2002 JEEP GRAND CHEROKEE brakes abs

ELECTRIC BRAKE
DESCRIPTION
The electronic brake distribution (EBD) functions
like a rear proportioning valve. The EBD system uses
the ABS system to control the slip of the rear wheels
in partial braking range. The braking force of the
rear wheels is controlled electronically by using the
inlet and outlet valves located in the HCU.
OPERATION
Upon entry into EBD the inlet valve for the rear
brake circuit is switched on so that the fluid supply
from the master cylinder is shut off. In order to
decrease the rear brake pressure the outlet valve for
the rear brake circuit is pulsed. This allows fluid to
enter the low pressure accumulator (LPA) in the
HCU resulting in a drop in fluid pressure to the rear
brakes. In order to increase the rear brake pressure
the outlet valve is switched off and the inlet valve is
pulsed. This increases the pressure to the rear
brakes. This will continue until the required slip dif-
ference is obtained. At the end of EBD braking (no
brake application) the fluid in the LPA drains back to
the master cylinder by switching on the outlet valve
and draining through the inlet valve check valve. At
the same time the inlet valve is switched on to pre-
vent a hydraulic short circiut in case of another
brake application.
The EBD will remain functional during many ABS
fault modes. If the red and amber warning lamps are
illuminated the EBD may have a fault.
FRONT WHEEL SPEED
SENSOR
DESCRIPTION
A wheel speed sensor is used at each wheel. The
front sensors are mounted to the steering knuckles.
The rear sensors are mounted at the outboard end of
the axle. Tone wheels are mounted to the outboard
ends of the front and rear axle shafts. The gear type
tone wheel serves as the trigger mechanism for each
sensor.
OPERATION
The sensors convert wheel speed into a small digi-
tal signal. The CAB sends 12 volts to the sensors.
The sensor has an internal magneto resistance
bridge that alters the voltage and amperage of the
signal circuit. This voltage and amperage is changed
by magnetic induction when the toothed tone wheel
passes the wheel speed sensor. This digital signal issent to the CAB. The CAB measures the voltage and
amperage of the digital signal for each wheel.
REMOVAL
(1) Raise and support the vehicle.
(2) Remove the front wheel sensor mounting bolt
(Fig. 1).
(3) Remove the sensor from the steering knuckle.
(4) Disengage the sensor wire from the brackets
(Fig. 1)on the steering knuckle.
(5) Disconnect the sensor from the sensor harness
(Fig. 2)and (Fig. 3).
(6) Remove the sensor and wire.
INSTALLATION
(1) Install the sensor on the steering knuckle.
(2) Apply Mopar Lock N' Seal or Loctitet242 to
the sensor mounting bolt. Use new sensor bolt if orig-
inal bolt is worn or damaged.
(3) Install the sensor mounting bolt and tighten
bolt to 12-14 N´m (106-124 in. lbs.).
(4) Engage the grommets on the sensor wire to the
steering knuckle brackets.
(5) Connect the sensor wire to the harness connec-
tor.
(6) Check the sensor wire routing. Be sure the
wire is clear of all chassis components and is not
twisted or kinked at any spot.
(7) Remove the support and lower vehicle.
Fig. 1 Sensor Location
1 - BRACKET
2 - BRACKET
3 - WHEEL SPEED SENSOR
4 - MOUNTING BOLT
WJBRAKES - ABS 5 - 43

G-SWITCH
DESCRIPTION
The G-switch (Fig. 4) is located under the rear
seat. The switch has directional arrow and must be
mounted with the arrow pointing towards the front
of the vehicle.
OPERATION
The switch is monitored by the CAB at all times.
The switch contains three mercury switches which
monitor vehicle deceleration rates (G-force). Sudden
changes in deceleration rates trigger the switch,
sending a signal to the CAB.
REMOVAL
(1) Fold the rear seat bottom assembly up for
access to the switch.
(2) Lift up the carpeting and disconnect the switch
harness (Fig. 5).
