Diagnostics

ABS

Introduction to ABS

The abbreviation ABS stands for anti-lock braking system. The purpose of this system is to prevent the

wheels from locking when the brakes are applied when the brake force generated exceeds that which

can be transmitted to the road via the tyre.

A great benefit with the ABS system is that it provides a maximum exploitation of the available road

surface friction when the brakes are applied. Consequently the braking distance is considerably shorter

and the vehicle is much easier to control under emergency braking.

If the vehicle is pulling a trailer the most efficient conditions are when both the tractor and trailer are

equipped with non-locking brakes.

The ABS system is governed by a control unit (micro-computer) which senses the wheel speeds with

the aid of a sensor and a pole wheel or exciter ring fitted to the brake drum or disc. The signals from

the sensor are processed by the control unit, which, via solenoid valves, governs the braking pressure

of the individual wheel brake actuators. Where axles form a bogie, it may only have sensors fitted to

one of the axles forming the bogie. The ABS system includes a number of control functions that test

the individual electrical circuits to ensure that they are functioning correctly. Should a fault occur the

driver is informed by a warning lamp on his instrument panel or, if a trailer is being drawn, via a lamp

visible to the driver on the front of the trailer.

Categories of ABS

Towing Vehicles

Category 1

The ABS will operate on at least one front axle and on one rear axle and have the ability

to utilise the higher adhesion when braking on a split friction surface. This will require one

of the axles, usually the rear axle, to have independent control. A typical system would

have four sensors and four modulators but could effectively have four sensors and 2 or 3

modulators. In the latter case the front axle could use one modulator with a “Select Low”

control philosophy where modulation action is taken on the first wheel to lock. This

system will protect the towing vehicle from brake induced jack-knifing and enable the

driver to steer during braking by preventing the directly controlled wheels from locking.

Category 2

The ABS will operate on at least one front axle and on one rear axle. The system will

have a minimum of four sensors and two modulators (one for each axle). The system will

generally operate on a “select Low” control philosophy where modulation action will be

taken on the first wheel to lock. This system will protect the towing vehicle from brake

induced jack-knifing and enable the driver to steer during braking by preventing the

directly controlled wheels from locking.

Category 3

The ABS operates only on the rear (drive) axle. The system will have two sensors and

one modulator and generally a “select Low” control philosophy where modulation action

will be taken on the first wheel to lock. This system will protect the towing vehicle from

brake induced jack-knifing by preventing the directly controlled wheels from locking.