Chevrolet Silverado / GMC Sierra. Service manual — part 728

Courtesy of GENERAL MOTORS CORP.

DESCRIPTION AND OPERATION

ABS DESCRIPTION AND OPERATION (WITH JL4)

This vehicle is equipped with a Bosch ABS/EBD/TCS/VSES brake system. The electronic brake
control module (EBCM) and the brake pressure modulator valve (BPMV) is 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)

Electronic Brake Distribution (EBD)

Power Brake Booster Solenoid Vacuum Supply

Traction Control System (TCS)

Vehicle Stability Enhancement System (VSES)

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

ABS pump motor-The ABS pump motor is part of the brake pressure modulator valve. The
ABS pump motor is active during ABS, VSES and base brake power assist functions.

System relays-There are two system relays internal to the EBCM. The solenoid relay is

Callout

Component Name

Preliminary Procedures:
Remove the passenger seat. Refer to Front Seat Replacement - Bucket .

1

Yaw Sensor Retaining Nuts (Qty: 2)

Tip: Pull back the carpet to gain access to the yaw sensor.

Tighten: 7 N.m (62 lb in)

NOTE:

Refer to Fastener Notice .

2

Yaw Sensor

3

Yaw Sensor Electrical Connector

2008 Chevrolet Silverado 1500

2008 BRAKES Antilock Brake System - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

energized when the ignition is ON. The ABS pump motor relay supplies a ground path
to the ABS pump motor when the EBCM commands the ABS pump motor on. The
system relays are non serviceable.

Solenoids-The solenoids are commanded ON and OFF by the EBCM to operate the
appropriate valves in the brake pressure modulator valve (BPMV).

Brake booster solenoid-The Power Brake Booster is solenoid operated, and applies a
mechanical force to brake master cylinder push rod to aid in brake pedal effort.

Brake booster vacuum sensor-The Brake Booster Vacuum Sensor is a input to EBCM, and
operates the ABS pump motor to precharge the brake system.

Brake pressure modulator valve (BPMV)-The BPMV uses a 4-circuit configuration to
control hydraulic pressure to each wheel independently.

The BPMV contains the following components:

ABS pump motor and pump

Four inlet valves

Four outlet valves

Two TC isolation valves

Two TC supply valves

A master cylinder pressure sensor

A front low-pressure accumulator

A rear low-pressure accumulator

Lateral accelerometer-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.

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 front brake
circuit at the master cylinder.

Power brake booster solenoid-Assist unit creates vacuum for the brake booster in case of a
vacuum loss or low vacuum to the brake booster. The power brake booster Solenoid is
active during increased brake booster assist, and base brake power assist functions.

Steering wheel position sensor-The EBCM receives several inputs from the steering wheel
position sensor. Three digital square wave signal inputs and one analog signal input are
wired directly to the EBCM harness connector. The EBCM uses the signals signals A and B
for determining position movement and uses the analog and index signals to determine
absolute center. All signals are monitored for plausibility to each other. The sensor is

2008 Chevrolet Silverado 1500

2008 BRAKES Antilock Brake System - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

provided ground and 5-volt power directly from the EBCM.

Traction control switch-VSES and the engine torque reduction function of TCS are
manually disabled or enabled by pressing 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.

Yaw rate sensor-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.

Antilock Brake System (ABS)

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

2008 Chevrolet Silverado 1500

2008 BRAKES Antilock Brake System - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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.

Electronic Brake Distribution (EBD)

The electronic brake distribution (EBD) is a control system that enhances the hydraulic
proportioning function of the mechanical proportioning valve in the base brake system. The EBD
control system is part of the operation software in the electronic brake control module (EBCM).
The EBD uses active control with existing ABS in order to regulate the vehicle's rear brake
pressure.

Brake Pressure Application

The EBCM uses brake pressure application to control traction by transferring torque through the
driveline to wheels which are not slipping. The ABS pump motor, and appropriate valve
solenoids are commanded ON and OFF to apply brake pressure to the slipping wheels. Brake
pressure application is used in an attempt to maintain equal wheel speed sensor (WSS) signals at
the driven wheels.

The EBCM does not allow excessive brake pressure application due to the fact that the solenoid
coils or the brakes may become overheated, damaging the EBCM or reducing the drivers ability
to stop the vehicle. Estimated coil and brake temperatures are determined by a calculation in the
EBCM software. Overheated solenoid coils cause all brake pressure application to become
disabled and the stability system disabled message to be displayed. Overheated brakes cause
brake pressure application during TCS events to disable, yet the VSES remains functional and as
long as the engine torque reduction is enabled, there is no indication to the driver when this
occurs and no DTC sets.

Vehicle Stability Enhancement System (VSES)

Vehicle stability enhancement system (VSES) provides added stability during aggressive
maneuvers. Yaw rate is the rate of rotation about the vehicle's vertical axis. The VSES is
activated when the electronic brake control module (EBCM) determines that the desired yaw rate

2008 Chevrolet Silverado 1500

2008 BRAKES Antilock Brake System - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado