System Description
ANTI-LOCK BRAKE SYSTEM - DESCRIPTIONAnti-lock Braking System (ABS)
NOTE: The Anti-lock Brake System (ABS) is a standard feature, with Electronic Traction Control (ETC) optional.
The brake system is hydraulically power assisted with an integrated, electronically controlled four channel ABS system.
The use of a power assisted brake system means that, during brake application, additional hydraulic energy is provided by the hydraulic power unit. This hydraulic power unit consists of an electrically driven pump and an accumulator which stores hydraulic energy in readiness for brake application. A pressure switch controls hydraulic pump operation to maintain fluid pressure in the accumulator.
Fluid pressure is distributed from the brake booster/ABS modulator unit to each of the four brake calipers. Foot pedal pressure is assisted by pressure created in the master cylinder and direct pressure from the power valve. The combination of the master cylinder and the power valve allows the driver to vary braking force by varying force applied to the brake pedal.
The hydraulic system comprises two completely independent circuits, with a vertical i.e. front/rear split. The combined POWER and HYDROSTATIC CIRCUIT supplies the front calipers. The POWER CIRCUIT supplies the rear calipers.
The purpose of ABS is to prevent the vehicle wheels locking during brake application, maintaining vehicle steerability and stability. This allows the vehicle to be steered while the brakes are applied, even under emergency conditions, and to avoid obstacles where there is sufficient space to redirect the vehicle.
The ABS ECU receives wheel speed information from four wheel speed sensors. The ECU monitors the deceleration of each wheel during braking and, in the event of one or more wheels being outside the expected values, controls via the hydraulic system the braking force available to that wheel.
When the wheel speed has returned to within the expected limits, the ECU restores the hydraulic pressure to the wheel. The ABS system allows optimal stopping distances to be achieved and prevents the wheels from locking.
The system is active after the ignition is switched to position II and has completed a bulb check. ABS and ETC warning lamps in the instrument pack warn of system operation and failure. Normal (non-ABS) braking remains available in the event of an ABS system failure.
When the ABS system operates, the driver will experience an audible noise from the ABS modulator and vibration transmitted through the brake pedal.
WARNING:
- 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.
Electronic Traction Control (ETC)
ETC is an option, available as an extension to ABS. The system operates on the rear axle only, to prevent loss of traction where one wheel has more grip than the other. The system works by applying the brake to a spinning rear wheel. This transfers torque to the wheel with grip. By applying the brake, ETC supplies the torque resistance which the wheel cannot.
An example of when the system would operate is where one side of the vehicle is on ice, with the other side on tarmac. ETC will control the spinning rear wheel.
If both wheels spin the system does not operate, as braking one wheel will not aid traction.
The system switches itself out at 50 km/h (30 mph). A vehicle travelling above this speed will not need ETC.
ETC system operation is smooth and continuous and will not affect the comfort of the vehicle.
ETC is inhibited when the brakes are applied. When the ETC feature is operating the 'TC' warning lamp in the instrument pack will illuminate for a minimum of two seconds, a single audible warning will sound and on earlier vehicles 'TRACTION' will be displayed in the message centre.