Most cars even small cars now come with an ABS safety system fitted.

What is ABS?

ABS stands for antilock braking system. 

The reason for the development of ABS is very simple.  Under braking conditions if one or more of the wheels lock then this has a number of consequences:

  1. Steering control is lost
  2. Braking distance increases
  3. Tyre wear is abnormal
  4. An accident is far more like to occur

Most cars even small cars now come with an ABS safety system fitted.

How does it work?

ABS is usually a central control unit with a number of inputs and outputs.

The most important and prone to failure are the wheel speed sensors.   Most of these are simple inductance sensors and work in conjunction with a toothed or magnetic wheel or bearing.

As the toothed wheel rotates this generates a signal, the frequency and voltage or which are proportional to wheel speed.  This signal is used by the ECU.  Most systems though now use hall effect sensors.

The Electronic contol unit is normally built into the Hydraulic Modulator although they can also be seperate items.  These are usually a very expensive item and can be prone to failure also.  The ECU records the data from the wheel speed sensors and calculates what action be taken by the Hydraulic Modulator.

The hydraulic modulator has 3 operating positions:

  1. Pressure build up brake line open to the pump
  2. Brake line closed/pressure hold
  3. Release of pressure/brake line closed to resertion.  The valves are controlled by solenoids.  The motor only runs when ABS is activated.

If a fault in the system occurs the ABS turns itself off and brings on the car’s ABS light. 

If this occurs you should have your car checked out by one of our technicians.


Vehicles now also come with an added safety system called ESP.  This means electronic stability program.

This works alongside the ABS system by keeping the car stable when loss of traction is detected.  It can do this by slowing down any particular wheel to keep the car balanced.


This system is simply used to prevent the wheels from spinning during acceleration.

Common ABS failures:

  • Wiring faults
  • Faulty sensors
  • Faulty brake switch
  • Faulty ABS pump
  • Mechanical failure, example – broken or damaged ABS ring or bearing
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