Antilock Brake System - Hybrid
ABS Description and Operation
The antilock brake system (ABS) consists of the following:
* Antilock brake system (ABS)
* Dynamic rear proportioning
* Hill start assist
* Traction control system
* Vehicle stability enhancement system (VSES)
The following components are involved in the operation of the above systems:
* Electronic brake control module (EBCM) - The EBCM controls the system functions and detects failures.
The EBCM contains the following components:
- System relay - The system relay is energized when the ignition is ON and no ABS DTCs are present. It supplies battery positive voltage to the solenoid valves and pump motor.
- Brake pressure sensors - The EBCM contains 2 brake pressure sensors that monitor applied master cylinder pressure to the wheels. The EBCM uses the brake pressure inputs to enhance VSES.
- Vent tube - The vent tube, located in the EBCM connector, is an opening to the internal cavity of the EBCM. It allows ventilation of the EBCM internals.
* The brake pressure modulator valve contains the hydraulic valves and pump motor that are controlled electrically by the EBCM. The brake pressure modulator valve uses a 4 circuit configuration with a diagonal split. The brake pressure modulator valve directs fluid from the reservoir of the master cylinder to the left front and right rear wheels and fluid from the other reservoir to the right front and left rear wheels. The diagonal circuits are hydraulically isolated so that a leak or malfunction in one circuit will allow continued braking ability on the other.
Note: There is a rubber isolator located under the brake pressure modulator valve and on the mounting studs. The rubber isolators protect the brake pressure modulator valve and the EBCM from vehicle vibrations.
* The brake pressure modulator valve contains the following components:
- Pump motor
- Inlet valves (one per wheel)
- Outlet valves (one per wheel)
- Master cylinder isolation valves (one per drive wheel)
- Prime valves (one per drive wheel)
- A master cylinder pressure sensor
* The EBCM sends a 12V reference voltage signal to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces a square wave DC signal voltage. The wheel speed sensor increases the signal frequency as the wheel speed increases, but does not increase the signal amplitude.
* Traction control switch is manually disabled or enabled using the traction control switch.
* Brake pedal position sensor - The EBCM receives a serial data message from the body control module (BCM) to indicate that the brake pedal is applied.
* Yaw rate sensor - The EBCM uses the yaw rate sensor to determine the rate of rotation along the vehicles vertical axis. The yaw rate sensor is packaged with the lateral accelerometer as a single component.
* Lateral accelerometer - The EBCM uses the lateral accelerometer to determine the sideways acceleration of the vehicle. The lateral accelerometer is packaged with the yaw rate sensor as a single component.
* The EBCM uses the power steering control module to determine the position and rotation of the steering wheel.
Initialization Sequence
The electronic brake control module (EBCM) performs on initialization test each ignition cycle. The initialization of the EBCM occurs when one set of the following conditions occur:
Both of the following conditions occur:
* The EBCM detects that there is a minimum of 500 RPM from the engine control module (ECM) via a serial data message.
* The brake pedal position sensor is not applied.
OR
Both of the following conditions occur:
* The vehicle speed is greater that 16 km/h (10 mph).
* The brake pedal position sensor is applied.
The initialization sequence may also be commanded with a scan tool.
The initialization sequence cycles each solenoid valve and the pump motor, as well as the necessary relays, for approximately 1.5 seconds to check component operation. The EBCM sets a DTC if any error is detected. The initialization sequence may be heard and felt while it is taking place, and is considered part of normal system operation.
The EBCM defines a drive cycle as the completion of the initialization sequence.
Antilock Brake System (ABS)
When wheel slip is detected during a brake application, the antilock brake system (ABS) enters antilock mode. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping. A separate hydraulic line and specific solenoid valves are provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressure to each wheel brake. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.
During antilock braking, a series of rapid pulsation is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the electronic brake control module responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during antilock operation.
Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.
Pressure Hold
The electronic brake control module closes the inlet valve and keeps the outlet valve closed in order to isolate the system when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
Pressure Decrease
The electronic brake control module decreases the pressure to individual wheels during a deceleration when wheel slip occurs. The inlet valve is closed and the outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cylinder.
Pressure Increase
The electronic brake control module increases the pressure to individual wheels during a deceleration in order to reduce the speed of the wheel. The inlet valve is opened and the outlet valve is closed. The increased pressure is delivered from the master cylinder.
Dynamic Rear Proportioning
The dynamic rear proportioning is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The dynamic rear proportioning control system is part of the operation software in the electronic brake control module. The dynamic rear proportioning uses active control with existing ABS in order to regulate the vehicles rear brake pressure.
The red brake warning indicator is illuminated when the dynamic rear proportioning function is disabled.
