ABS Description and Operation with LAT
The antilock brake system (ABS) consists of the following:
| • | Dynamic rear proportioning (DRP) |
| • | Traction control system (TCS) (NW9) |
| • | Vehicle stability enhancement system (VSES) (FX3) |
| • | Hill start assist (Hybrid RPO LAT) |
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. |
| • | Brake pressure modulator valve (BPMV) - The BPMV contains the hydraulic valves and pump motor that are controlled electrically by the EBCM. The BPMV uses a 4 circuit configuration with a diagonal split. The BPMV 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. |
| | Important: There is a rubber isolator located under the BPMV and on the mounting studs. The rubber isolators protect the BPMV and the EBCM from vehicle vibrations.
|
| • | The BPMV contains the following components: |
| - | Inlet valves (one per wheel) |
| - | Outlet valves (one per wheel) |
| - | Master cylinder isolation valves (one per drive wheel) |
| - | Prime valves (one per drive wheel) |
| • | Wheel speed sensors (WSS) - EBCM sends a 12-volt 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 - The TCS is manually disabled or enabled using the traction control switch. |
| • | Brake pedal position (BPP) 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 (w/FX3) - The EBCM uses the yaw rate sensor to determine the rate of rotation along the vehicle's vertical axis. The yaw rate sensor is packaged with the lateral accelerometer as a single component. |
| • | Lateral accelerometer (wFX3) - 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. |
| • | Steering wheel position sensor (SWPS) (w/FX3)--The EBCM uses the SWPS as an indication of the position and rotation of the steering wheel. |
Initialization Sequence
The electronic brake control module (EBCM) performs one 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 (BPP) is not applied. |
OR
Both of the following conditions occur:
| • | The vehicle speed is greater than 16 km/h (10 mph). |
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
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 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 (EBCM) 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 (EBCM) 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 (EBCM) 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 (EBCM) 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 (DRP)
The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the electronic brake
control module (EBCM). The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.
The red brake warning indicator is illuminated when the dynamic rear proportioning function is disabled.
Hill Start Assist (LAT)
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 (ECUs) 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 (IPC) |
| • | Transmission control module (TCM) |
| • | Starter generator control module (SGCM) |
The hill start assist feature minimizes vehicle rolling during the transition between brake release and engine restart. The brake pressure modulator valve (BPMV) maintains pressure in the brake lines, even though the pedal is released, until the engine
starts. Brake pressure is then released following engine restart.
The BPMV is actuated through the EBCM by a duty cycle from the ECM. The ECM also sends a serial data message to the body control module (BCM) to control the brake light signal during the transition between engine off, and engine start. The EBCM and BPMV
is a serviceable component.
Traction Control System (TCS) (NW9)
When drive wheel slip is noted while the brake is not applied, the electronic brake control module (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 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:
Vehicle Stability Enhancement System (VSES)
The vehicle stability enhancement system (VSES) adds an additional level of vehicle control to the electronic brake control module (EBCM).
Yaw rate is the rate of rotation about the vehicle's vertical axis. The lateral accelerometer measures the sideways acceleration of the vehicle. The VSES is activated when the EBCM determines that the desired yaw rate and lateral forces do not match the
actual attitude of the vehicle.
The desired attitude of the vehicle are 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 vehicle's 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 VSES activations generally occur during aggressive driving, in turns, or on poor road conditions, without much use of the accelerator pedal. When braking during VSES activation, the pedal pulsations feel different than the ABS pedal pulsations. The
brake pedal pulsates at a higher frequency during VSES activation.
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 (IPC) illuminates the brake warning indicator when the following occurs:
| • | The body control module (BCM) detects that the park brake is engaged. The IPC 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 (EBCM) detects a low brake fluid condition or a base brake pressure differential and sends a serial data message to the IPC requesting illumination. |
| • | The IPC performs the bulb check. |
| • | The EBCM detects an ABS-disabling malfunction which also disables dynamic rear proportioning (DRP) and sends a serial data message to the IPC requesting illumination. |
ABS Indicator
The IPC illuminates the ABS indicator when the following occurs:
| • | The EBCM detects an ABS-disabling malfunction and sends a serial data message to the IPC requesting illumination. |
| • | The IPC performs the bulb check. |
| • | The IPC detects a loss of serial data communication with the EBCM. |
| • | The EBCM sends a serial data message to the IPC requesting illumination. |
Stability System Off Indicator
The IPC illuminates the stability control off message when one or more of the following conditions exists:
| • | The EBCM disables engine torque reduction due to a malfunction and sends a serial data message to the IPC requesting illumination. |
| • | The driver manually disables the vehicle stability enhancement system (VSES) and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the IPC requesting illumination. |
Traction Disabled Indicator
The IPC illuminates the traction off indicator when the following occurs:
The IPC flashes the traction disabled indicator if wheel slip is detected while engine torque reduction and brake pressure application are both disabled.
