ABS (Anti-lock braking system): Everything you need to know

The anti-lock braking system (ABS) is a safety feature designed to stop your wheels from locking when you brake hard.

Since 2004, ABS has been a mandatory feature for all vehicles sold in the UK. This was deemed necessary because when a car’s wheels lock, its tyres can skid, leaving the driver with no control over the vehicle. An ABS really comes into its own on slippery roads or at fast speeds, when skidding can be particularly dangerous.

ABS works by automatically adjusting the brake pressure, so your wheels always have a good grip on the road. This reduces your stopping distance - and helps you maintain control of the steering.

This is accomplished with the help of the speed sensors on each wheel, which relay information to your car’s electronic control unit (ECU) so it can automatically adjust the brake pressure.

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A brief history of the ABS

Early incarnations of the ABS appeared in the early 20th century on trains and planes. Then, in the 1950s, Dunlop developed the Maxaret anti-skid system, which became widely used in the aviation industry.

However, it wasn’t until the late 1970s that car manufacturers embraced ABS technology, with the high-end Mercedes-Benz W116 being the first to feature it in 1978. By the 1980s, the technology was being utilised by many manufacturers across the industry.

In 2004, the EU passed legislation to make its inclusion mandatory for all vehicles.

How does an ABS work?

There are three main parts to an ABS: the wheel sensors (one on each wheel), the ECU, and the braking mechanism. The sensors keep track of how fast each wheel is turning and send this information to the ECU.

When the ECU is notified that one or more of the wheels have slowed down more than the others when the brake was applied, it will interpret this as a potential lock.

The ABS will then signal to the hydraulic pumps to repeatedly release and reapply the brakes to that wheel. Before ABS was widespread, drivers had to do this manually by ‘pumping’ the brakes. However, an ABS can do this multiple times per second.

You’ll know when the ABS kicks in, as you’ll feel the brake pedal pulsing or vibrating. You may also hear a scraping or grinding noise. All of this is normal and means your ABS is working properly, so don’t panic – just keep your foot on the pedal and let the ABS do its thing.

ABS warning light: What are the common causes??

While your ABS dashboard warning light should always come on briefly when you start your engine, if it remains lit, or appears while you are driving, this means there’s an issue. Common triggers for the ABS warning light include:

A malfunctioning ABS module

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

Additionally, your ABS module shares some important components with your 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 does malfunction, 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, meaning 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 of your car if you have to brake harshly.

What to do when your ABS light comes on

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’s 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 is 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).

Is it safe to drive with the ABS light on?

You can still drive if the ABS light is on, although you should be aware that you’ll only have the standard braking system to rely on.

Therefore, if you have to slam on your brakes, your wheels are likely to lock - and you won’t have complete control of your car. Your risk of being involved an accident will be much higher than if your ABS was working.

If your ABS is out of action, you’ll need to drive extremely carefully until you can get it fixed. We recommend taking your car to a garage as soon as possible.

In the meantime, you should drive more slowly than usual, keep more distance between yourself and other drivers – and make sure you steer and brake smoothly. If you do find yourself in a hard-braking scenario, you’ll have to resort to the old-fashioned manual brake pumping method:

Press and release the brake quickly and repeatedly until you gain control of your car. You’ll obviously never match the braking efficiency of an ABS, but you can produce a similar effect.

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.

How to prevent ABS issues

Regular maintenance is the key to avoiding ABS issues. Keeping your brake fluid topped up, changing worn brake pads, and regularly checking lines and hoses for leaks will keep your braking system in good working order.

Taking your car for routine brake inspections is essential too, especially if you’ve noticed anything unusual, such as grinding or squealing sounds – or the brake pedals feeling harder or softer than they should.

Maintaining the correct tyre pressure and tread depth is also important, as this can affect the information the sensors send to the ECU. 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.

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