ABS Description and Operation
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ABS Description and Operation
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This vehicle is equipped with a Continental Teves Mk25 brake system. The electronic brake control module (EBCM) and the brake pressure modulator valve assembly are serviced separately. The brake pressure modulator valve assembly uses a 4 circuit configuration to control hydraulic pressure to each wheel independently.
Depending on options, the following vehicle performance enhancement systems are provided.
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Antilock brake system (ABS)
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Dynamic rear proportioning
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Trailer stability assist
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Optimized hydraulic braking system
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The following components are involved in the operation of the above systems:
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EBCM-The EBCM controls the system functions and detects failures. It supplies voltage to the solenoid valves and pump motor.
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Brake pressure modulator valve assembly-The brake pressure modulator valve assembly contains the following components:
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Hydraulic pump with pump motor
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Two traction control/stability control supply valves
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Two traction control/stability control isolation valves
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High pressure attenuators
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Low pressure accumulator
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Multi-axis acceleration sensor-The yaw rate, lateral acceleration and longitudinal acceleration sensors are combined into one multi-axis acceleration sensor, external to the EBCM. The EBCM receives 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.
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Steering angle sensor-The EBCM receives serial data message inputs from the steering angle sensor. The steering angle sensor signal is used to calculate the desired yaw rate.
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Traction control switch-Traction control and stability control are manually disabled or enabled by pressing the traction control switch.
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Wheel speed sensors-The EBCM sends a 12V reference voltage to each wheel speed sensor. As the wheel spins, the wheel speed sensor produces an alternating current square wave signal. The EBCM uses the frequency of the square wave signal to calculate the wheel speed.
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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 EBCM 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 EBCM 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 EBCM 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 EBCM 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.
When drive wheel slip is noted, the EBCM will enter traction control mode.
First, the EBCM requests the engine control module (ECM) to reduce the amount of torque to the drive wheels via a serial data message. The ECM 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 EBCM 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 EBCM 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 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 EBCM 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 EBCM using the following inputs:
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Steering wheel position
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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 EBCM 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 the traction control switch for 5 s.
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Dynamic Rear Proportioning
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The dynamic rear proportioning is a control system that replaces the mechanical proportioning valve. Under certain driving conditions the EBCM will reduce the rear wheel brake pressure by commanding the appropriate solenoid valves ON and OFF.
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 EBCM 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 EBCM 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:
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Longitudinal acceleration
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Reverse gear information
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Accelerator pedal position
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The hydraulic brake assist function is designed to support the driver in emergency braking situations.
The EBCM receives inputs from the brake pressure sensor. When the EBCM senses an emergency braking situation, it will actively increase the hydraulic brake pressure to a specific maximum by turning the pump motor ON.
The trailer stability assist will detect any vehicle yaw instability, caused by an attached trailer. When instability is detected, the EBCM 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.
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Optimized hydraulic braking system
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With some engines the EBCM 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 EBCM 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.
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Driver Information Indicators
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Brake Warning Indicator
The instrument cluster turns the brake warning indicator ON when the following occurs:
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The instrument cluster performs the bulb check.
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The EBCM detects a fault and sends a serial data message to the instrument cluster requesting illumination.
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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.
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ABS Indicator
The instrument cluster turns the ABS indicator ON when the following occurs:
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The instrument cluster performs the bulb check.
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The EBCM detects a malfunction which disables the ABS and sends a serial data message to the instrument cluster requesting illumination.
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Traction Control/Stability Control Active Indicator
The instrument cluster turns the traction control/stability control-active indicator ON when the following occurs:
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The instrument cluster performs the bulb check.
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The ABS system is in traction control or stability control mode.
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The EBCM detects a malfunction which disables the traction control or stability control and sends a serial data message to the instrument cluster requesting illumination.
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Traction Control Off Indicator
The instrument cluster turns the traction control off indicator ON when the following occurs:
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The instrument cluster performs the bulb check.
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The driver manually disables the traction control by pressing the traction control switch. The EBCM sends a serial data message to the instrument cluster requesting lamp illumination.
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Stability Control Off Indicator
The instrument cluster turns the stability control off indicator ON when the following occurs:
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The instrument cluster performs the bulb check.
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The driver manually disables the stability control by pressing the traction control switch. The EBCM sends a serial data message to the instrument cluster requesting lamp illumination.
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