(3) Remove the switch mounting bolts and remove
the switch.
INSTALLATION
CAUTION: The mercury switch (inside the
G-Switch), will not function properly if the switch is
installed incorrectly. Verify that the switch locating
arrow is pointing to the front of the vehicle (Fig. 6).
(1) Note the position of the locating arrow on the
switch. Position the switch so the arrow faces for-
ward.
(2) Install the switch and tighten the mounting
bolts to 5.6 N´m (50 in. lbs.).
Fig. 2 Left Sensor Connector
1 - LEFT FRONT WHEEL SPEED SENSOR CONNECTOR
2 - ENGINE EXHAUST PIPE
3 - LEFT FRONT FRAME RAIL
4 - FRONT DRIVESHAFT
Fig. 3 Right Sensor Connector
1 - RIGHT FRONT WHEEL SPEED SENSOR CONNECTOR
2 - ENGINE EXHAUST Y-PIPE
3 - RIGHT FRONT FRAME RAIL
4 - RIGHT LOWER SUSPENSION ARM
Fig. 4 G-Switch
1 - SWITCH PART NUMBER
2 - ARROW INDICATES FRONT OF SWITCH FOR PROPER
MOUNTING
5 - 44 BRAKES - ABSWJ
FRONT WHEEL SPEED SENSOR (Continued)

(3) Connect the harness to the switch. Be sure the
harness connector is firmly seated.
(4) Place the carpet in position and fold the rear
seat back down.
REAR WHEEL SPEED SENSOR
DESCRIPTION
A wheel speed sensor is used at each wheel. The
front sensors are mounted to the steering knuckles.The rear sensors are mounted at the outboard end of
the axle. Tone wheels are mounted to the outboard
ends of the front and rear axle shafts. The gear type
tone wheel serves as the trigger mechanism for each
sensor.
OPERATION
The sensors convert wheel speed into a small digi-
tal signal. The CAB sends 12 volts to the sensors.
The sensor has an internal magneto resistance
bridge that alters the voltage and amperage of the
signal circuit. This voltage and amperage is changed
by magnetic induction when the toothed tone wheel
passes the wheel speed sensor. This digital signal is
sent to the CAB. The CAB measures the voltage and
amperage of the digital signal for each wheel.
REMOVAL
(1) Raise and fold the rear seat forward. Then
move the carpeting aside for access to the rear sensor
connectors.
(2) Disconnect the rear sensor wire at the harness
connectors (Fig. 7).
(3) Push the sensor wires and grommets through
the floorpan holes.
(4) Raise and support the vehicle.
(5) Disengage the sensor wire from the axle and
the chassis brackets and from the brake line retain-
ers.
(6) Remove the sensor mounting bolt from the rear
brake backing plate. (Fig. 8).
(7) Remove the sensor from the backing plate.
Fig. 5 G-Switch Mounting
1 - MOUNTING BOLTS
2 - CONNECTOR
3 - G-SWITCH
Fig. 6 G-Switch
1 - SWITCH PART NUMBER
2 - ARROW INDICATES FRONT OF SWITCH FOR PROPER
MOUNTING
Fig. 7 Rear Sensor Connector
1 - RIGHT REAR WHEEL SPEED SENSOR CONNECTOR
2 - LEFT REAR WHEEL SPEED SENSOR CONNECTOR
3 - G-SWITCH SENSOR
4 - PARKING BRAKE CABLES
WJBRAKES - ABS 5 - 45
G-SWITCH (Continued)

INSTALLATION
(1) Insert the sensor through the backing plate
(Fig. 9).
(2) Apply Mopar Lock N' Seal or Loctite 242tto
the original sensor bolt. Use a new bolt if the original
is worn or damaged.
(3) Tighten the sensor bolt to 12-14 N´m (106-124
in. lbs.).
(4) Secure the sensor wire in the brackets and the
retainers on the rear brake lines. Verify that the sen-
sor wire is secure and clear of the rotating compo-
nents.
