ABS Description and Operation without JL4
The electronic brake control module (EBCM) and the brake pressure modulator valve (BPMV) is serviced as one assembly. 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) |
| • | Hydraulic Brake Booster Power Supply |
| • | Traction Control System (TCS) (w/NW7) |
| • | Vehicle Stability Enhancement System (VSES) (w/JL4) |
The following components are involved in the operation of the above systems:
| • | ABS Pump Motor -- The ABS pump motor is part of the hydraulic brake booster. The ABS pump motor is active during ABS, VSES and base brake power assist functions. |
| • | Accumulator Pressure Sensor--The accumulator pressure sensor is located internally within the BPMV. The accumulator pressure sensor uses a 5-volt reference and generates an output signal proportionate to the hydraulic fluid pressure which is
present in the accumulator. |
| • | Backup Lamp Switch-- On vehicles equipped with manual transmissions, the EBCM receives a voltage signal on the backup lamp supply voltage circuit when the vehicle is in reverse gear. The EBCM uses the backup lamp supply voltage circuit to enhance
the operation of the ABS, VSES and TCS in reverse. |
| • | Yaw Rate Sensor/Lateral and Longitudinal Accelerometer (w/JL4)--The EBCM uses the lateral and longitudinal accelerometers to determine the sideways and front to back acceleration of the vehicle. The Yaw rate sensor determines the vehicle rotation.
The lateral accelerometer, yaw rate and longitudinal sensor are combined into one single component. |
| • | Longitudinal Accelerometer (w/o JL4)--The EBCM uses the longitudinal accelerometer to determine the actual straight-line acceleration of the vehicle. |
| • | Master Cylinder Pressure Sensor--The master cylinder pressure sensor is located within the BPMV. The master cylinder pressure sensor uses a 5-volt reference and generates an output signal proportionate to the hydraulic fluid pressure which is
present in the hydraulic brake circuit at the master cylinder. |
| • | Solenoids--The solenoids are commanded ON and OFF by the EBCM to operate the appropriate valves in the BPMV. |
| • | Steering Wheel Position Sensor (SWPS) (w/JL4)--The EBCM uses the SWPS as an indication of the position and rotation of the steering wheel. |
| • | System relays--There are two system relays internal to the EBCM. The solenoid relay is energized when the ignition is ON. The ABS pump motor relay supplies battery positive voltage to the ABS pump motor when the EBCM commands the ABS pump motor
on. The system relays are non serviceable. |
| • | Traction Control Switch --Engine traction Control and VSES is manually disabled or enabled using the traction control switch. |
| • | 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. |
| • | Stop Lamp Switch--The EBCM receives a voltage signal on the stop lamp supply voltage circuit when the brake pedal is applied. The EBCM applies a voltage to the center high mounted stop lamp (CHMSL) during a VSES event and sends a serial data
message to the body control module (BCM) requesting stop lamp activation. The EBCM uses the stop lamp supply voltage circuit to enhance the operation of the ABS, VSES and TCS. |
Antilock Brake System (ABS) Operation
When wheel slip 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 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 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. 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 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 reservoir.
Pressure Increase
After the wheel slip 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 noted while the brake is not applied, the electronic brake control module (EBCM) will enter traction control mode.
The EBCM sends pulse width modulated (PWM) signal to the powertrain control module (PCM) to reduce the amount of torque to the drive wheels. The PCM reduces torque to the drive wheels by retarding spark timing and by commanding the throttle actuator control.
The PCM uses a 12-volt PWM signal in order to report to the EBCM the amount of torque delivered to the drive wheels.
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.
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 vehicles yaw motion by applying differential
braking to the appropriate wheel. The amount of differential braking applied to the wheels 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.
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 6 km/h (4 mph). The solenoid relay is 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.
ECE 13 Response
The ABS indicator illuminates when a malfunction which disables ABS is detected. Often, the ABS indicator turns OFF following the instrument panel cluster (IPC) bulb check during the next ignition cycle unless the fault is detected during that ignition
cycle. However, the setting of certain DTCs may cause the ABS indicator to remain illuminated during the subsequent ignition cycle until the vehicle is operated at a speed greater than 10 km/h (7 mph) and no fault is detected. This is referred
to as an ECE 13 response. It is 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.
Driver Information Indicators and Messages
The following indicators are used to inform the driver of several different conditions:
Brake Warning Indicator
The instrument panel cluster (IPC) illuminates the brake warning indicator when any of the following instances occur.
| • | Body control module (BCM) detects that the park brake is engaged. The BCM sends a serial data message to the IPC requesting illumination. |
| • | EBCM detects a low brake fluid condition. The EBCM sends a serial data message to the IPC requesting illumination. |
| • | IPC performs the bulb check. |
| • | IPC detects a loss of serial data communication with the EBCM. |
| • | 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. |
| • | EBCM detects the loss of the brake assist function. |
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. |
| • | An ECE 13 response occurs at the beginning of an ignition cycle. The EBCM sends a serial data message to the IPC requesting illumination. |
Stability Caution Indicator
The stability caution indicator flashes when the VSES is active and illuminates solid when there is a TCS or VSES fault.
ABS Fault Message
The IPC displays the message whenever the ABS indicator is illuminated due to an ABS fault.
BRAKE SYSTEM Fault Message
The IPC displays the message whenever the red brake warning indicator is illuminated due to a base brake or DRP fault.
BRAKE FLUID Fault Message
The IPC displays the message when the EBCM sets a C0267 for low brake fluid level.
SERVICE STAB SYS fault Message
The IPC displays the traction fault message when either of the following instances occur:
| • | The EBCM detects a malfunction that requires VSES 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. |
SERVICE TRACTION fault Message
The IPC displays the traction fault 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. |
