Blog

Fault in ABS Braking System

ABS warning lights.

These three different ABS warning lights, indicate potential issues with the vehicle’s Anti-lock Braking System, such as a system malfunction or a problem with brake fluid pressure.

First let us understand how the ABS system works.

In the diagram above. The blue lines, containing brake fluid, start at the brake pedal in the car. As the pedal is depressed the pressure is exerted, via the brake fluid  on to the brake pads against the brake rotors to slow the car down.

At the centre of all four wheels you will notice a cog with gaps in it. The cog is made of f ferromagnetic material. About a millimetre away from the cogs are “Hall Effect Sensors” these sensors react to the difference between the raised portion of the cog and the gap between them. The Hall Effect sensors send a signal via the red wires to a computer which compares the signals from each of the hall effect sensors.

The ideal gap between the wheel speed sensor and the moving ring  is typically around 0.5 to 1.5 millimetres

Should one of the wheels lockup and send the car in to an uncontrollable slide, the Hall Effect sensor would stop sending fluctuating signals to the computer. The computer reacts instantaneously to reduce pressure on that wheel’s brake fluid allowing the wheel to start rotating again.

The pulsating release and applying pressure occurs at about 50 times per second.

The locked up slide is eliminated and the car is once again brought under control

Below is a brand new side shaft showing the gaps that the Hall Effect sensors

Below is an extreme close-up of the gaps which have been cut into the solid metal the ring.

Below is a picture of the same ring which has deteriorated to the weather and dirt it was subjected to. This expanded the diameter of the ring casing the gap between the ring and the Hall Effect sensor.

It is quite easy to see the impact marks on the solid portions of the upper portion of the cogged wheel. A close up of ring showing impact marks from the gapped ring which destroyed the sensor and activated the warning light on the dashboard.

Extreme closeup of the sensor end where you can see the wear marks. This end is normally a smooth finish asa can be seen outside of the grooved area

The incident that motivated this blog.

A new customer arrived. Over the last few months, he had three new sets of sensors fitted, and paid for, to his car. The root cause of this problem was not diagnosed.

The problem which we diagnosed was not the sensors that kept braking down but the deterioration of the gapped ring assembly.

Another safe car and a happy customer.