Antilock Brakes / Traction Control Systems: Description and Operation
ABS Description and Operation
This vehicle is equipped with a TRW EBC450 brake system. The electronic brake control module and the brake pressure modulator are serviced separately. The brake pressure modulator uses a four circuit configuration to control hydraulic pressure to each wheel independently.
Depending on options, the following vehicle performance enhancement systems are provided.
* ABS
* Traction control
* Stability control
* Dynamic rear proportioning
* Hill start assist
* Cornering brake control
* Hydraulic brake assist
* Trailer stability assist
The following components are involved in the operation of the above systems:
* Electronic brake control module - The electronic brake control module controls the system functions and detects failures. It supplies voltage to the solenoid valves and pump motor.
* Brake pressure modulator - The brake pressure modulator contains the following components:
- Hydraulic pump with pump motor
- Four isolation vales
- Four dump valves
- Two traction control/stability control supply valves
- Two traction control/stability control isolation valves
- Pressure sensor
- High pressure accumulator
- Low pressure accumulator
* Body control module monitors the brake pedal position sensor signal when the brake pedal is applied and sends a high speed serial data message to the electronic brake control module indicating the brake pedal position.
* Brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The sensor provides an analog voltage signal that will increase as the brake pedal is applied. The electronic brake control module monitors the brake pressure sensor which is integral to the brake pressure modulator.
* Instrument cluster - The instrument cluster displays the vehicle speed based on the information from the engine control module. The engine control module sends the vehicle speed information via a high speed serial data to the body control module. The body control module then sends the vehicle speed information via a low speed serial data to the instrument cluster in order to display the vehicle speed, either in kilometers or miles, based on the vehicle requirements.
* Multi-axis acceleration sensor - The yaw rate, lateral acceleration and longitudinal acceleration sensors are combined into one multi-axis acceleration sensor, external to the electronic brake control module. The electronic brake control module receives chassis serial data message inputs from the yaw rate, lateral acceleration and longitudinal acceleration sensor and activates stability control and hill start assist function depending on multi-axis acceleration sensor input.
* Multifunction switch - The traction control switch is a multifunction momentary switch. The body control module monitors the signal circuit from the traction control switch and sends a high speed serial data message to the electronic brake control module indicating the switch position. The traction control and stability control are manually disabled or enabled by pressing the traction control switch.
* Steering wheel angle sensor - The electronic brake control module receives chassis serial data message inputs from the steering angle sensor. The steering angle sensor signal is used to calculate the desired yaw rate.
* Transmission control module - The electronic brake control module receives high speed serial data message inputs from the transmission control module indicating the gear position of the transmission for hill start assist or hill hold functions.
* Wheel speed sensors - The electronic brake control module supplies a 12 V reference voltage to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces an alternating current square wave signal. The electronic brake control module uses the frequency of the square wave signal to calculate the wheel speed. The electronic brake control module sends the wheel speed information via high speed serial data to the engine control module.
Power-Up Self-Test
The electronic brake control module is able to detect many malfunctions whenever the ignition is ON. However, certain failures cannot be detected unless active diagnostic tests are performed on the components. Shorted solenoid coil or motor windings, for example, cannot be detected until the components are commanded ON by the electronic brake control module. Before, a power-up self-test is required at the beginning of each ignition cycle to verify correct operation of components before the various control systems can be enabled. The electronic brake control module performs the power-up self-test when the vehicle achieves a speed greater than 10 km/h (7 MPH). The system relay, solenoids, and the ABS pump motor are commanded ON and OFF to verify proper operation and the electronic brake control module verifies the ability to return the system to base braking in the event of a failure. The power-up self-test may sometimes be heard by the driver or passengers of the vehicle.
Initialization Sequence
The electronic brake control module performs one initialization test each ignition cycle. The initialization of the electronic brake control module occurs when the following conditions occur:
* The vehicle speed is greater than 16 km/h (10 MPH).
* The brake pedal is applied.
ABS
When wheel slip is detected during a brake application, an ABS event occurs. During ABS 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. The ABS does not, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.
During ABS braking, a series of rapid pulsations 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 ABS 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 ABS braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during ABS 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. The typical ABS activation sequence is as follows.
Pressure Hold
The electronic brake control module closes the isolation valve and keeps the dump valve closed in order to isolate the slipping wheel when wheel slip occurs. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
Pressure Decrease
If a pressure hold does not correct the wheel slip condition, a pressure decrease occurs. The electronic brake control module decreases the pressure to individual wheels during deceleration when wheel slip occurs. The isolation valve is closed and the dump valve is opened. The excess fluid is stored in the accumulator until the pump can return the fluid to the master cylinder or fluid reservoir.
Pressure Increase
After the wheel slip is corrected, a pressure increase occurs. The electronic brake control module increases the pressure to individual wheels during deceleration in order to reduce the speed of the wheel. The isolation valve is opened and the dump valve is closed. The increased pressure is delivered from the master cylinder.
