ABS Description and Operation (With F55)
ABS Description and Operation (With F55)
This vehicle is equipped with a Bosch ABS 8.0 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.
The following vehicle performance enhancement systems are provided.
* ABS
* Traction control
* Stability control
* Dynamic rear proportioning
* Hill start assist
* Hydraulic brake 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 solenoid vales
- Four dump solenoid valves
- Two traction control/stability control prime solenoid valves
- Two traction control/stability control isolation solenoid valves
- Pressure sensor
- High pressure accumulator
- Low pressure accumulator
* Multi-axis acceleration sensor - The yaw rate and lateral acceleration sensors are combined into one multi-axis acceleration sensor module, external to the electronic brake control module. The electronic brake control module receives serial data message inputs from the yaw rate, longitudinal, and lateral acceleration sensor and activates stability control depending on multi-axis acceleration sensor module input.
* Steering wheel angle sensor - The steering wheel angle sensor receives ignition voltage from the electronic brake control module. The electronic brake control module receives serial data message inputs from the steering wheel angle sensor identifying the position and direction of the steering wheel rotation. It utilizes this signal to calculate the driver intended driving direction. The steering wheel angle signal is also used to calculate the desired yaw rate.
* Traction control switch - The traction control switch is a momentary contact switch. The traction control can be manually disabled or enabled by pressing the traction control switch once. When the traction control switch is pressed twice the performance traction mode is available. There are five modes adjustable by the driver for changing track conditions based on driver confidence and skill level. When the traction control switch is pressed and held for five seconds the stability control can be manually disabled.
* Tour mode switch - The tour mode switch is a momentary contact switch that has two positions, TOUR 1 and TOUR 2. The electronic brake control module sends a serial data message to the suspension control module to modify the shock damper rates which will change the ride and handling characteristics of the vehicle. The body control module reads the performance mode from the electronic brake control module and requests the instrument panel via serial data to display the selected competitive mode message on the driver information center.
* Sport mode switch - The sport mode switch is a resistor ladder switch that has three positions, SPORT 1, SPORT 2 and TRACK. The electronic brake control module sends serial data messages to the suspension control module to modify the shock damper rates which will change the ride and handling characteristics of the vehicle. The electronic brake control sends serial data messages to the engine control module to manage engine torque based on the track conditions. The body control module reads the performance mode from the electronic brake control module and requests the instrument panel via serial data to display the selected competitive mode message on the driver information center.
* Wheel speed sensors - The wheel speeds are detected by active wheel speed sensors and encoder rings. The encoder rings consist of permanent magnets located in the hub. As the wheel spins the magnetic encoder ring rotates and passes by the wheel speed sensor head; which generates a sign wave in the wheel speed sensor. Each wheel speed sensor receives ignition voltage from the electronic brake control module and provides an alternating current square wave signal to the electronic brake control module. The electronic brake control module uses the frequency of the signals to calculate the wheel speeds.
Initialization Sequence
The initialization sequence cycles each solenoid valve and the pump motor, as well as the necessary relays, for approximately 30 milliseconds to check component operation. The electronic brake control module 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 self test is initiated by the electronic brake control module at the start of the ignition cycle and the speed of the fastest wheel exceeds 16 km/h (10 MPH).
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 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 brake pressure which is applied 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 by pressing the traction control switch for 5 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
Note: The Hill start assist feature will not operate if the stability control is disabled.
The hill start assist allows the driver to launch the vehicle without a roll back while moving their 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 hold feature:
* Engine torque
* Vehicle speed
* Brake pressure
* Clutch switch if equipped
* Longitudinal acceleration
* Accelerator pedal position
* Brake pedal position sensor
* Transmission gear information
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.
Performance Traction Management
When the traction control switch is pressed twice the performance traction mode is available. The performance traction management is used to enhance the performance of the vehicle on a race track. The performance traction management is divided into two components: corner exit control and launch control. The corner exit control will reduce race track lap times by modulating engine torque for maximum acceleration during vehicle cornering. The launch control will improve vehicle straight-line acceleration from a standstill by controlling the engine speed and modulating engine torque for maximum acceleration once the vehicle is moving. There are five modes adjustable by the driver for changing track conditions based on driver confidence and skill level.
The five performance traction management modes integrate magnetic ride control, launch control, traction control, stability control and road conditions are listed below.
Performance Traction Management Mode 1 WET
Mode 1 is intended for use during damp or wet conditions by drivers of all skill levels. The engine torque is managed based on the given track conditions along with the active handling and stability control.
When the traction control is set for wet conditions, with stability control on and magnetic ride control set on Tour, the driver information center displays:
* Perf Traction
* 1-WET
* Active Handling On
* Suspension Mode Tour
Performance Traction Management Mode 2 DRY
Mode 2 is intended for use on dry track conditions by less experienced drivers or those learning a new track. The engine torque is managed based on the given track conditions along with the active handling and stability control.
When the traction control is set for dry conditions, with stability control on and magnetic ride control set on Tour, the driver information center displays:
* Perf Traction
* 2-DRY
* Active Handling On
* Suspension Mode Tour
Performance Traction Management Mode 3
Mode 3 is intended for use on dry track conditions by drivers who are familiar with the track they are driving. The engine torque is managed based on the given track conditions along with the active handling and stability control.
When the traction control is set on Sport 1, with stability control on and magnetic ride control set on Sport, the driver information center displays:
* Perf Traction
* 3-SPORT 1
* Active Handling On
* Suspension Mode Sport
Performance Traction Management Mode 4
Mode 4 is intended for use on dry track conditions by drivers who are familiar with the track they are driving. The engine torque is managed based on the given track conditions along with the active handling and stability control.
When the traction control is set on Sport 2, with stability control off and magnetic ride control set on Sport, the driver information center displays:
* Perf Traction
* 4-SPORT 2
* Active Handling Off
* Suspension Mode Sport
Performance Traction Management Mode 5
Mode 5 is intended for use on dry track conditions by experienced drivers who are familiar with the track they are driving. Launch control is tuned for race or drag strip prepping. This mode will require more driver skill than all the other modes. The engine torque is maximized for cornering speed.
When the traction control is on Race, with stability control off and magnetic ride control set on Track, the driver information center displays:
* Perf Traction
* 5-RACE
* Active Handling Off
* Suspension Mode 1 TRACK
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 fault and sends a serial data message to the instrument cluster requesting illumination.
* The body control module (BCM) detects that the park brake is engaged. The instrument cluster receives a serial data message from the BCM 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 malfunction which disables the traction control or stability control 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 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 when the traction control switch is pressed and held for 5 seconds, the body control module will request the electronic brake control module to disable the electronic stability. The electronic brake control module sends a serial data message to the instrument cluster requesting illumination.