2002 JEEP GRAND CHEROKEE Transmission control unit

and therefore creates pressure in the pump. The tun-
ing of the front and rear axle orifices and valves
inside the gerotor pump is unique and each system
includes a torque-limiting pressure relief valve to
protect the clutch pack, which also facilitates vehicle
control under extreme side-to-side traction varia-
tions. The resulting pressure is applied to the clutch
pack and the transfer of torque is completed.
Under conditions in which opposite wheels are on
surfaces with widely different friction characteristics,
Vari-loktdelivers far more torque to the wheel on
the higher traction surface than do conventional
Trac-loktsystems. Because conventional Trac-lokt
differentials are initially pre-loaded to assure torque
transfer, normal driving (where inner and outer
wheel speeds differ during cornering, etc.) produces
torque transfer during even slight side-to-side speed
variations. Since these devices rely on friction from
this preload to transfer torque, normal use tends to
cause wear that reduces the ability of the differential
to transfer torque over time. By design, the Vari-lokt
system is less subject to wear, remaining more con-
sistent over time in its ability to transfer torque. The
coupling assembly is serviced as a unit. From a ser-
vice standpoint the coupling also benefits from using
the same lubricant supply as the ring and pinion
gears.
DIAGNOSIS AND TESTING
GEAR NOISE
Axle gear noise can be caused by insufficient lubri-
cant, incorrect backlash, tooth contact, worn/damaged
gears, or the carrier housing not having the proper
offset and squareness.
Gear noise usually happens at a specific speed
range. The noise can also occur during a specific type
of driving condition. These conditions are accelera-
tion, deceleration, coast, or constant load.
When road testing, first warm-up the axle fluid by
driving the vehicle at least 5 miles and then acceler-
ate the vehicle to the speed range where the noise is
the greatest. Shift out-of-gear and coast through the
peak-noise range. If the noise stops or changes
greatly:
²Check for insufficient lubricant.
²Incorrect ring gear backlash.
²Gear damage.
Differential side gears and pinions can be checked
by turning the vehicle. They usually do not cause
noise during straight-ahead driving when the gears
are unloaded. The side gears are loaded during vehi-
cle turns. A worn pinion mate shaft can also cause a
snapping or a knocking noise.
BEARING NOISE
The axle shaft, differential and pinion bearings can
all produce noise when worn or damaged. Bearing
noise can be either a whining, or a growling sound.
Pinion bearings have a constant-pitch noise. This
noise changes only with vehicle speed. Pinion bearing
noise will be higher pitched because it rotates at a
faster rate. Drive the vehicle and load the differen-
tial. If bearing noise occurs, the rear pinion bearing
is the source of the noise. If the bearing noise is
heard during a coast, the front pinion bearing is the
source.
Worn or damaged differential bearings usually pro-
duce a low pitch noise. Differential bearing noise is
similar to pinion bearing noise. The pitch of differen-
tial bearing noise is also constant and varies only
with vehicle speed.
Axle shaft bearings produce noise and vibration
when worn or damaged. The noise generally changes
when the bearings are loaded. Road test the vehicle.
Turn the vehicle sharply to the left and to the right.
This will load the bearings and change the noise
level. Where axle bearing damage is slight, the noise
is usually not noticeable at speeds above 30 mph.
LOW SPEED KNOCK
Low speed knock is generally caused by a worn
U-joint or by worn side±gear thrust washers. A worn
pinion shaft bore will also cause low speed knock.
VIBRATION
Vibration at the rear of the vehicle is usually
caused by a:
²Damaged drive shaft.
²Missing drive shaft balance weight(s).
²Worn or out-of-balance wheels.
²Loose wheel lug nuts.
²Worn U-joint(s).
²Loose/broken springs.
²Damaged axle shaft bearing(s).
²Loose pinion gear nut.
²Excessive pinion yoke run out.
²Bent axle shaft(s).
Check for loose or damaged front-end components
or engine/transmission mounts. These components
can contribute to what appears to be a rearend vibra-
tion. Do not overlook engine accessories, brackets
and drive belts.
All driveline components should be examined
before starting any repair.
