Antilock Brakes / Traction Control Systems: Description and Operation

ABS Description and Operation

This vehicle is equipped with an TRW 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) (w/NW9)

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. 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-There are 2 system relays internal to the EBCM. The system relays are energized when the ignition is ON. The system relays supply battery positive voltage to the valve solenoids and to the ABS pump motor. This voltage is referred to as system voltage
* Traction control switch (w/NW9 and w/JL4)-TCS and vehicle stability enhancement system (VSES) is manually disabled or enabled using the traction control switch.
* Wheel speed sensors (WSS) - As the wheel spins, the wheel speed sensor produces an AC signal. The EBCM uses this AC signal to calculate wheel speed. The wheel speed sensors are replaceable only as part of the wheel hub and bearing assemblies.
* 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.

Initialization Sequence

The electronic brake control module (EBCM) performs 1 initialization test each ignition cycle. The initialization of the EBCM occurs when 1 set 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

* The vehicle speed is greater than 16 km/h (10 mph).
* The brake pedal is applied.

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 traction control system (TCS) controls vehicle traction by applying brakes and limiting engine torque to the drive wheels. The EBCM performs the fallowing 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 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 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 electronic brake control module (EBCM) 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.

Vehicle Dynamics Caution (VDC) Indicator

The IPC illuminates the VDC indicator when there is a traction event, or if the traction control system (TCS) has been disabled due to a malfunction.

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 CONTROL OFF Message

The IPC displays this message when the traction control system (TCS) has been turned off by the driver.

The IPC also displays this message momentarily on the DIC on an initial Key up after the power moding times out and is reset.