Antilock Brakes / Traction Control Systems: Description and Operation

ABS Description and Operation

This vehicle is equipped with the Continental Teves MK-70 antilock braking system.

The vehicle is equipped with the following braking systems:

* Antilock Brake System (ABS).
* Engine Traction System (ETS) (w/NW7)

The following components are involved in the operation of the above system.

* Electronic brake control module (EBCM)-The EBCM controls the system functions and detects failures.
The EBCM contains the following components:
* Solenoids-The solenoids are commanded by the EBCM to operate the appropriate valves in the brake pressure modulator valve.
The EBCM contains the following components:
* Brake pressure modulator valve (BPMV)-The BPMV uses a 4-circuit configuration to direct fluid to the left front, right front, left rear, and right rear wheels independently.
The BPMV contains the following components:
- Pump motor
- Four isolation valve for each hydraulic circuit
- Four dump valve for each hydraulic circuit
* Wheel speed sensors (WSS)-As the wheels spin, a toothed ring located at each wheel hub interrupts a magnetic field in the wheel speed sensors. This causes the wheel speed sensors to generate an AC signal. The EBCM uses this AC signal to calculate the wheel speed. Any imperfections in the toothed ring, such as a missing or damaged tooth, can cause an inaccurate WSS signal.

Initialization Sequence

The electronic brake control module (EBCM) performs one initialization test each ignition cycle. The initialization of the EBCM occurs when the following conditions are met:

* The ignition is ON.
* The bulb check has been completed.
* Vehicle speed is greater than 13 km/h (8 mph).

The initialization sequence briefly cycles each solenoid and the pump motor to verify proper operation of the components. The EBCM sets one or more DTCs in accordance with any malfunction that is detected.

Antilock Brake System

When wheel slip is detected during a brake application, the 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 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 (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 electronic brake control module (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.

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.

Engine Traction System (ETS) (NW7)

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 requests the powertrain control module (PCM) to reduce the amount of torque to the drive wheels via GMLAN messages. The PCM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The PCM reports the amount torque delivered to the drive wheels via GMLAN messages.

Brake Warning Indicator

The instrument panel cluster (IPC) illuminates the brake warning indicator when any of the following occurs.

* The IPC detects that the park brake is engaged.
* The Body control module (BCM) detects a low brake fluid level condition, and sends a GMLAN message to the EBCM and sets a DTC. The BCM will request illumination of the brake warning indicator.
* The IPC performs the bulb check.
* An ABS disabling malfunction may also disables dynamic rear proportioning (DRP).

ABS Indicator

The instrument panel cluster (IPC) illuminates the ABS indicator when any of the following occurs:

* The electronic brake control module (EBCM) detects an ABS disabling malfunction. The IPC receives a GMLAN message from the EBCM requesting illumination.
* The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
* The IPC detects a loss of GMLAN communications with the EBCM.

ECE 13 Response

The EBCM illuminates the ABS indicator when a malfunction which disables ABS is detected. 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 a wheel speed sensor or pump motor related DTC causes 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. 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.

Engine Traction System Indicator

TRAC OFF

The instrument panel cluster (IPC) illuminates the TRAC OFF indicator when the electronic brake control module (EBCM) inhibits the Engine Traction System due to a malfunction in the Engine Traction System. The IPC receives a GMLAN message from the EBCM requesting illumination.