(Refer to 22 - TIRES/WHEELS - DIAGNOSIS AND
TESTING)
3 - 92 REAR AXLE - 226RBAWJ
REAR AXLE - 226RBA (Continued)

CONDITION POSSIBLE CAUSES CORRECTION
COOLANT LEVEL
CHANGES IN COOLANT
RESERVE/OVERFLOW
TANK. TEMPERATURE
GAUGE IS IN NORMAL
RANGE1. Level changes are to be expected
as coolant volume fluctuates with
engine temperature. If the level in
the tank was between the FULL and
ADD marks at normal operating
temperature, the level should return
to within that range after operation
at elevated temperatures.1. A normal condition. No repair is necessary.
FAN RUNS ALL THE
TIME1. Fan control sensors inoperative. 1. Check for DTC's. Verify sensor readings.
2. Fan control solenoid stuck9on9. 2. Check fan operation speeds. Refer to fan
speed operation table.
3. Fan control solenoid harness
damaged.3. Check for DTC 1499. Repair as required.
4. Transmission temperature too
high.4. Check for transmission over temp. DTC.
5. Engine coolant temperature too
high.5. (a) Check coolant level. Correct level as
required.
(b) Thermostat stuck. Replace thermostat.
(c) Water pump failed. Replace water pump.
(d) Coolant flow restricted. Clean radiator.
(e) Air flow over radiator obstructed.Remove
obstruction.
DIAGNOSIS AND TESTING - COOLING SYSTEM
LEAKS
ULTRAVIOLET LIGHT METHOD
A leak detection additive is available through the
parts department that can be added to cooling sys-
tem. The additive is highly visible under ultraviolet
light (black light). Pour one ounce of additive into
cooling system. Place heater control unit in HEAT
position. Start and operate engine until radiator
upper hose is warm to touch. Aim the commercially
available black light tool at components to be
checked. If leaks are present, black light will cause
additive to glow a bright green color.
The black light can be used in conjunction with a
pressure tester to determine if any external leaks
exist (Fig. 5).
PRESSURE TESTER METHOD
The engine should be at normal operating temper-
ature. Recheck the system cold if cause of coolant
loss is not located during the warm engine examina-
tion.
WARNING: HOT, PRESSURIZED COOLANT CAN
CAUSE INJURY BY SCALDING.
Fig. 5 Leak Detection Using Black LightÐTypical
1 - TYPICAL BLACK LIGHT TOOL
7 - 10 COOLINGWJ
COOLING (Continued)

(6) Start the engine and operate at 2400 rpm.
Within ten minutes the air temperature (indicated on
the dial thermometer) should be up to 93É C (200É F).
Fan driveengagementshould have started to occur
at between 91É to 96É C (195É to 205É F). Engage-
ment is distinguishable by a definiteincreasein fan
flow noise (roaring). The timing light also will indi-
cate an increase in the speed of the fan.
(7) When the air temperature reaches 93É C (200É
F), remove the plastic sheet. Fan drivedisengage-
mentshould have started to occur at between 62É to
85É C (145É to 185É F). A definitedecreaseof fan
flow noise (roaring) should be noticed. If not, replace
the defective viscous fan drive unit.
PWM FAN CONTROL MODULE -
4.0L
DESCRIPTION
The pulse width modulated (PWM) radiator cooling
fan relay is located behind the front bumper fascia
below the right headlamp.
OPERATION
The PWM relay is used to control the speed of the
electric radiator cooling fan. It allows for multiple fan
speeds. This allows for improved fan noise and A/C
performance, better engine cooling, and additional
vehicle power.
PWM relay operation is controlled by the Power-
train Control Module (PCM). To operate the PWM
relay, the PCM looks at inputs from:
²Engine coolant temperature
²Ambient temperature from the body controller
²Vehicle speed
²Transmission oil temperature
²A/C switch position (A/C request)
REMOVAL
The Pulse Width Modulated (PWM) cooling fan
relay is located below the right headlamp behind the
bumper fascia (Fig. 28).
(1) Remove front bumper and grill assembly.
(2) Remove 1 support bolt near front of reservoir
(Fig. 28).
(3) Remove 2 reservoir mounting bolts.
(4) Remove reservoir from vehicle to gain access to
vacuum hose (Fig. 29). Disconnect vacuum hose from
reservoir fitting at rear of reservoir.
(5) Disconnect electrical connector at relay (Fig.
30).
(6) Remove 2 relay mounting bolts (Fig. 30) and
remove relay.
