Module, Anti-Lock Brake System (Abs): Operation

The Anti-lock Brake System (ABS) module is mounted to the Hydraulic Control Unit (HCU). The ABS module is a micro-controller based device which monitors the ABS system during normal braking and controls it when the vehicle is in an ABS stop. The ABS function avoids wheels lock during braking maneuvers, maintaining the directionality of the vehicle, and optimizing the reproducibility of stop distance.

The primary functions of the ABS are to:

• Monitor the anti-lock brake system for proper operation.
• Detect wheel locking or wheel slipping tendencies by monitoring the speed of all four wheels of the vehicle.
• Control fluid modulation to the wheel brakes while the system is in an ABS mode.
• Store diagnostic information.
• Provide communication to the scan tool while in diagnostic mode.
• Illuminate the ABS, Electronic Stability Control (ESC), or BRAKE warning indicator lamp when the system is faulted.
• Illuminate the ESC function lamp when a vehicle dynamic event is occurring.
• Provide vehicle speed information, by monitoring the wheel speed sensors, to all modules on the Controller Area Network-Chassis (CAN-C) bus.

The ABS module constantly monitors the anti-lock brake system for proper operation. If the ABS module detects a fault, it will turn on a warning indicator lamp and disable the anti-lock braking system. The normal base braking system will remain operational.

The ABS module continuously monitors the speed of each wheel through the signals generated by the wheel speed sensors to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the ABS module commands the coils to actuate. The coils then open and close the valves in the HCU that modulate brake fluid pressure in some or all of the hydraulic circuits. The ABS module continues to control pressure in individual hydraulic circuits until a locking tendency is no longer present. The ABS module contains a self-diagnostic program that monitors the ABS system for system faults. When a fault is detected, a warning indicator lamp is turned on and the fault Diagnostic Trouble Code (DTC) is then stored in a diagnostic program memory. A latched fault will disable certain system functionality for the current ignition cycle. An unlatched fault will disable certain system functionality until the fault condition disappears. These DTC's will remain in the ABS module memory even after the ignition has been turned off. The DTC's can be read and cleared from the ABS module memory by a technician using a scan tool. If not cleared with a scan tool, the fault occurrence and DTC will be automatically cleared from the ABS module memory after the identical fault has not been seen during the next 100 ignition cycles may be required for the amber ABS warning indicator lamp to extinguish on the next ignition cycle.

The ABS module covers ABS braking functions, Cornering Brake Control (CBC), Drag Torque Control Motor [Schleppmomenten Regelung] (MSR), and Off-Road ABS (ORA) ESC functions.

The ESC:

• Attempts to avoid wheel lock for ABS operation.
• Reduces over steer tendencies for CBC operation.
• Attempts to avoid drive axle locking for MSR operation.
• Optimally decelerate off-road for ORA operation.