Electronic Brake Control Module: Description and Operation
Anti-Lock Control - Traction Control - 4.2LCOMPONENT LOCATIONS - SHEET 1 OF 2
NOTE: LHD (left-hand drive) installation shown, RHD (right-hand drive) installation similar
COMPONENT LOCATIONS - SHEET 2 OF 2
NOTE: RHD (right-hand drive) installation shown, LHD (left-hand drive) installation similar
GENERAL
The anti-lock control - traction control system is based on the 4 channel Bosch 8.0 system and provides the following brake functions:
^ ARM (active roll mitigation).
^ ABS (anti-lock brake system).
^ CBC (corner brake control).
^ DSC.
^ EBD (electronic brake force distribution).
^ ETC (electronic traction control).
^ EBA (emergency brake assist).
^ EDC (engine drag-torque control).
^ HDC.
The system consists of the following components:
^ A DSC switch.
^ An HDC switch.
^ A stoplamp switch.
^ Four wheel speed sensors.
^ A yaw rate and lateral acceleration sensor.
^ A steering angle sensor.
^ Four warning indicators.
^ A HCU (hydraulic control unit) with attached ABS (anti-lock brake system) module.
ABS MODULE
The ABS (anti-lock brake system) module controls the brake functions using the HCU (hydraulic control unit) to modulate hydraulic pressure to the individual wheel brakes.
The ABS (anti-lock brake system) module is attached to the HCU (hydraulic control unit), on the driver side inner fender in the engine compartment. A 46 pin connector provides the electrical interface between the ABS (anti-lock brake system) module and the vehicle wiring.
ABS Module Harness Connector C1585
SYSTEM OPERATION
ARM
The ARM function uses the brakes and the engine to attempt to restore stability if the vehicle is forced into such a harsh manoeuver that it risks tipping over.
The ABS (anti-lock brake system) module monitors driver inputs and vehicle behavior using various powertrain signals and the inputs from the wheel speed sensors, the steering angle sensor and the yaw rate and lateral acceleration sensor. These are compared with modeled behavior and, if vehicle behavior reaches a given risk level, the ABS (anti-lock brake system) module cuts the engine power, or brakes one or more wheels, just enough to help the vehicle regain its poise and help the driver remain in control.
While the ignition is on, ARM is permanently enabled, even when DSC has been selected off.
ABS
ABS (anti-lock brake system) controls the speed of all road wheels to ensure optimum wheel slip when braking at the adhesion limit. This prevents the wheels from locking, which helps to retain effective steering control of the vehicle.
On the front axle, the brake pressure is modulated separately for each wheel. On the rear axle, brake pressure is modulated by select low. Select low applies the same pressure to both rear brakes, with the pressure level being determined by the wheel on the lower friction surface. This maintains rear stability on split friction surfaces.
CBC
CBC influences the brake pressures, below the DSC and ABS (anti-lock brake system) thresholds, to counteract the yawing moment produced when braking in a corner. CBC produces a correction torque by limiting the brake pressure on one side of the vehicle.
DSC
DSC uses the brakes and powertrain torque control to help maintain the lateral stability of the vehicle. While the ignition is on the DSC function is permanently enabled unless selected off by the DSC switch. Even if DSC is deselected, driving maneuvers with extreme yaw or lateral acceleration may trigger DSC activity to assist vehicle stability.
DSC enhances driving safety in abrupt maneuvers and in understeer or oversteer situations which may occur in a bend. The ABS (anti-lock brake system) module monitors the yaw rate and lateral acceleration of the vehicle, and the steering input, then selectively applies individual brakes and signals for powertrain torque adjustments to reduce understeer or oversteer.
In general: in an understeering situation, the inner wheels are braked to counteract the yaw movement towards the outer edge of the bend-, in an oversteering situation, the outer wheels are braked to prevent the rear end of the vehicle from pushing towards the outer edge of the bend.
