What is an anti-lock braking system (ABS) and how does it work?

The anti-lock braking system (ABS) is a key safety feature found in virtually all modern cars.

When a car’s brakes are applied forcefully, the wheels can lock, causing a loss of traction and steering control. ABS prevents this by rapidly applying and releasing the brakes, which helps to maintain grip and control.

This system allows drivers to steer whilst braking in emergencies, reducing the risk of skidding and improving overall safety.

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How does an ABS work?

An ABS comprises four key components:

Speed sensors: These sensors are located on each wheel and constantly monitor wheel speed.
ECU: The electronic control unit (ECU) continually receives information from the speed sensors. If it detects that any of the wheels are slowing down more than the others during braking, it sends a signal to activate the valves.
Valves: The valves located within the brake lines are controlled by the ECU. They can be opened, closed, or partly closed as required to regulate the brake pressure applied to each wheel.
Pump: When an ABS valve releases pressure, the pump restores it by pushing brake fluid back into the system, allowing brake force to be reapplied quickly. This helps the car to maintain its manoeuvrability.

Before ABS technology became widespread, drivers were taught to pump their brakes with a technique known as ‘cadence braking’ to prevent wheel lock-up and maintain as much steering control as possible. This required skill and exertion in a stressful situation.

An ABS can perform this function automatically - and often with greater precision than a human driver could.

When the system detects that the wheels are about to lock, it rapidly applies and releases the brakes (up to 15 times per second). This controlled pulsing slows the car down, improves manoeuvrability, and helps to reduce the stopping distance.

How do I know when the ABS is active?

You’ll usually know when your ABS system activates, as you’ll feel a juddering sensation from the brake pedal, accompanied by a grinding, scraping, clicking, or buzzing sound.

ABS warning light: What are the common causes?

Although the ABS dashboard warning light should always appear briefly when you start your engine, if it remains lit, or appears while you are driving, this means there’s a potential issue.

A persistent ABS light will also cause your car to fail its MOT, so it’s important to deal with this proactively.

Common triggers for the ABS warning light include:

A malfunctioning ABS module

Your ABS module could stop working for numerous reasons, including corrosion, wiring problems, loose connections, and out-of-date software.

The ABS module also shares some important components with the traction control system (TCS). So, if anything goes wrong with your TCS, your ECU may not be able to differentiate between that system and your ABS.

If the ABS module malfunctions, the system won’t work as it should. It won’t be able to modulate the brake pressure, potentially leading to the wheels locking up and sending you into a skid, with little control over the steering.

Low brake fluid levels

Your braking system is an integral part of the ABS, so any issues with your brakes can cause the ABS warning light to appear.

If your brake fluid is low, there won’t be enough hydraulic pressure for the ABS to pump and release the brakes when it needs to. As such, it won’t be able to stop your wheels from locking.

Faulty wheel speed sensors

The ABS relies on accurate information being sent to the ECU from each of the sensors.

If one or more of these sensors stops working – whether due to dirt build-up, electrical issues, physical damage, or general wear and tear, then the data received by the ECU may be incomplete, or none may be sent in the first place.

Without the right information, the ECU won’t know when to apply the ABS, which means there’s nothing to prevent your wheels from locking up when you brake.

A blown ABS fuse

Although the fuse is primarily designed to protect the system’s electrical components from breaking if there’s a power surge, if it blows, the entire ABS will shut down.

This will leave you with only the standard braking system. So, you’ll be at a much higher risk of skidding and losing control should you have to brake harshly.

Accidentally turning off the ABS

Finally, many cars have the option to deactivate the ABS and TCS. If you switch either of these off, you should see a light on the dashboard warning you they’re inactive.

So, if you see either of these warning lights, make sure you haven’t accidentally caught a switch. Check your owner’s manual if you’re unsure where they’re located.

ABS benefits

Better steering control

ABS prevents wheel lock-up, allowing you to steer around obstacles, even when braking forcefully, which can be invaluable in emergency scenarios.

Enhanced safety

By preventing skids, maintaining wheel grip, and steering control, ABS can significantly cut the risks of road accidents.

Reduced stopping distances

ABS technology significantly reduces stopping distances on many road surfaces, as it can pump the brakes faster than a typical human driver.

Less tyre wear

When wheels lock up, this can cause excessive wear and flat spots (localised areas with more wear than the rest of the tyre).

ABS can be highly effective at preventing these issues, helping to prolong the lifespan of your tyres and braking system.

Works together with other safety systems:

ABS works in synergy with other safety systems such as electronic stability control (ESC) and the traction control system (TSC).

The ESC utilises the ABS’ sensors to detect and remedy traction loss. This provides greater stability during sharp and sudden manoeuvrers.

The TSC can prevent wheel spin whilst accelerating by using the ABS to apply the brakes on specific wheels - or reducing the engine power.

ABS drawbacks

Limited performance in some conditions

Whilst ABS works well in most conditions, performance may be compromised in certain environments:

On dry asphalt, stopping distances may be slightly higher than an expertly executed manual brake.
Extremely slippery surfaces such as lightly packed ice and snow may also increase stopping distances, as the ABS can prevent the wheels from gripping them.
Mud or deep gravel can also present challenges for an ABS due to the lack of grip.

Driver preference

If you’re used to a driving without ABS technology, the pulsing sensation in the brake pedal when it activates might take some getting used to.

In an emergency scenario, you should ignore the temptation to pump the brakes and instead maintain constant pressure on the pedal.

Maintenance complexity and cost

ABS systems are more sophisticated than older, simpler braking systems. Repairs can be more expensive due to the additional sensors and electronic components.

