Functional Description - Anti-Lock Brake System (Abs) Module

The ABS module incorporates the Electronic Stability Control (ESC) system. On vehicles equipped with Hill Start Assist (HSA), the ABS module also incorporates the HSA strategy.

On vehicles that are not equipped with HSA,  there is a brake hold feature designed for the ESS system that should prevent the vehicle from lurching forward during an ESS auto-start on downhill grades. The ESC will attempt to hold the brake system pressure when the ESS system has determined that the vehicle is stopped on a grade and the ABS module has received a bus message indicating the ESS auto-stop engine shutdown is in an active state. The system holds brake system pressure until either a bus message is received indicating that the engine is running again or a message indicating that the accelerator pedal position is greater than 0%. No drive torque balancing is required. Brake system pressure is ramped out quickly as possible when either of those two signals meet the specified values.

On vehicles equipped with the HSA function for the ESS system,  this functions only when the engine is running as well as the transmission shifter is in a specified gear location and downhill grade criteria is met. The ESS application is designed to shut off the engine as vehicle braking is applied by the driver. To avoid unintentional rollback, the HSA functionality is required while the engine is off and the vehicle is stopped on a shallower grade. The HSA will remain functional through the supply voltage dip experienced during warm engine cranking.

When the vehicle comes to a stand still and the ESS system state changes to indicate that an ESS auto-stop event is pending, the ABS Module activates the ESS HSA providing that the ESS HSA grade criteria is met and an adequate amount of brake master cylinder pressure is available. Note that the ESS HSA functionality will have a different HSA activation grade threshold to meet the vehicle application. When the ESS HSA function is active, the ESC will isolate the appropriate amount of brake system pressure needed. If the grade is greater than the ESS HSA grade threshold but less than the standard HSA grade threshold, the ESS HSA functions depending on the vehicle application and specified by the ESC system. The ESC system will:

• Deactivate when the ESS engine state changes to engine running.
• Remain active until the HSA timeout or engine drive off torque balancing values are met.

The ESC broadcasts the HSA state and the calculated HSA holding pressure over the Controller Area Network-Chassis (CAN-C) network bus. This signal identifies the following states of the HSA system:

• HSA is not active/pending. This indicates that the HSA activation requirements are yet to be determined or the vehicle is not at a stand still.
• Vehicle stand still has been detected and the HSA is not holding braking system pressure.
• A stand still and grade have been detected and the HSA signal is indicating what the minimum braking system pressure required is needed to hold the vehicle on the grade. This signal must compensate for loss of engine creep/idle torque when the engine shuts down. This signal must quickly transition to the required braking system hold pressure after determining vehicle stand still and detection of the grade. This signal must quickly transition directly to the calculated braking system hold pressure if the grade is greater then the ESS HSA calibrated threshold. The signal can differ from the actual pressure that the HSA system is holding. The PCM will compare the brake master cylinder pressure to the signal to ensure that enough brake system pressure is in the brake system to hold the vehicle before an ESS auto-stop event is performed. If the brake pressure signal is Signal Not Acquired (SNA), all HSA functionality is disabled due to a system fault.
• ESS HSA will not disable when the HSA feature is disabled by the customer using the programmable options menu to turn the HSA system off.

The ABS/ESC module:

• is responsible for reading and reporting the state of the Vehicle Speed Sensor (VSS) as well as the wheel speed Revolution Per Minute (RPM) calculations.
• reports the rolls test bench mode state.
• reports out the current state of the brake pedal position.
• reports out the Yaw rate.
• reports out vehicle longitudinal acceleration and vehicle lateral acceleration readings.