Fig. 28 Radiator Cooling Fan Relay Location
1 - RADIATOR FAN RELAY
2 - VACUUM RESERVOIR
3 - BOLT
4 - RIGHT FRONT FENDER
Fig. 29 Vacuum Reservoir Removal/Installation
1 - VACUUM HOSE
2 - RADIATOR
3 - VACUUM RESERVOIR
WJENGINE 7 - 41
FAN DRIVE VISCOUS CLUTCH - 4.0L (Continued)

REMOVAL - EXHAUST-TO-BODY GROUND
STRAP
(1) Raise and support vehicle.
(2) Remove fasteners from exhaust, crossmember
and transmission/transfer case.
(3) Remove the ground strap.
INSTALLATION
INSTALLATION - ENGINE TO BODY GROUND
STRAP
(1) Position the engine-to-body ground strap into
the engine compartment.
(2) Install the engine-to-body ground strap eye-
let(s) onto the stud(s) on the engine.
(3) On models with a 4.0L engine, install and
tighten the nut that secures the engine-to-body
ground strap eyelet to the stud on the right rear side
of the engine cylinder head. Tighten the nut to 5.6
N´m (50 in. lbs.).
(4) On models with a 4.7L engine, install and
tighten the two nuts that secure the engine-to-body
ground strap eyelets to the studs on the right and
left rear sides of the engine intake manifold. Tighten
the nuts to 11.3 N´m (100 in. lbs.).(5) Install and tighten the screw that secures the
engine-to-body ground strap eyelet to the lower ple-
num panel. Tighten the screw to 4.5 N´m (40 in. lbs.).
INSTALLATION - EXHAUST-TO-BODY GROUND
STRAP
(1) Install the ground strap to the exhaust, cross-
member and transmission/transfer case.
(2) Install the fasteners.
(3) Lower the vehicle.
REMOTE SWITCHES
DESCRIPTION
Remote radio control switches are included on
models equipped with the optional leather-wrapped
steering wheel. The two rocker-type switch units
(Fig. 15) are mounted in the upper spoke covers of
the rear (instrument panel side) steering wheel trim
cover. The switch unit on the left side is the seek
switch and has seek up, seek down, and preset sta-
tion advance switch functions. The switch unit on the
right side is the volume control switch and has vol-
ume up, volume down, and mode advance switch
functions.
Fig. 13 Engine-To-Body Ground Strap Remove/
Install - 4.0L Engine
1 - SCREW
2 - LOWER PLENUM PANEL
3 - ENGINE
4 - STUD
5 - NUT
6 - GROUND STRAP
Fig. 14 Engine-To-Body Ground Strap Remove/
Install - 4.7L Engine
1 - GROUND STRAP
2 - SCREW
3 - ENGINE
4 - STUD (2)
5 - NUT (2)
6 - LOWER PLENUM PANEL
WJAUDIO 8A - 19
RADIO NOISE SUPPRESSION GROUND STRAP (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)

Programmable Communications Interface (PCI) data
bus network.
For diagnosis of the HSM, MHSM or the PCI data
bus, a DRBIIItscan tool and the proper Diagnostic
Procedures manual are recommended. The HSM or
MHSM cannot be repaired and, if faulty or damaged,
it must be replaced.
OPERATION
The MSM receives hard wired inputs from the
power seat switch and the potentiometers on each of
the driver side power seat motors. The MSM receives
messages over the PCI data bus from the Driver
Door Module (DDM) (memory switch status), the
Powertrain Control Module (PCM) (vehicle speed sta-
tus), and the Body Control Module (seat belt switch
status).The MSM will prevent the seat memory recall
function from being initiated if the driver side seat
belt is buckled, if the transmission gear selector lever
is not in the Park or Neutral positions, or if the vehi-
cle is moving.
DIAGNOSIS AND TESTING
DIAGNOSIS AND TESTING - HEATED SEAT
MODULE
Visually inspect the related wiring harness connec-
tors. Look for broken, bent, pushed out, or corroded
terminals. If any of the above conditions are present,
repair as necessary. If not, use a DRBIIItscan tool
and the proper Diagnostic Procedures Manual to test
the HSM or MHSM. For complete circuit diagrams,
refer toPower Seat Premium I/IIIin Wiring Dia-
grams.
REMOVAL
(1) Disconnect and isolate the battery negative
cable.
