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44-4
ANTI°LOCK BRAKE SYSTEM (ABS)
120°3
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 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 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 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.
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 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.
Traction Control System (TCS)
When drive wheel slip is noted while the brake is not
applied, the EBCM will enter traction control mode.
The EBCM uses a 5-volt pulse-width modulated (PWM)
signal to request the 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 5-volt PWM signal in order to report to the EBCM
the amount of torque delivered to the drive wheels.
Brake Warning Indicator
The instrument panel cluster (IPC) illuminates the
brake warning indicator when the following occurs:
S
The body control module (BCM) detects that the
park brake is engaged. The IPC receives a class 2
message from the BCM requesting illumination.
S
The EBCM detects a low brake fluid condition and
sends a class 2 message to the IPC.
S
The IPC performs the bulb check.
S
An ABS-disabling malfunction also disables
dynamic rear proportioning (DRP).
Circuit Operation
44-5
ANTI°LOCK BRAKE SYSTEM (ABS)
120°3
ABS Indicator
The IPC illuminates the ABS indicator when the
following occurs:
S
The electronic brake control module (EBCM)
detects an ABS-disabling malfunction. The IPC
receives a class 2 message from the EBCM
requesting illumination.
S
The IPC performs the bulb check.
S
The IPC detects a loss of class 2 communications
with the EBCM.
Traction Control Indicators (w/NW7)
LOW TRACTION
The low traction indicator is part of the traction control
switch. The EBCM applies ground to the traction control
active signal circuit in order to illuminate the low traction
indicator during a traction control event.
TRACTION OFF
The traction off indicator, which is part of the traction
control switch, turns OFF when ground is applied to the
service traction control signal circuit. The EBCM
illuminates the traction off indicator if any of the
following conditions are present.
S
The EBCM inhibits the traction control system.
S
The driver manually disables the traction control
system by pressing the traction control switch.
S
The automatic transmission shift lever is in the low
(1) position.
The EBCM inhibits the traction control system when a
TCS-disabling malfunction occurs, or when the
automatic engagement feature is programmed to
disable the TCS when the ignition is turned ON. Refer to
Programming
the
Traction
Control
Automatic
Engagement Feature.
Programming the Traction Control Automatic
Engagement Feature
The
automatic
engagement
feature
may
be
programmed so that the traction control system
activates or does not activate automatically at the start
of each ignition cycle. In order to change the status of
the automatic engagement feature, perform the
following procedure:
Important
Failure to follow the correct procedure may cause
DTC C0283 to set in EBCM memory.
1.
Park the vehicle and apply the parking brake.
2.
Unlock the ignition and shift the transmission into
NEUTRAL (N).
3.
Turn the ignition ON, engine OFF.
4.
Press and hold the brake pedal and the accelerator
pedal.
5.
Press and hold the traction assist switch for 5
seconds.
6.
Release the brake and accelerator pedals and the
traction control switch.
7.
Turn the ignition OFF.
Circuit Operation
44-6
ANTI-LOCK BRAKE SYSTEM (ABS)
(Refer to Section 201 for photographs.)
Component
Photo No.
Brake Fluid Level Switch
Left side of engine compartment
3
. . . . .
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Electronic Brake Control
Module (EBCM)
Inner left side frame rail (near transmission)
41
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Front Axle Actuator
Lower right front of engine (on oil pan)
39
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fuse Block-Rear
Under the left rear seat
52
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Fuse Block-Underhood
Left side of engine compartment
3
. . . . . . .
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Instrument Panel Cluster
(IPC)
On the left side of the IP, above the steering column
59
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Powertrain Control Module
(PCM) (4.2L)
Left front of engine
5
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Powertrain Control Module
(PCM) (5.3L)
Left front of engine compartment
25
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SP201
Near right front of lower console
80
. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stop Lamp Switch
On brake pedal support
64
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Traction Control Switch
(2WD)
In center of the I/P
92
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Wheel Speed Sensor
(WSS)-LF
Left front wheel housing
30
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Wheel Speed Sensor
(WSS)-RF
Right front wheel housing
30
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Connector
C101 (38-BLK)
Left side of engine compartment
3
. . . . . . . . . . . . . .
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C107 (5.3L) (38-BLK)
Left rear of engine compartment
18
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Ground
G107 (4.2L)
Lower left side of engine
37
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G107 (5.3L)
Lower left side of engine
38
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G201
Near right front of lower console
80
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G304
Outside of left frame rail, near electronic brake control module
35
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Component Location Index
47
SUPPLEMENTAL RESTRAINT (SRS)
Electronic Frontal Sensor (EFS)-Left
Electronic Frontal Sensor (EFS)-Right
Fuse Block-Underhood
Inflatable Restraint I/P Module
Inflatable Restraint Sensing and Diagnostic Module
(SDM)
Inflatable Restraint Side Impact Module-LF
Inflatable Restraint Side Impact Module-RF
Inflatable Restraint Side Impact Sensor (SIS)-Left
Inflatable Restraint Side Impact Sensor (SIS)-Right
Inflatable Restraint Steering Wheel Module
Inflatable Restraint Steering Wheel Module Coil
Seat Belt Buckle Switch-Driver
Component Index
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