Description And Operation - Abs: Notes
The Anti-Lock Brake System (ABS) includes the following components:
| 1. | Refer to WHEEL SPEED SENSORS (WSS) . |
| 2. | Refer to INTEGRATED CONTROL UNIT (ICU) |
| 3. | Refer to INSTRUMENT PANEL CLUSTER (IPC) |
| 4. | Refer to STEERING COLUMN CONTROL MODULE (SCCM) |
The Electronic Stability Control (ESC) may also be referred to as Electronic Stability Program (ESP) depending on the vehicle model year and configuration. Certain components may also reference ESP, ESC, or use the traction control symbol.
OPERATION
The ABS module is mounted to the Hydraulic Control Unit (HCU) together forming the ICU. The ABS module is a micro-controller based device which monitors the anti-lock brake system during normal braking and controls it when the vehicle is in an ABS stop. The ABS function avoids wheel lock during braking maneuvers, maintaining the direction of the vehicle, and optimizing stopping 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.
If the vehicle is equipped with traction control, the ESC function lamp will illuminate anytime the amber ABS warning indicator lamp illuminates.
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 anti-lock brake system for system faults. When a fault is detected, a warning indicator lamp is turned on and a Diagnostic Trouble Code (DTC) fault 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 5633 km/h (3, 500 miles). Drive-off may be required for the amber ABS warning indicator lamp to go out on the next ignition cycle.
The ABS has multiple subsystems including, Traction Control System (TCS), Brake Assist System (BAS), Electronic Stability Control (ESC), Hill Start Assist (HSA), Electronic Roll Mitigation (ERM), Trailer Sway Control (TSC), and Hill Descent Control (HDC), Cornering Brake Control (CBC), Drag Torque Control [Motor Schleppmomenten Regelung] (MSR), and Off-Road ABS (ORA) ESC. All of these systems work together to enhance vehicle stability and control in various driving conditions.
A brief explanation of each subsystem follows:
TRACTION CONTROL
This system monitors the amount of wheel spin of each of the driven wheels. If wheel spin is detected, brake pressure is applied to the slipping wheel(s) to provide enhanced acceleration and stability.
BRAKE ASSIST SYSTEM
The BAS is designed to optimize the vehicle's braking capability during emergency braking maneuvers. The system detects an emergency braking situation by sensing the rate and amount of brake application and then applies optimum pressure to the brakes.
ELECTRONIC STABILITY CONTROL
The ESC system enhances directional control and stability of the vehicle using steering wheel angle, yaw (turning) rate and lateral acceleration sensors (combined into Dynamics Sensor). Using signals from these sensors, in addition to individual wheel speed sensor signals the ESC corrects for over/under steering of the vehicle by applying the brake of the appropriate wheel to assist in counteracting an over/under steer condition. Engine power may also be reduced to help the vehicle maintain the desired path. Certain components may also reference ESP, ESC, or use the traction control symbol.
HILL START ASSIST
The HSA system does not operate on an automatic transmission vehicle in the neutral position.
The HSA system is designed to maintain the level of brake pressure for a short period of time after the brake pedal is released so that the vehicle does not roll on a hill between releasing the brake pedal and applying the accelerator pedal. The following must be true for HSA to activate:
- The vehicle must be stopped with the brake applied
- The vehicle must be at a eight percent or greater incline (three percent for manual transmissions)
- Gear selection must match uphill direction (forward gear for uphill, reverse for backing uphill)
ELECTRONIC ROLL MITIGATION
The ERM system monitors steering wheel input and the speed of the vehicle. When the rate of change of the steering wheel angle and vehicle's speed are sufficient to potentially cause wheel lift, the appropriate brake will apply, and engine power may be reduced to lessen the chance that wheel lift will occur. ERM will only intervene during very severe or evasive driving maneuvers.
TRAILER SWAY CONTROL
The TSC system uses sensors in the vehicle to recognize an excessively swaying trailer and will take the appropriate actions to attempt to stop the sway. The system may reduce engine power and apply the brake of the appropriate wheel(s) to counteract the sway of the trailer. TSC will become active automatically once an excessively swaying trailer is recognized.
HILL DESCENT CONTROL
The HDC system maintains vehicle speed while descending hills in off-road driving conditions by applying the brakes when necessary. HDC can only be enabled when the transfer case is in the "4WD LOW" position and the vehicle speed is less than 48 km/h (30 mph). If these conditions are not met while attempting to use the HDC feature, the HDC indicator light will flash on/off. The HDC indicator lamp will be on solid when HDC is enabled. When enabled, HDC senses the terrain and activates when the vehicle is descending a hill. HDC is only intended for low speed off-road driving. The accelerator pedal will override HDC at any time.
CORNERING BRAKE CONTROL
The CBC function reduces the over steer tendency of the vehicle during an ABS braking maneuver in a curve by providing an anti-yaw moment through reduction of front inner wheel pressure. The ESC shall pay attention to possible torque steering disturbance and noise actuations. CBC is active during braking in curve maneuvers or when ABS function is active. CBC is not active when ABS function is not active.
DRAG TORQUE CONTROL [MOTOR SCHLEPPMOMENTEN REGELUNG]
The MSR function tries to avoid locking of the driving axle during throttle-off maneuvers due to engine braking torque. MSR is active during throttle-off maneuvers. MSR activates when the locking tendency of a driving wheel exceeds the target value.
OFF-ROAD ABS (ORA) ESC
On deformable surfaces (gravel, sand, etc), a specific Off-Road ABS calibration with higher wheel slip targets may be used to improve the ABS stopping distance. A larger wheel slip will help take advantage of the wedge effect of the deformable surface in front of the tires. Off-road ABS shall be automatically enabled if the vehicle has terrain switch setting(s) for deformable surfaces, or it may be enabled if the transfer case is switched to low range. The ORA logic may also be used if the ESC detects a deformable surface based on information from the wheel speed sensors, longitudinal acceleration sensor, and brake torque model. The deeper wheel slip targets used in ORA shall be dependent on vehicle speed, steering wheel angle, and surface estimation. The ORA logic is not intended to be used on dry or wet asphalt, packed snow, or ice.