The ABS (anti-lock brake system) module monitors the tracking stability of the vehicle using inputs from the wheel speed sensors, the steering angle sensor and the yaw rate and lateral acceleration sensor. The tracking stability is compared with stored target data and, whenever the tracking stability deviates from the target data, the ABS (anti-lock brake system) module intervenes by applying the appropriate brakes. On vehicles with an automatic transmission, when the DSC function is active, the ABS (anti-lock brake system) module also signals the TCM (transmission control module) to prevent gear shifts. If necessary, the ABS (anti-lock brake system) module also signals:
^ The ECM (engine control module), to reduce engine torque.
^ The transfer box control module, to adjust the locking torque of the center differential.
^ The rear differential control module, to adjust the locking torque of the rear differential.
The DSC function overrides the differential locking torque requests from the terrain response system.
If DSC is selected off, a warning chime sounds and a system off message is shown in the message center.
EBD
EBD (electronic brake force distribution) limits the brake pressure applied to the rear wheels. When the brakes are applied, the weight of the vehicle transfers forwards, which reduces the ability of the rear wheels to transfer braking effort to the road surface. This can cause the rear wheels to slip and make the vehicle unstable.
EBD (electronic brake force distribution) uses the anti-lock braking hardware to automatically optimize the pressure of the rear brakes, below the point where anti-lock braking would be invoked. Only the rear axle is under EBD (electronic brake force distribution) control.
ETC
ETC attempts to optimize forward traction by reducing engine torque or braking a spinning wheel until it regains grip.
ETC is activated if an individual wheel speed is above that of the vehicle reference speed (positive slip) and the brake pedal is not pressed. The spinning wheel is braked, allowing the excess torque to be transmitted to the non spinning wheels through the drive line. If necessary, the ABS (anti-lock brake system) module also sends a high speed CAN (controller area network) bus message to the ECM (engine control module) to request a reduction in engine torque. Torque reduction requests are for either a slow or fast response: a slow response requests a reduction of throttle angle-, a fast response requests an ignition cut-off.
When the DSC function is selected off with the DSC switch, the engine torque reduction feature is disabled.
When the ETC function is active the ABS (anti-lock brake system) module also signals the TCM (transmission control module) to prevent gear shifts.
EBA
EBA (emergency brake assist) assists the driver, in emergency braking situations, by automatically maximizing the braking effort. There are two situations when the ABS (anti-lock brake system) module will invoke EBA (emergency brake assist) : when the brake pedal is pressed very suddenly and when the brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation.
When the brake pedal is pressed very suddenly, the ABS (anti-lock brake system) module increases the hydraulic pressure to all of the brakes until they reach the threshold for ABS (anti-lock brake system) operation, thus applying the maximum braking effort for the available traction. The ABS (anti-lock brake system) module monitors for the sudden application of the brakes using the inputs from the stoplamp switch and from the pressure sensor in the HCU (hydraulic control unit). With the brake pedal pressed, if the rate of increase of hydraulic pressure exceeds the predetermined limit, the ABS (anti-lock brake system) module invokes emergency braking.
When the brake pedal is pressed hard enough to bring the front brakes into ABS (anti-lock brake system) operation, the ABS (anti-lock brake system) module increases the hydraulic pressure to the rear brakes up to the ABS (anti-lock brake system) threshold.
EBA (emergency brake assist) operation continues until the driver releases the brake pedal enough for the hydraulic pressure in the HCU (hydraulic control unit) to drop below a threshold value stored in the ABS (anti-lock brake system) module.
EDC
EDC prevents wheel slip caused by any of the following:
^ A sudden decrease in engine torque when the accelerator is suddenly released.
^ A downshift using the Command Shift.
When the ABS (anti-lock brake system) module detects the onset of wheel slip without the brakes being applied it signals the ECM (engine control module), on the high speed ECM (engine control module) bus, to request a momentary increase in engine torque.
HDC
HDC uses brake intervention to control vehicle speed and acceleration during low speed descents in off-road and low grip on-road conditions. Generally, equal pressure is applied to all four brakes, but pressure to individual brakes can be modified by the ABS (antilock brake system) and DSC functions to retain stability. Selection of the HDC function is controlled by the HDC switch on the center console. HDC operates in both high and low ranges, at vehicle speeds up to 50 km/h (31.3 mph).