(2) Remove the driver side front bucket seat from
the power seat track unit. Refer toBucket Seat
Track Adjusterin Body for the procedure.
(3) Lift the heated seat module off of the power
seat track and disconnect the power seat wire har-
ness connectors (Fig. 10).
(4) Remove the module from the bracket.
Fig. 9 Heated Seat Module Remove/Install
1 - NUT (4)
2 - SEAT CUSHION FRAME
3 - POWER SEAT TRACK
4 - STUD (4)
5 - MODULE
6 - BRACKET
Fig. 10 Heated Seat Module Remove/Install
1 - NUT (4)
2 - SEAT CUSHION FRAME
3 - POWER SEAT TRACK
4 - STUD (4)
5 - MODULE
6 - BRACKET
WJELECTRONIC CONTROL MODULES 8E - 11
MEMORY HEATED SEAT/MIRROR MODULE (Continued)

(2) If equipped, remove Transmission Control Mod-
ule (TCM).
(3) Remove coolant reserve/overflow tank.
(4) Remove cover over electrical connectors. Cover
snaps onto PCM.
(5) Carefully unplug three 32±way connectors at
PCM.
(6) Remove three PCM bracket-to-body mounting
nuts (Fig. 13).
(7) Remove PCM/PCM bracket assembly from
vehicle.
(8) Remove 3 PCM-to-PCM bracket bolts (screws)
(Fig. 14).
INSTALLATION
USE THE DRBIIItSCAN TOOL TO REPRO-
GRAM THE NEW POWERTRAIN CONTROL
MODULE (PCM) WITH THE VEHICLES ORIGI-
NAL IDENTIFICATION NUMBER (VIN) AND
THE VEHICLES ORIGINAL MILEAGE. IF THIS
STEP IS NOT DONE, A DIAGNOSTIC TROUBLE
CODE (DTC) MAY BE SET.
The PCM is located on the cowl panel in right/rear
side of engine compartment (Fig. 12).
(1) Check pins in three 32±way electrical connec-
tors for damage. Repair as necessary.
(2) Install PCM to its mounting bracket. Tighten
three mounting bolts to 3 N´m (25 in. lbs.) torque.
(3) Install PCM/PCM bracket to body. Install 3
nuts and tighten 9 N´m (80 in. lbs.) torque.
(4) Install three 32±way connectors.(5) Install cover over electrical connectors. Cover
snaps onto PCM.
(6) Install coolant reserve/overflow tank.
(7) If equipped, install Transmission Control Mod-
ule (TCM).
(8) Connect negative cable to battery.
(9) Use the DRBIIItscan tool to reprogram new
PCM with vehicles original Identification Number
(VIN) and original vehicle mileage.
SENTRY KEY IMMOBILIZER
MODULE
DESCRIPTION
The Sentry Key Immobilizer Module (SKIM) is the
primary component of the Sentry Key Immobilizer
System (SKIS) (Fig. 15). The SKIM is located in the
steering column, below the ignition lock cylinder
housing. The SKIM has an integral halo-like antenna
ring that extends from one side.
The SKIM cannot be adjusted or repaired. If faulty
or damaged, the entire SKIM unit must be replaced.
OPERATION
The Sentry Key Immobilizer Module (SKIM) con-
tains a Radio Frequency (RF) transceiver and a
microprocessor. The SKIM transmits RF signals to,
and receives RF signals from the Sentry Key tran-
Fig. 14 Powertrain Control Module (PCM) Mounting
Bracket
1 - PCM BRACKET
2 - PCM
3 - PCM-TO-BRACKET SCREWS (3)
Fig. 15 Sentry Key Immobilizer Module
1 - STEERING COLUMN
2 - SKIM
3 - MOUNTING SCREW
WJELECTRONIC CONTROL MODULES 8E - 17
POWERTRAIN CONTROL MODULE (Continued)

certain conditions or inputs to provide the vehicle
operator with an audible alert to supplement a visual
indication.
