Antilock Brakes / Traction Control Systems: Description and Operation

CAUTION: ABS is an aid to retaining steering control and stability while braking.
^ ABS cannot defy the natural laws of physics acting on the vehicle.
^ ABS will not prevent accidents resulting from excessive cornering speeds, following another vehicle too closely or aquaplaning, i.e. where a layer of water prevents adequate contact between tire and road surface.
^ The additional control provided by ABS must never be exploited in a dangerous or reckless manner which could jeopardize the safety of driver or other road users.
^ The fitment of ABS does not imply that the vehicle will always stop in a shorter stopping distance.

INTRODUCTION
The purpose of ABS is to prevent vehicle wheels locking during brake application, thus maintaining vehicle steerability and stability. This allows vehicle to be steered whilst brakes are applied, even under emergency conditions, and to avoid obstacles where there is sufficient space to redirect the vehicle.

DESCRIPTION
The brake system consists of dual hydraulic circuits in a front rear split. That is individual circuits to front and rear axles.
Wheel speed signals are provided by the fitting of exciter rings to axles and speed sensors.
An ABS stop light switch is fitted to provide a braking signal to the ECU as well as operate the stop lights.
Electrical wiring is provided as necessary with system relays and fuses. Discovery Electrical Troubleshooting Manual gives the location and wiring circuits of these components.
Visual warning of system malfunction is provided via a warning light in the instrument panel.

Hydraulic Circuit:

OPERATION
During normal braking the feel of the brake pedal on vehicles equipped with ABS will be the same as that on non ABS vehicles. During ABS cycling the driver will experience feedback in the form of a pulsating brake pedal and solenoid/pump motor noise from the Modulator.
The system is fully automatic in operation both on and off road, and will only act when the ECU detects that ABS cycling is necessary. The ECU continually monitors the operation of the system.
When starting the ABS system will go through a series of self checks, including a check of the solenoid valves/pump motor.

Brake Warning Symbol:

Warning Light
The warning light will be on during the self check (1.3-2 secs). If no faults are stored in the memory the light goes out for 0.5 sec, then comes on again. It will then remain on until the vehicle is driven faster than 7 Kph (5 mph). This is the only time that the light will be on without indicating a problem.
Any faults detected by the ECU will cause the warning light to illuminate indicating that the vehicle may not have full ABS control.