Antilock Brakes / Traction Control Systems: Description and Operation
ABS Description and Operation
This vehicle is equipped with a Continental Teves Mk100 ABS, traction control, electronic stability control and a hydraulic brake system with diagonal brake circuit split.
The electronic brake control module and the brake pressure modulator is 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
* Auto dry brakes
* Traction control
* Stability control
* Dynamic rear proportioning
* Hill hold start assist
* Hydraulic brake assist
* Intelligent brake assist
* Optimized hydraulic braking system
The following components are involved in the operation of the above systems:
* Electronic brake control-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 valves
- Four dump valves
- Two traction/stability control supply valves
- Two traction/stability control isolation valves
- Brake pressure sensor
* Multi-axis acceleration sensor in vehicles equipped (without UGN)-The yaw rate, lateral acceleration and longitudinal acceleration sensors are combined into one multi-axis acceleration sensor, internal to the inflatable restraint sensing and diagnostic module.
* Multi-axis acceleration sensor in vehicles equipped (with UGN)-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 serial data message inputs from the yaw rate, lateral acceleration and longitudinal acceleration sensor and activates stability control depending on multi-axis acceleration sensor input.
* Steering wheel angle sensor-This sensor measures the rotation angle, angle velocity and direction of the steering wheel, providing serial data message inputs to the electronic brake control module. The steering wheel angle sensor signal is used to calculate the intended driving direction.
* Traction control switch-The traction control system is manually disabled or enabled by pressing the traction control switch.
* Traction/Stability control switch-The electronic stability control is manually disabled or enabled by pressing and holding the traction control switch for five seconds.
* Active wheel speed sensors-The electronic brake control module sends a signal 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.
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. Therefore, a power-up self-test is performed to verify correct operation of system components. The electronic brake control module performs the first phase of the power-up self-test when the ignition is first turned ON. This phase consists of internal self-testing of the electronic brake control module along with electrical checks of system sensors and circuits.
Initialization Sequence
The initialization sequence cycles each solenoid valve and the pump motor, as well as the necessary relays, for approximately thirty 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 active test is initiated by the electronic brake control module at the start of the ignition cycle and the speed of the fastest wheel exceeds 10 km/h (7 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.
Auto Dry Brakes
The auto dry brakes work in tandem with the automatic rain sense windshield wiper system. The auto dry brakes automatically activates when the windshield wipers are in operation, and the vehicle has been traveling over 32 km/h (20 MPH) continuously for more than 6.4 m (4 mi) without using the cruise control. The operation is initiated by the stability system, when a slight amount of pulsing hydraulic pressure is applied to the brake calipers which in turn applies the brake pads to wipe away water from the brake rotors while the vehicle is in motion.
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 over steer or under steer. 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 or enabled by pressing and holding the traction control switch 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 Hold Start Assist
The hill hold start assist allows the driver to launch the vehicle without a roll back while moving the 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 hold start assist is activated when the electronic brake control module determines that the driver wishes to move the vehicle up-hill, either backwards or forwards.
The following inputs are used for hill hold start assist feature:
* Accelerator pedal position
* Brake switch
* Brake pressure
* Clutch switch, if equipped
* Engine torque
* Longitudinal acceleration
* Reverse gear information
* Vehicle speed
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, it will actively increase the hydraulic brake pressure to a specific maximum by turning the pump motor ON.
Intelligent Brake Assist
The intelligent brake assist function is designed to provide limited braking to help prevent front and rear low speed collisions.
The electronic brake control module receives inputs from the brake pedal position sensor, wheel speed sensors, short range radar and ultrasonic sensors to detect a collision. When the electronic brake control module senses a possible collision, it will actively increase the hydraulic brake pressure to apply the brakes.
Optimized Hydraulic Braking System
With some engines the electronic brake control module monitors the vacuum in the brake booster with a vacuum sensor and controls a brake booster vacuum pump depending on vacuum sensor input. It also has a hydraulic brake boost feature which supplements the brake system to maintain consistent brake performance under conditions of low brake booster vacuum. Low brake booster vacuum conditions can include initial start up after the vehicle has been parked for several hours, very frequent brake stops, or high altitude driving. The hydraulic brake boost system activates only during a brake apply under low vacuum conditions. In this case the electronic brake control module will actively increase and control the hydraulic brake pressure by turning the pump motor ON and the appropriate solenoid valves ON and OFF. When hydraulic brake boost is active, a series of rapid pulsations is felt in the brake pedal.
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 detects that the park 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/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 malfunction which disables the traction/stability control and sends a serial data message to the instrument cluster requesting illumination.
* The driver manually disables the traction/stability control by pressing and holding the traction control switch for five seconds. The electronic brake control module 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 electronic brake control module sends a serial data message to the instrument cluster requesting illumination.
* The driver manually disables the stability control by pressing and holding the traction control switch for five seconds. The electronic brake control module sends a serial data message to the instrument cluster requesting illumination.