Service Hill Start Message
The driver information center (DIC) displays SRVC HILL START when it receives a serial data message from the ECM that a fault is detected that affects the hybrid or hill start assist system.
ABS Description and Operation without LAT
This vehicle is equipped with a TRW EBC 440 brake master cylinder assembly. The electronic brake control module (EBCM) and the brake pressure modulator valve (BPMV) can be serviced separately. The BPMV uses a 4 circuit configuration to control
hydraulic pressure to each wheel independently.
The following vehicle performance enhancement systems are provided.
| • | Antilock brake system (ABS) |
| • | Dynamic rear proportioning (DRP) |
| • | Traction control system (TCS) |
| • | Vehicle stability enhancement system (VSES) (w/FX3) |
The following components are involved in the operation of the above systems:
| • | Brake pressure modulator valve (BPMV)--The BPMV contains the hydraulic valves and pump motor that are controlled electrically by the EBCM. Additionally, with VSES, the BPMV contains an internal brake fluid pressure sensor monitoring the primary
supply hydraulic circuit. The BPMV uses a 4 circuit configuration with a diagonal split. The BPMV 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. |
| • | Solenoids--The solenoids are commanded ON and OFF by the EBCM to operate the appropriate valves in the BPMV. |
| • | System relay--The system relays are internal to the EBCM. The system relays are energized when the ignition is ON. The system relays supply battery voltage to the valve solenoids. The ABS pump motor is connected to fused battery. |
| • | Traction control switch (w/FX3)--TCS and vehicle stability enhancement system (VSES) is manually disabled or enabled using the traction control switch. |
| • | Wheel speed sensors (WSS) - The wheel speed sensor receives ignition voltage from the EBCM and provides a DC square wave signal back to the module. As the wheel spins, the EBCM uses the frequency of the square wave signal to calculate the wheel speed. |
| • | Brake pedal position (BPP) 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 (w/FX3)--The EBCM uses the yaw rate sensor to determine the rate of rotation along the vehicle's vertical axis. The yaw rate sensor is packaged with the lateral accelerometer as a single component. |
| • | Lateral accelerometer (wFX3)--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. |
| • | Steering wheel position sensor (SWPS) (w/FX3)--The hydraulic power steering equipped vehicles have a SWPS discrete wired sensor providing information to the EBCM. The EBCM uses the 2 different SWPS signals as an indication of the position and
rotation of the steering wheel. Electric power steering (EPS) systems provide SWPS information via serial data communication to the EBCM. |
Initialization Sequence
The electronic brake control module (EBCM) performs one 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 is not applied. |
OR
Both of the following conditions occur:
| • | The vehicle speed is greater than 16 km/h (10 mph). |
| • | The brake pedal is applied. |
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) Operation
When wheel skidding is detected during a brake application, an ABS event occurs. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from skidding. 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 antilock 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 (EBCM) responds to wheel speed
sensor inputs and attempts to prevent wheel skid. 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 skidding. 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. 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 skidding wheel when an ABS event 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 skid condition, a pressure decrease occurs. The isolation valve remains closed and the dump valve is opened. This allows a small amount of brake fluid to escape from the hydraulic circuit into the low pressure
accumulator (LPA) reservoir.
Pressure Increase
After the wheel skid is corrected, a pressure increase occurs. The isolation and dump valves are returned to their off states and brake pressure in the hydraulic circuit is once again dependant on the amount of force applied to the brake pedal by the driver.
If the driver applies too much pressure causing wheel slip, the ABS activation sequence reoccurs.
Dynamic Rear Proportioning (DRP) Operation
The dynamic rear proportioning (DRP) is a control system that enhances the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the electronic
brake control module (EBCM). The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.