The EMIC circuitry operates on battery current
received through fused B(+) fuses in the Power Dis-
tribution Center (PDC) and the Junction Block (JB)
on a non-switched fused B(+) circuit, and on battery
current received through a fused ignition switch out-
put (run-start) fuse in the JB on a fused ignition
switch output (run-start) circuit. This arrangement
allows the EMIC to provide some features regardless
of the ignition switch position, while other features
will operate only with the ignition switch in the On
or Start positions. The EMIC circuitry is grounded
through two separate ground circuits of the instru-
ment panel wire harness. These ground circuits
receive ground through take outs of the instrument
panel wire harness with eyelet terminal connectors
that are secured by a nut to a ground stud located on
the floor panel transmission tunnel beneath the cen-
ter floor console, just forward of the Airbag Control
Module (ACM).
The EMIC also has a self-diagnostic actuator test
capability, which will test each of the PCI bus mes-
sage-controlled functions of the cluster by lighting
the appropriate indicators (except the airbag indica-
tor), sweeping the gauge needles across the gauge
faces from their minimum to their maximum read-
ings, and stepping the odometer display sequentially
from all zeros through all nines. (Refer to 8 - ELEC-
TRICAL/INSTRUMENT CLUSTER - DIAGNOSIS
AND TESTING). The self-diagnostic actuator test
can be initialized manually or using a DRBIIItscan
tool. Refer to the appropriate diagnostic information.
See the owner's manual in the vehicle glove box for
more information on the features, use and operation
of the EMIC.
GAUGES
All gauges receive battery current through the
EMIC circuitry when the ignition switch is in the On
or Start positions. With the ignition switch in the Off
position battery current is not supplied to any
gauges, and the EMIC circuitry is programmed to
move all of the gauge needles back to the low end of
their respective scales. Therefore, the gauges do not
accurately indicate any vehicle condition unless the
ignition switch is in the On or Start positions. All of
the EMIC gauges, except the odometer, are air core
magnetic units. Two fixed electromagnetic coils are
located within each gauge. These coils are wrapped
at right angles to each other around a movable per-
manent magnet. The movable magnet is suspended
within the coils on one end of a pivot shaft, while the
gauge needle is attached to the other end of the
shaft. One of the coils has a fixed current flowingthrough it to maintain a constant magnetic field
strength. Current flow through the second coil
changes, which causes changes in its magnetic field
strength. The current flowing through the second coil
is changed by the EMIC circuitry in response to mes-
sages received over the PCI data bus. The gauge nee-
dle moves as the movable permanent magnet aligns
itself to the changing magnetic fields created around
it by the electromagnets.
The gauges are diagnosed using the EMIC self-di-
agnostic actuator test. (Refer to 8 - ELECTRICAL/
INSTRUMENT CLUSTER - DIAGNOSIS AND
TESTING). Proper testing of the PCI data bus, and
the data bus message inputs to the EMIC that con-
trol each gauge requires the use of a DRBIIItscan
tool. Refer to the appropriate diagnostic information.
Specific operation details for each gauge may be
found elsewhere in this service information.
VACUUM-FLUORESCENT DISPLAY
The Vacuum-Fluorescent Display (VFD) module is
soldered to the EMIC circuit board. The display is
active with the ignition switch in the On or Start
positions, and inactive when the ignition switch is in
any other position. The illumination intensity of the
VFD is controlled by the EMIC circuitry based upon
electronic dimming level messages received from the
BCM over the PCI data bus, and is synchronized
with the illumination intensity of other VFDs in the
vehicle. The BCM provides dimming level messages
based upon internal programming and inputs it
receives from the control knob and control ring on
the control stalk of the left (lighting) multi-function
switch on the steering column.
The VFD has several display capabilities including
odometer and trip odometer information. An odome-
ter/trip odometer switch on the EMIC circuit board is
used to control the display modes. This switch is
actuated manually by depressing the odometer/trip
odometer switch button that extends through the
lower edge of the cluster lens, just right of the speed-
ometer. Actuating this switch momentarily with the
ignition switch in the On position will toggle the
VFD between the odometer and trip odometer modes.
The EMIC microprocessor remembers which display
mode is active when the ignition switch is turned to
the Off position, and returns the display to that
mode when the ignition switch is turned On again.
Depressing the switch button for about two seconds
while the VFD is in the trip odometer mode will
reset the trip odometer value to zero. Holding this
switch depressed while turning the ignition switch
from the Off position to the On position will initiate
the EMIC self-diagnostic actuator test. Refer to the
appropriate diagnostic information for additional
details on this VFD function.
WJINSTRUMENT CLUSTER 8J - 5
INSTRUMENT CLUSTER (Continued)