Chevrolet Silverado / GMC Sierra. Service manual — part 729

does not match the actual yaw rate as measured by the yaw rate sensor.

The desired yaw rate is calculated by the EBCM using, primarily, the following inputs.

The position of the steering wheel

The speed of the vehicle

The lateral, or sideways acceleration of the vehicle

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. When a yaw rate error is detected, the EBCM
attempts to correct the vehicle's yaw motion by applying brake pressure to one or more of the
wheels. The amount of brake pressure which is applied varies, depending on the correction
required. The engine torque may be reduced also, if it is necessary to slow the vehicle while
maintaining stability.

VSES activations generally occur in turns during aggressive driving. When braking during VSES
activation, the pedal may pulsate. The brake pedal pulsates at a higher frequency during VSES
activation than during ABS 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 first phase of the
power-up self-test when the ignition is first turned ON. 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 master cylinder pressure
sensor performs a self-test by sending a series of specific voltage signals to the EBCM, each for a
predetermined amount of time. This phase of the power-up self-test may be heard by the driver,
depending on how soon the engine is cranked and started after turning ON the ignition. The
second phase of the power-up self-test begins when the vehicle is driven at a speed greater than
12 km/h (7.5 mph) and the EBCM has not detected any traction control module (TCS)/vehicle
stability enhancement system (VSES) related malfunctions thus far. When the brake switch
indicates that the brake is not applied and the master cylinder pressure is detected as being low,
the EBCM proceeds with the test. The EBCM isolates all of the wheels by closing the 4 isolation
valves. Due to the fact that all of the wheels are isolated during the second phase of the test, the
test must be aborted if the brake is applied while the test is being performed. Occasionally, the

2008 Chevrolet Silverado 1500

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

driver may detect this by experiencing a momentary hard pedal.

VSES Sensors Initialization

The vehicle stability enhancement system (VSES) sensors values may vary slightly due to
differences in temperature, sensor mounting, connector resistances, manufacturing, etc. Since
VSES is a very sensitive and precise control system, it is imperative that the electronic brake
control module (EBCM) be able to accurately equate a given sensor voltage with an actual unit of
measurement. For example, the yaw rate signal of one vehicle may be 2.64 volts at +18.0 deg/sec
yaw rate while the yaw rate signal of another vehicle may be 2.64 volts at +17.5 deg/sec yaw
rate. Therefore, at the beginning of each ignition cycle, the EBCM must perform an initialization
procedure to observe how the VSES sensors are correlated with each other and also to determine
what each sensor value is when the applicable unit of measurement equals 0. This voltage is
referred to as the sensor bias voltage. Although some activation of the VSES system may occur if
required to prior to full initialization, the system does not give optimum performance until the
sensors are fully initialized.

The following VSES sensors require initialization:

The yaw rate sensor

The lateral accelerometer

The master cylinder pressure sensor

The steering wheel position sensor

When the vehicle speed is greater than 25 km/h (15 mph), full sensor initialization must occur
during 3 km (1.8 mi) of driving or 1 km (0.6 mi) of straight and stable driving, whichever occurs
first. Although an attempt at initialization may fail due to driving conditions, such as driving on a
very winding road, failed initialization is usually caused by a sensor bias voltage which is not
within an acceptable range. Often, a DTC sets soon after a failed initialization attempt. The
message center displays the stability system disabled message when sensor initialization fails.

ECE 13 Response

The electronic brake control module (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 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. This reaction 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

2008 Chevrolet Silverado 1500

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

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.

Active Vacuum Booster

The power brake booster assist unit creates mechanical force for the brake booster in case of a
vacuum loss or low vacuum to the brake booster. The power brake booster system consists of:

The sensor circuits and pressure sensor

The check valve manifold

The vacuum solenoid valve

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

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

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 electronic brake
distribution (EBD) and sends a serial data message to the IPC requesting illumination.

ABS Indicator

The IPC illuminates the ABS indicator when the following occurs.

2008 Chevrolet Silverado 1500

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

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.

A DTC is set during the previous ignition cycle which requires an ECE 13 response at the
beginning of the current ignition cycle. The EBCM sends a serial data message to the IPC
requesting illumination.

Traction Control Off Indicator

The IPC illuminates the traction off indicator when the following occurs.

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 VSES and engine torque reduction by pressing the traction
control switch. The EBCM sends a serial data message to the IPC requesting illumination.

Service Brake Booster Message

The service brake system message is displayed whenever the red brake warning indicator is
illuminated.

Stabilitrak Off Message

The message center displays the stabilitrak off message when one or more of the following
conditions exists.

The transfer case is shifted into 4 LO. The EBCM sends a serial data message to the IPC
requesting illumination.

The driver manually disables the VSES and engine torque reduction by pressing the traction
control switch. The EBCM sends a serial data message to the IPC requesting illumination.

The estimated temperature of any solenoid coil exceeds an acceptable limit. The EBCM
sends a serial data message to the IPC requesting this display.

The EBCM detects a failed brake switch. The EBCM sends a serial data message to the IPC
requesting this display. A DTC sets when this condition exists.

VSES sensor initialization time is excessive. The EBCM sends a serial data message to the
IPC requesting this display.

Serial data communication between the EBCM and any of several other control modules is
interrupted. The EBCM sends a serial data message to the IPC requesting this display or the

2008 Chevrolet Silverado 1500

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