important to note that your standard brake setup requires maintenance - see our post here for more information.

Will I know whether my ABS system activates?

Yes. If your ABS system activates to prevent a skid under braking, you will feel a pulsation through the brake pedal and hear grinding sounds. This is perfectly normal and shows the system is doing what it should.

How will I know whether something is wrong with the ABS system?

The ABS light on the car's dashboard will illuminate if a fault is detected anywhere in the system. Avoid harsh braking and have it attended to by a qualified mechanic immediately. Assume that your ABS doesn't work and be even more cautious on the road.

Can I switch off my ABS?

Some sports cars might offer a function where racing drivers can switch off driver-assistance features like traction control, but ABS is generally always active. Unless you find a way around this by pulling fuses, for example, the ABS system can't be switched off.

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