(5) Route the sensor wires to the rear seat area.
(6) Feed the sensor wires the through floorpan
access hole and seat the sensor grommets into the
floorpan.
(7) Remove the support and lower the vehicle.(8) Fold the rear seat and carpet forward for
access to the sensor wires and connectors.
(9) Connect the sensor wires to the harness con-
nectors.
(10) Reposition the carpet and fold the rear seat
down.
HCU (HYDRAULIC CONTROL
UNIT)
DESCRIPTION
The HCU consists of a valve body, pump motor,
and wire harness.
OPERATION
Accumulators in the valve body store extra fluid
released to the system for ABS mode operation. The
pump is used to clear the accumulator of brake fluid
and is operated by a DC type motor. The motor is
controlled by the CAB.
The valves modulate brake pressure during
antilock braking and are controlled by the CAB.
The HCU provides three channel pressure control
to the front and rear brakes. One channel controls
the rear wheel brakes in tandem. The two remaining
channels control the front wheel brakes individually.
During antilock braking, the solenoid valves are
opened and closed as needed. The valves are not
static. They are cycled rapidly and continuously to
modulate pressure and control wheel slip and decel-
eration.
During normal braking, the HCU solenoid valves
and pump are not activated. The master cylinder and
power booster operate the same as a vehicle without
an ABS brake system.
During antilock braking, solenoid valve pressure
modulation occurs in three stages, pressure increase,
pressure hold, and pressure decrease. The valves are
all contained in the valve body portion of the HCU.
PRESSURE DECREASE
The outlet valve is opened and the inlet valve is
closed during the pressure decrease cycle.
A pressure decrease cycle is initiated when speed
sensor signals indicate high wheel slip at one or
more wheels. At this point, the CAB closes the inlet
then opens the outlet valve, which also opens the
return circuit to the accumulators. Fluid pressure is
allowed to bleed off (decrease) as needed to prevent
wheel lock.
Once the period of high wheel slip has ended, the
CAB closes the outlet valve and begins a pressure
increase or hold cycle as needed.
Fig. 8 Sensor Mounting Bolt
1 - WHEEL SPEED SENSOR
2 - MOUNTING BOLT
Fig. 9 Wheel Speed Sensor
1 - WHEEL SPEED SENSOR
2 - BACKING PLATE
5 - 46 BRAKES - ABSWJ
REAR WHEEL SPEED SENSOR (Continued)

PRESSURE HOLD
Both solenoid valves are closed in the pressure
hold cycle. Fluid apply pressure in the control chan-
nel is maintained at a constant rate. The CAB main-
tains the hold cycle until sensor inputs indicate a
pressure change is necessary.
PRESSURE INCREASE
The inlet valve is open and the outlet valve is
closed during the pressure increase cycle. The pres-
sure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.
REMOVAL
(1) Remove the negative battery cable from the
battery.
(2) Remove the air cleaner housing,(Refer to 9 -
ENGINE/AIR INTAKE SYSTEM/AIR CLEANER
HOUSING - REMOVAL).
(3) Pull the CAB harness connector release up and
remove connector (Fig. 10).(4) Remove the brake lines from the HCU.
(5) Remove the HCU/CAB side mounting bolt and
the two rear mounting bolts. (Fig. 11).
(6) Remove the HCU/CAB assembly from the vehi-
cle.
INSTALLATION
(1) Install HCU/CAB assembly into the mounting
bracket and tighten mounting bolts to 12 N´m (9 ft.
lbs.).
(2) Install the brake lines to the HCU and tighten
to 16 N´m (12 ft. lbs.).
(3) Install CAB harness connector and push down
connector release.
(4) Install air cleaner housing,(Refer to 9 -
ENGINE/AIR INTAKE SYSTEM/AIR CLEANER
HOUSING - INSTALLATION).
(5) Install negative battery cable to the battery.