Hill Start Assist
When a conventional vehicle is stopped on a hill, the brakes will hold the vehicle in place until the brake pedal is released. When a hybrid vehicle is completely stopped on a hill, the engine may shut off. When the driver releases the brake pedal, and applies the accelerator, the engine will restart. During the restart sequence, the vehicle may roll. Inputs and outputs from the following electronic control modules are required in order to accomplish a safe smooth transition during vehicle stops and starts:
* Engine control module (ECM)
* Electronic brake control module (EBCM)
* Instrument panel cluster
* Transmission control module
* Starter generator control module
The hill start assist feature minimizes vehicle rolling during the transition between brake release and engine restart. The brake pressure modulator valve maintains pressure in the brake lines, even though the pedal is released, until the engine starts. Brake pressure is then release following engine restart.
The starter generator control module hill assist valves are actuated through the ECM controls. The ECM also commands the brake light signal required to the body control module (BCM) during the transition between engine off and engine start. The brake hill start assist valves are serviceable components.
Traction Control
When drive wheel slip is noted while the brake is not applied, the electronic brake control module will enter traction control mode.
First, the electronic brake control module (EBCM) request the engine control module (ECM) to reduce the amount of torque to the drive wheels via the requested torque signal circuit. The ECM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The ECM reports the amount torque delivered to the drive wheels via the delivered torque signal circuit.
If the engine torque reduction does not eliminate drive wheel slip, the EBCM will actively apply the drive wheel brakes. During traction control braking, hydraulic pressure in each drive wheel circuit is controlled to prevent the drive wheels from slipping. The master cylinder isolation valve closes in order to isolate the master cylinder from the rest of the hydraulic system. The prime valve then opens in order to allow the pump to accumulate brake fluid in order to build hydraulic pressure for braking. The drive wheel inlet and outlet solenoid valves then open and close in order to perform the following functions:
* Pressure hold
* Pressure increase
* Pressure decrease
Vehicle Stability Enhancement System
The vehicle stability enhancement system adds an additional level of vehicle control to the electronic brake control module (EBCM)
Yaw rate is the rate of rotation about the vehicles vertical axis. The lateral accelerometer measures the sideways acceleration of the vehicle. The vehicle stability enhancement system is activated when the EBCM determines that the desired yaw rate and lateral forces do not match the actual altitude of the vehicle.
The desired altitude of the vehicle is calculated from the following inputs:
* The position of the steering wheel
* The speed of the vehicle
* The lateral, or sideways acceleration of the vehicle
* Yaw rate of the vehicle
* Master cylinder brake pressure
The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. If the yaw rate error becomes too large, the EBCM attempts to correct the vehicles yaw motion by applying differential braking to the appropriate wheel. The amount of differential braking applied to the left or right front wheel is based on both the yaw rate error and side slip rate error.
The vehicle stability enhancement system activations generally occur during aggressive driving, in turns, or on poor road conditions, without much use of the accelerator pedal. When braking during vehicle stability enhancement activation, the brake pedal pulsates at a higher frequency than the base braking system.
Driver Information Indicators and Messages
The following indicators are used to inform the driver of several different factors.
Brake Warning Indicator
the instrument panel cluster illuminates the brake warning indicator when the following occurs:
* The body control module detects that the park brake is engaged. The instrument panel cluster receives a serial data message from the BCM requesting illumination. The brake warning indicator flashes at a rate of approximately twice per second when the park brake is engaged.
* The electronic brake control module detects a low brake fluid condition or a base brake pressure differential and sends a serial data message to the instrument panel cluster requesting illumination.
* The instrument panel cluster performs the bulb check.
* The EBCM detects an ABS disabling condition which also disables the dynamic rear proportioning and sends a serial data message to the instrument panel cluster requesting illumination.
ABS Indicator
The instrument panel cluster illuminates the ABS indicator when the following occurs:
* The EBCM detects an ABS disabling malfunction and sends a serial data message to the instrument panel cluster requesting illumination.
* The instrument panel cluster performs the bulb check.
* The instrument panel cluster detects a loss of serial data communication with the EBCM.
* The EBCM sends a serial data message to the instrument panel cluster requesting illumination.
Stability System Off Indicator
The instrument panel cluster illuminates the stability control off message when one or more of the following conditions exist:
* The EBCM disables engine torque reduction due to a malfunction and sends a serial data message to the instrument panel cluster requesting illumination.
* The driver manually disables the vehicle stability enhancement system and engine torques reduction by pressing the traction control switch. The EBCM sends a serial data message to the instrument panel cluster requesting illumination.
Traction Control System Indicator - TRAC OFF
The instrument panel cluster illuminates the TRAC OFF indicator when the following occurs:
* The electronic brake control module (EBCM) inhibits the traction control system due to a malfunction in the traction control system. The instrument panel cluster receives a serial data message from the EBCM requesting illumination.
* The body control module (BCM) detects that the traction control switch has bee pressed (signal circuit is low). The BCM sends a serial data message to the EBCM in order to disable traction control. The instrument panel cluster receives a serial data message from the EBCM requesting illumination. The BCM sends a serial data message tot he radio in order to activate an audible warning.
* The battery voltage is low.
Service Hill Start Message
The instrument panel cluster illuminates the SERVICE HILL HOLD when it receives a serial data message from the ECM or starter generator control module that a fault is detected in the hybrid or hill start assist system.