Traction Control System (TCS)
When drive wheel slip is detected while the brake is not applied, the electronic brake control module (EBCM) will enter traction control mode. The traction control system (TCS) controls vehicle traction by applying brakes and limiting engine
torque to the drive wheels. The EBCM performs the following TCS operations:
| • | The EBCM requests the engine control module (ECM) to reduce the amount of torque to the drive wheels via serial data messages. |
| • | The ECM reduces torque to the drive wheels by retarding spark timing and throttle actuator control (TAC). |
| • | The ECM reports the amount torque delivered to the drive wheels via serial data messages. |
| • | The EBCM applies brake pressure to the slipping wheel in order to transfer torque to the gripping wheel. |
Power-Up Self-Test
The electronic brake control module (EBCM) 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 EBCM. Therefore, 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 EBCM 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 EBCM 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.
Vehicle Stability Enhancement System (VSES)
The vehicle stability enhancement system (VSES) adds an additional level of vehicle control to the electronic brake control module (EBCM).
Yaw rate is the rate of rotation about the vehicle's vertical axis. The lateral accelerometer measures the sideways acceleration of the vehicle. The VSES is activated when the EBCM determines that the desired direction of the vehicle does not match the
actual direction of the vehicle.
The desired response of the vehicle is calculated from the following inputs:
| • | The position of the steering wheel |
| • | The speed of the vehicle |
The actual response of the vehicle is calculated from the following outputs:
| • | Actual lateral, or sideways acceleration of the vehicle |
| • | Actual yaw rate of the vehicle |
| • | Actual 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. This is determined by steering wheel position rate of change. If the yaw rate error becomes too large, the EBCM
attempts to correct the vehicle's 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 VSES activations generally occur during aggressive driving, in turns, or on poor road conditions, without much use of the accelerator pedal. When braking during VSES activation, the pedal pulsations feel different than the ABS pedal pulsations. The
brake pedal pulsates at a higher frequency during VSES activation.
Driver Information Indicators and Messages
The following indicators are used to inform the driver of several different conditions:
Brake System Warning Indicator
The instrument panel cluster (IPC) illuminates the brake warning indicator when any of the following instances occur.
| • | The body control module (BCM) detects that the park brake is engaged. The BCM sends a serial data message to the IPC requesting illumination. |
| • | The BCM detects a low brake fluid condition. The EBCM sends a serial data message to the IPC requesting illumination. |
| • | The IPC performs the bulb check. |
| • | The IPC detects a loss of serial data communication with the EBCM. |
| • | The EBCM detects an ABS-disabling malfunction which also disables dynamic rear proportioning (DRP). The EBCM sends a serial data message to the IPC requesting illumination. |
ABS Indicator
The IPC illuminates the ABS indicator when any of the following instances occur.
| • | The EBCM detects an ABS-disabling malfunction. The EBCM sends a serial data message to the IPC requesting illumination. |
| • | The IPC performs the bulb check. |
| • | The IPC detects a loss of serial data communication with the EBCM. |
Usually, the ABS indicator is turned OFF during the following ignition cycle unless the fault is detected during that ignition cycle. However, the setting of certain DTCs will cause the ABS indicator to remain illuminated during the following ignition
cycle until the vehicle is operated at a speed greater than 13 km/h (8 mph). This allows the EBCM to verify that no malfunction exists, before turning OFF the ABS indicator, in order to comply with the United Nations Economic Commission for Europe (ECE)
regulation no. 13. This response occurs even if the ABS indicator turns OFF when the scan tool is used to clear the DTCs. When repairing these vehicles, it is important to ensure that the ECE 13 response has occurred and that the ABS indicator
does not illuminate after returning the vehicle to the customer. It is also important to verify that ECE 13 is not the cause of an ABS indicator which is illuminated when no DTCs are set, before attempting to diagnose other possible causes.
SERVICE TRACTION Message
The IPC displays the SERVICE TRACTION message when either of the following instances occur:
| • | The EBCM detects a malfunction that requires traction control to be disabled. The EBCM sends a serial data message to the IPC requesting that the message be displayed. |
| • | The IPC detects a loss of serial data communication with the EBCM. |
TRACTION OFF Message
The IPC displays this message when the traction control system (TCS) has been turned off by the driver.
