Principles of Operation

Anti-Lock Control - Hybrid

Principles of Operation

Regenerative Braking

The regenerative brake system utilizes the electric motor portion of the hybrid electric powertrain as a generator to create electrical current. This recharges the High Voltage Traction Battery (HVTB) and delivers brake torque to be used in place of or in combination with the conventional friction brakes to slow the vehicle. The amount of brake torque provided by the electric motor is dependent upon the state of charge of the HVTB. When the HVTB is almost fully charged, the amount of regenerative braking is limited to avoid overcharging, and the requested deceleration is produced by conventional friction braking alone.

Conventional Friction Braking

If the HVTB is almost fully charged, or the requested amount of braking cannot be achieved by regenerative braking alone, the ABS module applies the conventional friction brakes. The friction brakes utilize components similar to those found in a conventional vacuum assisted brake system with ABS. For a list of components, refer to Brake System - General Information Testing and Inspection.

Brake-By-Wire

During a normal brake application, the brake pedal feel simulator allows the pressure applied to the brake pedal by the driver's foot to act against a spring instead of the brake booster push rod. The brake pedal angle sensor is mounted to the brake pedal assembly and provides the ABS module with an analog signal indicating the amount of deceleration the driver is demanding. If it is necessary for the conventional friction brakes to be applied, the ABS module applies a Pulse Width Modulated (PWM) voltage to a vacuum solenoid inside the active brake booster. The solenoid allows vacuum from either the combustion engine (if the engine is running) or the electric vacuum pump to move the booster push rod which in turn pushes the piston in the master cylinder and create hydraulic brake pressure. If the brake pedal is applied quickly and forcefully enough, the driver can overcome the brake pedal feel simulator spring pressure, allowing a direct connection between the brake pedal, booster push rod and master cylinder.

Brake Pedal Feel Simulator and Cut-Off Device

When the ignition is turned on, the ABS module runs a series of checks to make sure the system is running properly. Once these checks are complete, the ABS module closes the valve in the simulator cut-off device which puts the brake system into brake-by-wire mode. Brake-by-wire mode is when the pedal feedback to the driver is provided by the pedal feel simulator and not the pressure in the base brake system. The simulator uses spring pressure to provide a familiar brake pedal sensation. When the driver presses the brake pedal, the ABS module interprets the pedal movement through the brake pedal angle sensor. Based on the pedal angle and the amount of brake torque that can be provided through regenerative braking, the ABS module determines how much brake torque is provided by the power train and how much is provided by the hydraulic brakes. The brake pedal is uncoupled from the brake booster push rod allowing the system to hold off hydraulic pressure when regenerative braking is providing the necessary brake torque. When hydraulic braking is required, the ABS module activates the booster solenoid, this allows the vacuum booster to push on the master cylinder creating pressure in the hydraulic system. In the event a fault occurs which inhibits the brake-by-wire mode, the ABS module opens the cut-off device valve. This puts the system into conventional mode with vacuum boosted hydraulic brakes and no regenerative braking. In this mode, the pedal feedback to the driver is provided by the booster. When the driver pushes the pedal, the uncoupling between the pedal and the booster push rod is overcome, and the pedal arm pushes on the booster push rod as in a conventional brake system.

Active Brake Booster

The active brake booster is similar to a conventional vacuum assisted brake booster with the addition of a brake booster solenoid, a brake booster travel sensor and a vacuum pressure sensor. If it is necessary to apply the friction brakes due to inadequate deceleration from regenerative braking, the ABS module sends a PWM voltage to the brake booster solenoid that is proportional to the amount of brake torque requested. The booster solenoid opens, which allows the vacuum booster push rod to push on the master cylinder creating pressure in the hydraulic system.

The ABS module uses the signals produced by the vacuum pressure sensor to verify that vacuum is being maintained in the brake booster. The ABS module uses the signals produced by the brake booster travel sensor to monitor brake booster performance.

Electric Vacuum Pump

When the combustion engine is not running, an electric vacuum pump is used to maintain vacuum inside the brake booster. The ABS module turns the electric vacuum pump on and off through the use of a mechanical relay and a solid state relay. If the ABS module detects a vacuum pump motor circuit concern, a vacuum system leak or a vacuum pump performance concern, a DTC sets in the ABS module and the combustion engine runs to provide vacuum for the brake booster.

Anti-Lock Braking

The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 4 active wheel speed sensors, one for each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel speed sensor tone rings pass through the magnetic field generated by the active wheel speed sensor. This creates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.

