Hummer H3. Manual — part 653

DESCRIPTION AND OPERATION

ABS DESCRIPTION AND OPERATION

This vehicle is equipped with an Advics HB-CI brake master cylinder assembly. 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.

2007 Hummer H3

2007 BRAKES Antilock Brake System - H3

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

2007 Hummer H3

2007 BRAKES Antilock Brake System - H3

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 dependent 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

2007 Hummer H3

2007 BRAKES Antilock Brake System - H3

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

2007 Hummer H3

2007 BRAKES Antilock Brake System - H3