1
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Hydraulic control unit (HCU)
2
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Rear wheel speed sensor
3
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Front wheel speed sensor
In addition to the conventional brake system with two diagonally split brake circuits, the Anti-lock Brake System (ABS) consists of an Hydraulic Control Unit (HCU) with integrated electronic control unit (ABS module), four wheel sensors and the ABS and Electronic Brake Distribution (EBD) warning lamps in the instrument cluster.
EBD is integrated in the software of the ABS control unit, which electronically regulates and optimises the pressure applied to the wheel brakes on the rear axle in accordance with the operating conditions.
Function
The ABS is a closed loop control system in the brake system which prevents the wheels from locking up under braking and therefore maintains steerability and driving stability in extreme situations.
ABS control unit
The ABS control unit continuously compares a reference deceleration value with those detected at individual wheels. This allows the lock-up tendency of each wheel to be detected separately.
If the ABS control unit detects that a wheel is threatening to lock up, then the corresponding hydraulic valves in the valve block are either closed or opened.
The ABS control unit also passes on the wheel speed signals obtained from the wheel sensors to the Powertrain Control Module (PCM).
The warning lamps in the instrument cluster are actuated by the ABS control unit after switching on the ignition. They remain on up to the end of the ABS self-test, and are also actuated in the event of system errors.
The ABS broadcasts its lamp control signals onto the high speed CAN bus. The high speed CAN bus signals are converted to medium speed CAN bus signals by the PJB. The medium speed CAN bus signals are read by the instrument cluster.
The ABS control unit continuously monitors the system for possible faults and stores these as and when they occur. The corresponding trouble codes can be interrogated with the aid of Worldwide Diagnostic System (WDS) via the Data Link Connector (DLC).
The ABS control unit self-configures to the exact vehicle specification using Central Car Configuration data broadcast by the PJB. This allows the correct brake control calibration for the vehicle to be selected. If the Central Car Configuration is incorrect, or the wrong ABS module is fitted, a calibration will not be found and the ABS will not function. In this case, a limp-home EBD function is provided - but both ABS and EBD warning lamps will be illuminated and a DTC stored.
The ABS control unit is directly connected via a diagnostic data link to the DLC.
The HCU is pre-filled for service.