The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 4 normally open inlet valves inside the HCU and through the outlet ports of the HCU to the brake calipers. If the ABS module senses that a wheel is about to lock up, based on wheel speed sensor data, the module closes the normally open inlet valve for that wheel. This prevents more fluid from getting to the brake caliper. The ABS module then reads the wheel speed sensor signal from the affected wheel again. If the wheel is still decelerating (locking up), the module opens the normally closed outlet valve. This allows a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. The ABS pump motor runs during an ABS braking event so that sufficient brake fluid pressure is consistently available to the HCU valves. This procedure of closing and opening valves is repeated several times per second until the speed of the affected wheel matches the speeds of the other wheels. Once this has been accomplished, the ABS module returns the solenoid valves to their normal positions.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module carries out a preliminary electrical check of the wheel speed sensor circuitry by sending voltage through the sensor and checking for the voltage to return. At speeds above 20 km/h (12 mph), without the brake pedal being pressed, the pump motor is commanded ON for less than 2 seconds to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors. In the event that the ABS module detects a concern, the module sets a DTC and send a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) bus to illuminate the yellow ABS warning indicator, the stability/traction control indicator (also known as the sliding-car icon) and/or the red brake warning indicator. If the concern is severe enough, ABS may be disabled. Normal power-assisted braking, however, remains.

Electronic Brake Distribution (EBD)

The ABS module incorporates a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain threshold, the ABS module closes the appropriate solenoid valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

Some drivers may feel a slight bump sensation in the brake pedal when EBD is active.

If there is a DTC in the ABS module specifically for the HCU or there are 2 or more wheel speed sensor DTCs, EBD is disabled. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and the sliding-car icon is illuminated.

AdvanceTrac(R) Stability Enhancement System

The AdvanceTrac(R) system provides the following stability enhancement features for certain driving situations:

- Traction Control System - helps to avoid excessive drive-wheel spin and loss of traction.
- Electronic Stability Control (ESC) System - helps to avoid skids and lateral slides.

The traction control system uses the ABS and the PCM to limit wheel spin. When the ABS module detects a wheel spinning excessively, brake pressure to the appropriate brake caliper is modulated. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a traction control event is taking place. When the IPC receives this message, it flashes the sliding-car icon (twice per second). If the event is severe enough, the ABS module also sends a message to the PCM over the HS-CAN bus to assist with traction control. When the PCM receives this message, it adjusts engine timing and decreases fuel injector pulses. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the sliding-car icon.

Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through PCM intervention.

Certain DTCs in the ABS module disables the traction control system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon are illuminated.

Electronic Stability Control (ESC) System

The ESC system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for anti-lock braking. The ESC system also uses input from the steering wheel rotation sensor and the stability control sensors (yaw rate, roll rate, longitudinal acceleration and lateral acceleration), which are internal to the Restraints Control Module (RCM). Additional information from other modules is sent over the HS-CAN bus to help maintain vehicle stability.

The ABS module uses the various sensors to continuously monitor vehicle direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a stability event is taking place. When the IPC receives this message, it flashes the sliding-car icon. Under certain conditions during a stability event, the PCM may be requested to reduce engine torque as it does for traction control. Once the stability condition has been corrected, the ABS module sends another message over the HS-CAN bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the sliding-car icon.

Certain DTCs being present in the ABS module may disable the ESC system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon is illuminated.

Traction Control Switch

Unlike conventional ABS, the traction control portion of the AdvanceTrac(R) can be deactivated by the driver through the use of the traction control switch. The switch is hardwired to the IPC. When pressed, the switch sends a ground signal to the IPC. The IPC then sends a message over the HS-CAN bus indicating that the driver has requested that the system be deactivated and illuminates the sliding-car OFF icon. Once disabled, the system remains in that state until the driver presses the switch again or the ignition is cycled.

Stability Control Sensors

The stability control sensors for the AdvanceTrac(R) system consist of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over the dedicated Controller Area Network (CAN) bus. If any of the sensors are defective, a new RCM must be installed. Refer to Supplemental Restraint System Service and Repair for additional information on the RCM.

- The yaw rate sensor measures the relative vehicle motion about the vertical axis through the vehicle center of gravity.
- The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.

Lateral acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.

Steering Wheel Position Information

The steering wheel position information is sent to the ABS module from the Power Steering Control Module (PSCM) along the HS-CAN bus. The PSCM calculates steering position based on motor position within the Electronic Power Assist Steering (EPAS) steering gear. For information on the PSCM and EPAS system, refer to Steering System - Electronic Power Assist Steering (EPAS) Testing and Inspection.

ABS Module Calibration (Multi-Calibration Routine)

When an ABS fault has been corrected or an ABS component has been replaced, it is necessary to calibrate the ABS module. The calibration procedure is required for the brake-by-wire sensors to learn the "zero-position" of the vehicle which means the vehicle must not be moving and that the brake pedal must not be applied.

To carry out the calibration procedure, connect the scan tool, proceed to the Multi-Calibration routine and follow the scan tool instructions.

ABS Module Configuration

When a new ABS module is installed, it must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).