(6) Bleed base and ABS brake systems,(Refer to 5 -
BRAKES - STANDARD PROCEDURE) OR (Refer to
5 - BRAKES - STANDARD PROCEDURE).
Fig. 10 CAB Connector Release
1 - CONNECTOR RELEASE
2 - CAB
Fig. 11 HCU/CAB Assembly
1 - SIDE MOUNTING BOLT
2 - REAR MOUNTING BOLTS
WJBRAKES - ABS 5 - 47
HCU (HYDRAULIC CONTROL UNIT) (Continued)

(5) Connect the two instrument panel wire harness
connectors to the BCM.
(6) Reinstall the instrument panel fuse cover to
the bottom of the BCM and JB unit. (Refer to 8 -
ELECTRICAL/POWER DISTRIBUTION/FUSE
COVER - INSTALLATION).
(7) Connect the battery negative cable.
COMMUNICATION
DESCRIPTION
The Programmable Communication Interface (PCI)
data bus system is a single wire multiplex system
used for vehicle communications. Multiplexing is a
system that enables the transmission of several mes-
sages over a single channel or circuit.
Many of the control modules in a vehicle require
information from the same sensing device. Multiplex-
ing reduces wire harness complexity, sensor current
loads and controller hardware because each sensing
device is connected to only one controller, which
reads and distributes the sensor information to the
other controllers over the data bus. Also, because
each controller on the data bus can access the con-
troller sensor inputs to every other controller on the
data bus, more function and feature capabilities are
possible.
A multiplex system allows the information flowing
between controllers to be monitored using a diagnos-
tic scan tool. This system allows a control module to
broadcast message data out onto the bus where all
other control modules can read the messages that are
being sent. When a module reads a message on the
data bus that it requires, it relays that message to
its microprocessor. Each module ignores the mes-
sages on the data bus that it dosen't recognize.
OPERATION
Data exchange between modules is achieved by
serial transmission of encoded data over a single wire
broadcast network. The PCI data bus messages are
carried over the bus in the form of Variable Pulse
Width Modulated (VPWM) signals. The PCI data bus
speed is an average 10.4 Kilo-bits per second (Kbps).
The voltage network used to transmit messages
requires biasing and termination. Each module on
the PCI data bus system provides its own biasing
and termination. Each module (also referred to as a
node) terminates the bus through a terminating
resistor and a terminating capacitor. The Powertrain
Control Module (PCM) is the only dominant node for
the PCI data bus system.
The PCI bus uses low and high voltage levels to
generate signals. The voltage on the buss varies
between zero and seven and one-half volts. The lowand high voltage levels are generated by means of
variable-pulse width modulation to form signals of
varying length.
When a module is transmitting on the bus, it is
reading the bus at the same time to ensure message
integrity.
Each module is capable of transmitting and receiv-
ing data simultaneously.
The PCI data bus can be monitored using the
DRBIIItscan tool. It is possible for the bus to pass
all DRBIIIttests and still be faulty if the voltage
parameters are all within the specified range and
false messages are being sent.
CONTROLLER ANTILOCK
BRAKE
DESCRIPTION
The Controler Antilock Brake (CAB) is mounted to
the Hydraulic Control Unit (HCU) and operates the
ABS system (Fig. 4).
OPERATION
The CAB voltage is supplied by the ignition switch
in the RUN position. The CAB contains dual micro-
processors. A logic block in each microprocessor
receives identical sensor signals. These signals are
processed and compared simultaneously. The CAB
contains a self check program that illuminates the
ABS warning light when a system fault is detected.
Faults are stored in a diagnostic program memory
and are accessible with the DRBIIItscan tool. ABS
faults remain in memory until cleared, or until after
the vehicle is started approximately 50 times. Stored
Fig. 4 Controller Antilock Brakes
1 - HCU
2 - MOTOR
3 - CAB
8E - 6 ELECTRONIC CONTROL MODULESWJ
BODY CONTROL MODULE (Continued)