Learn about ABS sensors, and how they work with your vehicle’s anti-lock braking system (ABS) to help you stay in control during emergency braking.
ABS sensors are an integral part of your car's anti-lock braking system (ABS), helping to prevent your wheels from locking when the brakes are applied sharply.
The system helps prevent skidding and loss of steering control, making it a powerful piece of safety equipment that helps keep you, your passengers, and other road users safe.
After brakes are manually applied, the ABS detects if your wheels are about to lock and, if they are, gently 'pumps' the brakes. It does this much more rapidly than a human driver would be able to do. This pulsating brake action means the driver is more likely to stay in control of the vehicle, because the wheels keep moving rather than locking up.
The sensors can determine wheel speed, acceleration and deceleration.
Under UK law, all new cars placed on the market since 2004 have been fitted with ABS.
Where are ABS sensors located?
An ABS sensor is situated around each wheel, mounted over a toothed ferrous-metal cog. The sensor measures how quickly the cog teeth pass under the sensor using magnet-based technology.
After the driver has applied the brakes, data from the sensors is passed to the ABS control unit, which then determines if - and by how much - the brakes should be automatically pumped.
The controller connects to a pump and a valve on each wheel. The pump can increase pressure on the brake line, while the valve can release pressure. These two components work in quick succession to maintain control of the wheels, creating a 'juddering' sensation in your brake pedal.
Under what conditions will my ABS be triggered?
Your car’s ABS system won’t be triggered in normal braking situations, or when you’re driving slowly. It only works if you brake suddenly and sharply - suggesting an emergency braking situation.
Using your ABS properly
You don't need to pump the brakes yourself; the ABS will do this for you. Simply apply the brakes firmly and maintain that pressure.
You'll feel a juddering or pulsating sensation through the brake pedals - this is perfectly normal. You may also hear a grinding sound.
Avoid jerky or sudden movements of the steering wheel; keep your hands firmly planted on the wheel and face the direction you wish to travel in.
Maintain brake pressure until your vehicle comes to a stop.
Don’t rely too heavily on your ABS
ABS is a powerful safety feature in emergencies, but you’ll still need to maintain full control of your vehicle.
How to stop a car without ABS?
As mentioned, all new cars have had ABS fitted for the last 20 years. If, however, you drive an older car, you'll need to pump the brakes manually. To do this, apply pressure fully, then quickly release the brake. Repeat this 'pumping' action until the vehicle comes to a complete standstill.
You can tell if your vehicle has ABS by checking the dashboard: there should be a symbol with 'ABS' inside a circle. The light beneath this symbol will illuminate if there is a problem with your ABS system.
Your owner’s manual will also tell you if you have ABS.
When might my ABS warning light come on?
Common reasons drivers see their ABS warning light illuminated:
- The ABS system is off
- One or all wheel speed sensors are malfunctioning
- The ABS controller module is malfunctioning
- Reservoir fluid is low
Refer to your handbook for more information.
How do I maintain my ABS system?
The handbook that came with your car will tell you how often you need to get your ABS system checked over. It is usually every four years, or every 30,000 to 40,000 miles.
If you see the ABS warning light come on, have it checked over promptly. As well as reducing levels of safety, a faulty ABS will cause your vehicle to fail its next MOT.
You can however usually use your vehicle if the ABS isn’t working, but as mentioned you should have it looked over by a professional as soon as you can.
Note that the ABS light may come on briefly before you start driving. If it stays on during travel, have it checked out.
Brake fluid checks
The brake fluid in your brake line transfers hydraulic pressure to your brakes. Regular checks and fluid replacement are therefore essential for the effective functioning of your ABS system - and your braking system more generally.
Signs your ABS might be malfunctioning
Aside from an illuminated ABS warning light, there may be other clues your ABS system is not working as it should.
Abnormal brake pedal feedback
If your brakes, when applied, feel spongy, soft, or especially strong, or if they pulse in an irregular fashion, then it could be a sign your ABS sensors are not functioning properly.
Pulsating sensation when braking
When the brakes are applied, you might have a pulsating feeling through your steering wheel or brake pedal, in combination with some unexpected or unusual braking behaviour. Irregular or frequent pulsing may also suggest one, some or all of your ABS sensors are suffering a malfunction.
Increased stopping distances
When braking on uneven or slick surfaces, a faulty ABS sensor may cause stopping distances to increase.
Is it illegal to drive without ABS?
It is not technically illegal to drive without ABS activated, but it will help to keep you safe in an emergency braking scenario.
When might you turn off your ABS?
Some drivers choose to turn off their ABS during off-road driving. However it should be turned on again when driving on normal surfaces.
How does ABS affect stopping distances?
Your car's ABS system works best on dry, solid surfaces or when it is wet - in which case stopping distances will likely be shorter. However, ABS might actually increase stopping distances on dirt or gravel, or after it has snowed (but steering control will still be improved).
ABS is also less effective on icy surfaces, when wheel lock can occur.
If you simply applied the brakes manually without ABS, the stopping distances may be shorter, but the likelihood of losing control of the vehicle would be increased.
In very wet situations, the teeth in the sensor cogs can fill with water, making the sensor readings less reliable (because the magnetic action doesn't work as well). The ABS will work properly again when this water has cleared.
