What is ABS? How to check for an ABS fault

What is ABS? How to check for an ABS fault

According to a study conducted in 2004, cars fitted with ABS are 48% less likely to be involved in road traffic accidents than those without. 

It’s hard to put a number on the number of lives saved, but anyone with experience trying to stop in wet weather in a car without ABS knows how brilliant the technology is. It enables inexperienced drivers to brake safely in wet conditions, with a much-reduced risk of losing control or skidding. 

In this article, we’ll discuss how it works and how to tell if your car’s ABS system has a fault.  

What is ABS? 

An Anti-Lock braking System consists of a few key components not found in non-ABS-equipped vehicles. Its basic purpose is to prevent the wheels from locking or skidding under hard braking. 

Modern brakes are very powerful, without ABS it would be easy for them to lock up the wheels under emergency braking conditions. This is especially important on wet or damp roads. 

Why prevent skidding? 

It may seem counter-intuitive, to release the brakes when trying to slow down, but a wheel that is ‘locked up’ or skidding, provides much less to slow the vehicle than one that is still rotating.  

Further to this, a car's steering only works when the wheels are turning, if they’re stationary in a cloud of smoke, the car will understeer and be generally uncontrollable, ABS allows you to brake and turn. 

How does ABS work 

The ABS system consists of three main components, an Electronic Control Unit (ECU), an ABS pump/modulator, and ABS sensors. All three parts, along with the standard braking system, need to be in good condition for the system to function properly. 

Wheel Speed or ABS Sensors 

ABS sensors, aka Wheel speed sensors are typically used at each wheel, positioned behind the brake disc or drum. They remain stationary, whilst an ABS ring on the drive shaft, axle or brake component spins at road speed close to them.  

Wheel speed sensors use a magnetic field to detect the ABS ring, also known as a reluctor ring, pulse ring or pulse wheel. This allows the ECU to detect the speed at which each wheel is spinning. 

With this information, the ECU can detect if a wheel has locked up under braking, when this happens, the ABS controller or ECU will signal the ABS modulator to activate brake modulation. 

ABS Controller/Modulator/Pump 

The ABS modulator, sometimes called the ABS pump, is a brake component in-line with the standard braking system. It uses a high-pressure electric pump and complex valve systems to rapidly turn on and off the brake pressure to the offending wheel.  

All this happens within a split-second, the ABS pump will modulate the braking force until the brakes are released or traction is regained. In many cars, the ECU, and pump will be built into the same unit. 

How do I know if my ABS is working? 

If you recently had to brake very harshly, particularly on a wet or gravelled surface, and you felt the brake pedal and front of the car shake slightly as you came to a stop, accompanied by a fast juddering sound, this is your ABS system working.  

Symptoms of an ABS fault are hard to pinpoint, though, and if in any doubt you should always take the car to a mechanic to have the system checked, before continuing to drive. 

Bumper can help you split the costs of any car repairs with 0% interest monthly payments.

ABS warning light  

The wheel speed sensors are often used by other systems such as Traction control, so the car's ECU is constantly monitoring the output of them, if any should become faulty the car should display an ABS warning light on the dashboard. 

Wheels locking up 

One symptom is if your front wheels lock and skid upon hard braking, this can be heard easily if the windows are down, but can sometimes be hard to hear in the cabin. The car will feel like it has suddenly hit ice, steering will be heavy but unresponsive until the brakes are released. 

Brake power reduces under hard braking

In some cars, if the ABS pump is faulty, the system can seem fine until the ABS is needed. Then, when the brakes are applied and the ABS is activated, the braking power disappears. This can be a symptom of a failing ABS pump.

This can often remain unnoticed until you really need your brakes, it is important to have the system checked by a professional if you have any concerns at all.

Find a trusted garage here.

Common causes of ABS failure

ABS, like any other system on a car, can fail prematurely. Because of the relatively low number of parts involved, it is usually easy to diagnose but can be expensive.

Wheel Sensor Fault

Common if the car has just had work done on a suspension component, wheel bearing or wheel, the ABS sensors can be fragile devices, and a small knock whilst removing other parts can cause them to send faulty readings. 

An ECU diagnostic check should detect this issue, most cars will also warn of some kind of issue with a general ABS warning on the dashboard. ABS sensors range from £30-£150 each, and would usually take less than an hour to change.

ABS pump or modulator fault

An internal fault with the ABS pump may be harder to detect through software, but most cars would store an error code after the first failed attempt at initiating the ABS system.

ABS Modulators can become faulty due to a number of things such as age or build quality issues, but one thing they are sensitive to is poor brake fluid. Contaminants that get into the fluid over time can cause blockages in the small valves inside the pump. This is why it's important to check your brake fluid regularly.

ABS pumps can be very expensive, up to and exceeding £1,000 depending on the type of car, there may be options for refurbishing your old one for less, but your mechanic would be best to advise you.

Braking system

For the ABS to work effectively, the rest of the conventional braking system needs to be in good working order. 

Common faults with the braking system can be a failed master cylinder, old or insufficient brake fluid, excessively worn pads and discs, seized callipers and more.

What to do if you are unsure about your Anti-Lock Braking System 

If for any reason you suspect your ABS, get your car to a garage as soon as possible, they will be able to perform diagnostic checks as well as ‘field tests’ to see if the ABS is working as it should.

To find a garage near you enter your vehicle registration and postcode on our website. Plus, apply for a Bumper Credit Limit and split the cost of your repair bill into monthly instalments, at no extra cost.

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