HDC may be used in D, R and Command Shift 1 in high range, and in D, R and all Command Shift gears in low range. When in D, the transmission control module will automatically select the most appropriate gear. The vehicle should not be driven with HDC active and the transmission in N.
HDC can be selected at speeds up to 80 km/h (50 mph), but will only be enabled at speeds below 50 km/h (31.3 mph). When HDC is selected:
^ At speeds up to 50 km/h (31.3 mph), the HDC information indicator is permanently illuminated if a valid gear is selected.
^ At speeds from >50 to 80 km/h (>31.3 to 50 mph) the HDC information indicator flashes and a message advising that the speed is too high is displayed in the message center.
If the HDC switch is pressed while vehicle speed is more than 80 km/h (50 mph), the HDC information indicator will not illuminate and HDC will not be selected.
^ If the speed increases to 80 km/h (50 mph), the HDC function is switched off, the information indicator goes off, a warning chime sounds and a message advising that HDC has been switched off is displayed in the message center.
When HDC is enabled, the ABS (anti-lock brake system) module calculates a target speed and compares this with the actual vehicle speed. The ABS (anti-lock brake system) module then operates the HCU, in the active braking mode, as required to achieve and maintain the target speed. During active braking for HDC, the ABS (anti-lock brake system) module also energizes the HDC relay to operate the stoplamps. Applying the foot brakes during active braking may result in a pulse through the brake pedal, which is normal.
The target speed varies, between minimum and maximum values for each gear and transmission range, depending on driver input with the accelerator pedal. If the accelerator pedal is not operated, the ABS (anti-lock brake system) module adopts a default target speed.
As well as varying the target speed with the accelerator pedal, the target speed can also be varied by pressing the speed control '+' and '-' buttons (where fitted).
During changes of target speed, the ABS (anti-lock brake system) module limits deceleration and acceleration to -0.5 m/s 2 (-1.65 ft/S2 ) and +0.5 m/S2 (+1.65 ft/S2 ) respectively.
To provide a safe transition from active braking to brakes off, the ABS (anti-lock brake system) module invokes a fade out strategy, which gradually discontinues the braking effort, if it detects any of the following during active braking:
^ HDC selected off with the HDC switch.
^ Failure of a component used by HDC, but not critical to the fade out function.
^ Accelerator pedal pressed when transmission is in neutral.
^ Brake overheat.
If fade out is invoked because of deselection or component failure, the HDC function is cancelled by the ABS (anti-lock brake system) module. If fade out is invoked because the accelerator pedal is pressed with the transmission in neutral, or because of brake overheat, the HDC function remains in standby and resumes operation when the accelerator pedal is released or the brakes have cooled.
The fade out strategy increases the target speed, at a constant acceleration rate of 0.5 m/s2 (1.65 ft/S2), until the maximum target speed is reached or until no active braking is required for 0.5 s. If the accelerator pedal is positioned within the range that influences target speed, the acceleration rate is increased to 1.0 m/s 2 (3.3 ft/S2).
When fade out is invoked because of component failure, a warning chime sounds, the HDC information indicator is extinguished and a message advising there is a fault is displayed in the message center.
When fade out is invoked because of brake overheat, a message advising that HDC is temporarily unavailable is displayed. At the end of fade out, the HDC information indicator flashes. The message is displayed, while HDC remains selected, until the brakes have cooled.
To monitor for brake overheat, the ABS (anti-lock brake system) module monitors the amount of braking activity and, from this, estimates the temperature of each brake. If the estimated temperature of any brake exceeds a preset limit, the ABS (anti-lock brake system) module invokes the fade out strategy. After the fade out cycle, the HDC function is re-enabled when the ABS (anti-lock brake system) module estimates that all of the brake temperatures are at less than 64% of the temperature limit.
When HDC is selected off, a warning chime sounds and a system off message is shown in the message center.