What to do when your ABS light comes on (step-by-step)

When your ABS warning light comes on, there are a few things you can try yourself before calling a mechanic.

First of all, check the brake fluid levels:

Park your car and turn off the engine. Make sure the car iIs on a level surface.
Open the bonnet and find the brake fluid reservoir. You should be able to see from the outside how much fluid is in there. There are ‘minimum’ and ‘maximum’ fluid level markers.
If you can’t see the level, or if it’s more towards the ‘minimum’ mark, you’ll need to open the cap. Give it a good clean so that nothing falls in, then carefully open it. Top the fluid up to the ‘maximum’ line, making sure you use the one that’s recommended for your car.
Put the cap back on and close the bonnet.

If this doesn’t resolve the warning light, it’s time to check the ABS fuse:

Park your car and turn the engine off. Open the bonnet and look for the fuse. Your owner’s manual will have a diagram showing where the ABS fuse is.
Have a look for any damage. This is unlikely to be visible from the outside, so you’ll usually have to take the fuse out and check the metal strip. If it’s broken, this means the fuse has blown, so you’ll have to fit a new one.
Replace the fuse with one that has the same amperage, making sure it’s properly installed. Close the bonnet and start the engine to see if the ABS light stays on.

If neither of these methods work, you may have to reset the system. To do this, you’ll need to disconnect the battery:

Park your car and turn off the engine.
Open the bonnet and look for the negative (-) terminal of the battery. Disconnect this and leave it for around 15 minutes. This will reset the car’s entire system.
Re-connect the terminal and start the engine. Check to see whether this has solved the issue.
If the light is still on, it’s time to take your car to a professional mechanic, as you may be dealing with a more complex issue involving components such as the sensors, module, or braking system (including the hydraulics).

What are the different types of ABS?

Four-channel, four-sensor ABS

Featured as standard in most modern cars, this is the most modern and effective ABS system, with separate speed sensors and valves for each of the four wheels. This maximises braking force and allows individual wheels to be monitored.

Three-channel, three-sensor ABS

Usually found in pickup trucks with four-wheel ABS. Although both front wheels have their own sensors and valves, the rear wheels share a sensor and a valve, which is located on the rear axle.

One-channel, one-sensor ABS

Typically featured in older cars and light-duty trucks with rear-wheel ABS, one-channel, one-sensor ABS uses a single sensor and valve for both rear wheels.

Whilst a four-wheel ABS also aids steering, a rear-wheel-only ABS is only designed to prevent spinning on slippery services.

Is it safe to drive with the ABS light on?

Driving without ABS does increase your risk of losing control and being involved in a collision, so you should drive cautiously.

When you’re driving with the ABS light on, you’ll have to rely solely on the standard braking system. Should you need to slam on the brakes, the risk of wheel-lock and losing control of your car is increased.

If you’re dealing with a persistent ABS light, we recommend taking your car to a garage as soon as possible.

Tips for driving with an ABS light

Drive more slowly than usual.
Keep more distance between yourself and other drivers.
Steer and brake in a smooth, controlled manner.

If you need to stop suddenly, pump the brakes quickly and repeatedly until you regain control. Whilst this won’t typically match the braking efficiency of an ABS system, it’s the next best option.

Please note: If your brake warning light appears alongside your ABS light, this means your whole braking system is faulty - and your car isn’t safe to drive.

In this case, you should pull over as soon as you can, gradually decreasing your speed and applying very gentle pressure to your brakes. Then, turn your engine off and call for assistance.

ABS maintenance tips

Keep up with your car’s servicing and maintenance schedule.
Ensure brake fluid is topped up.
Change worn brake pads.
Check brake lines and hoses for leaks.
Maintain the correct tyre pressure and optimal tread depth (at least 3mm), as this can affect the information sent to the ECU.

Take your car to a mechanic if you notice grinding, or squealing sounds – or the brake pedals feeling harder or softer than they should.

Your mechanic should test your ABS during your car’s regular services too, using specialised diagnostic tools to find, check, and fix specific error codes.

The history of ABS

The first ABS technology was developed for aircraft in the 1920s to prevent wheel lock-up when landing.

In the 1950s, Dunlop developed the Maxaret anti-skid system, which became widely used in the aviation industry.

ABS was first used for automotive applications in the 1960s. The 1966 Jensen Interceptor FF was one of the first models to feature a hydraulic ABS system. By the 1980s, the technology was widely utilised by car manufacturers.

In 2004, the EU passed legislation to make its inclusion mandatory for all new vehicles (incorporating the UK market). It was also made a federal requirement for the US market from September 2011.

Frequently Asked Questions

Anti-lock brakes work best on dry roads, as they can prevent wheel lock-up without significantly increasing stopping distances.

They also perform well in wet conditions and help to maintain steering control when the car would otherwise struggle.

Anti-lock brakes are less effective on ice, especially if you don’t have winter tyres. As there’s so little grip, even light braking can affect traction.

In snowy conditions, ABS can sometimes reduce braking effectiveness.

In fact, a locked wheel could be advantageous in this scenario, as it digs into the snow, causing a build-up in front of the tyre, which creates resistance!

No, if your car is equipped with ABS (and the system is activated), you should not pump the brakes.

If you do, the ABS may struggle to keep up, which could increase your stopping distance and risk of a collision.

Your ABS brakes should be checked as part of your regular service – typically every 12,000 miles or once every 12 months.

However, if the warning light appears between service intervals, you should take your car to a mechanic promptly.

Although disabling the ABS isn’t generally recommended, some drivers will do so in certain scenarios.

Off-road driving may be easier for an experienced driver when they have complete control of the brakes. When off-roading with ABS, the wheels may spin and lose traction.

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