The Anti-Lock Braking System (ABS) & How It Works
The Anti-Lock Braking System or Anti-Skid Braking System is commonly abbreviated as ABS and is an advanced braking technology installed on motor vehicles to prevent the wheels from locking up (ceasing motion) and skidding uncontrollably. It does so by pulsating the braking force delivered to the rear wheels and the front wheels similar to threshold braking, which is a technique commonly used in motor racing and the idea is to control the brake pedal pressure and apply maximum braking force to a point just before the wheels are about to slip.
The ABS system improves vehicle control and will greatly reduce the stopping distance on dry and slippery surfaces, however, it can significantly increase the stopping distance on loose gravel and snow-covered roads where traction between the tyres and road surface is limited.
ESP (Electronic Stability Control) works seamlessly integrated with ABS to further maintain tractive contact with the road surface by electronically controlling the front-to-rear brake bias. The ESP system, depending on the specific capabilities and implementation is also known as Electronic Brakeforce Distribution (EBD), Traction Control System (TCS), Electronic Stability Control (ESC) and Dynamic Traction Control (DTC).
Components of an ABS System
There are four main components that make up an ABS system; wheel speed sensors, hydraulic valves, a pump and an ABS controller, which is also known as an ABS Module or ABS ECU.
Wheel Speed Sensors
The wheel speed sensor is used to determine the acceleration or deceleration of the wheel. These sensors use an ABS ring / reluctor ring which spins with the wheel/differential and creates a magnetic field around the wheel speed sensor, which in turn will generate the voltage in the sensor.
In each of the brake lines, there is a hydraulic valve which is controlled by the ABS module. On some setups, the valve will have three positions:
- Position One – The valve is open and any pressure from the brake master cylinder passes directly through to the brake.
- Position Two – The valve is closed, blocking the brake line and isolating the brake from the master cylinder. This prevents the pressure from rising further if the driver presses harder on the brake pedal.
- Position Three – The valve opens partially, releasing some of the pressure from the brake.
The majority of issues with the valve system occur because of clogged valves. A clogged inoperable valve will be unable to open, close or change position and the system will fail in trying to modulate the valves and control the pressure supplied to the brakes.
The pump in the ABS unit is used to restore the loss in pressure to the hydraulic brakes after the hydraulic valves have released it. Signals from the wheel speed sensors are processed by the ABS ECU Module and if a wheel slip is detected, the module will signal the valve to be released to avoid the wheel slip. After the valve is released and the pressure is lost, the pump is used to restore the lost pressure to the braking system. The ABS controller will modulate the status of the pump to control and provide the desired amount of pressure and reduce slipping.
ABS ECU / ABS Module / ABS Controller
The ABS controller, also commonly known as an ABS ECU receives input signals from all the wheel speed sensors and can interpret the information received to identify if there is a wheel slip on any one wheel. If wheel spin is detected and a wheel loses traction, the controller will limit the braking force (EBD) and send the signal to actuate the hydraulic braking valves on and off.
Symptoms Of A Failing ABS Pump
Fortunately, there are signs that you can use to detect whether your ABS pump is failing.
ABS Warning Light or Engine Light: This is the most common sign that you have an issue with your pump. If you have an ABS orange light illuminated on the drivers’ control panel, you have an issue that needs addressing. If you own an older vehicle, it is possible the check engine light will illuminate instead, to warn you that there may be a problem with the ABS system.
Unresponsive Brake Pedal: If your brake pedal is not as responsive as it used to be, or it takes longer to depress, this can indicate that you have an issue with your pump. Do not leave it until your brake pedals become completely unresponsive, as this is a danger to yourself and other road users.
Forceful Braking: You may find that when you put your foot down on your brake pedal, it requires more force than usual, or that a lighter braking manoeuvre no longer works when you try and stop the vehicle. Having a hard brake pedal is not adequate for safe driving, and your braking distance will be affected as the pressure sensor may be faulty.
Brakes Locking Up: If your brakes are locking up when you driving normally, then there is an issue with your ABS pump. This is because, as we mentioned before, it is designed to ensure that your brakes don’t lock up and there is enough traction between the tyres and the ground.
With this issue, you should immediately get it checked by a professional, especially if you start to hear strange clicking noises.
Speedometer: On rare occasions, the speedometer will show that you are doing 0MPH, or will show you incorrect speeds when you are driving. Usually, this issue is accompanied by the check engine light or the ABS light coming on when you start your engine. However, if you experience this problem and there are no lights illuminated on your dashboard, you should still get your vehicle inspected by a professional.
The ABS Pump + ABS ECU / Module is a very common failing part, which here at Sinspeed we have extensive knowledge in testing and repairing. If your ABS unit has failed, check out our ABS Repair category for some of the most common repairs we perform. The units we provide exceed OE specifications and they come with a lifetime warranty! If you have any questions, you can call us on 0203 815 9441.