Traction Control
When drive wheel slip is noted, the electronic brake control module will enter traction control mode.
First, the electronic brake control module requests the engine control module to reduce the amount of torque to the drive wheels via a serial data message. The engine control module reduces torque to the drive wheels and reports the amount of delivered torque.
If the engine torque reduction does not reduce drive wheel slip, the electronic brake control module will actively apply the brakes on the slipping drive wheel. During traction control braking, hydraulic pressure in each drive wheel circuit is controlled to prevent the drive wheels from slipping. The electronic brake control module commands the pump motor and appropriate solenoid valves ON and OFF to apply brake pressure to the slipping wheel.
Traction control can be manually disabled or enabled by pressing the traction control switch.
Stability Control
Stability control provides added stability during aggressive maneuvers. Yaw rate is the rate of rotation about the vehicle's vertical axis. The stability control is activated when the electronic brake control module determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.
The desired yaw rate is calculated by the electronic brake control module using the following inputs:
* Steering wheel position
* Vehicle speed
* Lateral acceleration
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. When a yaw rate error is detected, the electronic brake control module attempts to correct the vehicle's yaw motion by applying brake pressure to one or more of the wheels. The amount of applied brake pressure varies, depending on the correction required. The engine torque may be reduced also, if it is necessary to slow the vehicle while maintaining stability.
Stability control activations generally occur in turns during aggressive driving. When braking during stability control activation, the brake pedal may pulsate.
Stability control can be manually disabled or enabled by pressing and holding the traction control switch down for five seconds.
Dynamic Rear Proportioning
The dynamic rear proportioning is a control system that replaces the mechanical proportioning valve. Under certain driving conditions the electronic brake control module will reduce the rear wheel brake pressure by commanding the appropriate solenoid valves ON and OFF.
Hill Start Assist
The hill start assist allows the driver to launch his vehicle without a roll back while he is moving his foot from the brake pedal to the accelerator pedal. The electronic brake control module calculates the brake pressure, which is needed to hold the vehicle on an incline and locks that pressure for a certain time by commanding the appropriate solenoid valves ON and OFF when the brake pedal is released. Hill start assist is activated when the electronic brake control module determines that the driver wishes to move his vehicle up-hill, either backwards or forwards.
The following inputs are used for hill start assist feature:
* Brake switch
* Brake pressure
* Longitudinal acceleration
* Engine torque
* Reverse gear information
* Clutch switch
* Accelerator pedal position
* Vehicle speed
Cornering Brake Control
Cornering brake control is a slip control function that is intended to improve the vehicle's lateral/yaw stability during combined braking and cornering situations.
The electronic brake control module will reduce the brake pressure to the inside wheels by commanding the appropriate solenoid valves ON and OFF.
Hydraulic Brake Assist
The hydraulic brake assist function is designed to support the driver in emergency braking situations.
The electronic brake control module receives inputs from the brake pressure sensor. When the electronic brake control module senses an emergency braking situation, the electronic brake control module will actively increase the brake pressure to a specific maximum.
Trailer Stability Assist
The trailer stability assist will detect any vehicle yaw instability, caused by an attached trailer. When instability is detected, the electronic brake control module attempts to correct the vehicle's yaw motion by applying brake pressure to one or more of the wheels. The engine torque may be reduced also, if it is necessary to slow down the vehicle.
Driver Information Indicators
Brake Warning Indicator
The instrument cluster turns the brake warning indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The electronic brake control module detects a faulty pump motor, solenoid valves, an internal hardware failure or a too high or too low battery voltage and sends a serial data message to the instrument cluster requesting illumination.
* The body control module detects that the parking brake is engaged. The instrument cluster receives a serial data message from the body control module requesting illumination.
ABS Indicator
The instrument cluster turns the ABS indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The electronic brake control module detects a malfunction which disables the ABS and sends a serial data message to the instrument cluster requesting illumination.
Traction Control/Stability Control Active Indicator
The instrument cluster turns the traction control/stability control-active indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The ABS system is in traction control or stability control mode.
* The electronic brake control module detects a traction control/stability control occurring and sends a serial data message to the instrument cluster requesting illumination.
Traction Control Off Indicator
The instrument cluster turns the traction control off indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The driver manually disables the traction control by pressing and releasing the traction control switch. The electronic brake control module sends a serial data message to the instrument cluster requesting illumination.
Stability Control Off Indicator
The instrument cluster turns the stability control off indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The driver manually disables the stability control by pressing and holding the traction control switch down for five seconds. The electronic brake control module sends a serial data message to the instrument cluster requesting illumination.
Traction/Stability Control Indicator
The instrument cluster turns the traction/stability control indicator ON when the following occurs:
* The instrument cluster performs the bulb check.
* The electronic brake control module detects a fault or malfunction in the traction control/stability control system and sends a serial data message to the instrument cluster requesting illumination.