2002 JEEP GRAND CHEROKEE Message

HEATED MIRRORS
TABLE OF CONTENTS
page page
HEATED MIRRORS
DESCRIPTION..........................8
OPERATION............................8DIAGNOSIS AND TESTING - HEATED
MIRRORS............................8
HEATED MIRRORS
DESCRIPTION
Electrically heated outside rear view mirrors are
optional equipment on this model. These mirrors fea-
ture an electric heating grid located behind the mir-
ror glass of each power operated outside rear view
mirror. These heating grids consist of a single resis-
tor wire routed in a grid-like pattern and captured
between two thin sheets of plastic. When electrical
current is passed through the resistor wire, it pro-
duces enough heat energy to clear the outside mirror
glass of ice, snow or fog. Battery current is directed
to the outside mirror heating grid only when the rear
window defogger switch is in the On position.
If the outside mirror heating grids and the rear
window heating grid are all inoperative, (Refer to 8 -
ELECTRICAL/HEATED GLASS - DIAGNOSIS AND
TESTING - REAR WINDOW DEFOGGER SYSTEM).
If the outside mirror heating grids are inoperative,
but the rear window heating grid is operating as
designed, (Refer to 8 - ELECTRICAL/HEATED MIR-
RORS - DIAGNOSIS AND TESTING)
The heating grid behind each outside mirror glass
cannot be repaired and, if faulty or damaged, the
entire power mirror unit must be replaced. Refer to
Power Mirrors for the procedures.
OPERATION
The outside mirror heating grids are energized and
de-energized by the Driver Door Module (DDM) and
the Passenger Door Module (PDM) based upon the
rear window defogger switch status. The Body Con-
trol Module (BCM) monitors the rear window defog-ger switch. When the BCM receives an input from
the switch, it sends a defogger switch status message
to the DDM and the PDM over the Programmable
Communications Interface data bus. The DDM and
PDM respond to the defogger switch status messages
by energizing or de-energizing the battery current
feed to their respective outside rear view mirror
heating grids.
DIAGNOSIS AND TESTING - HEATED MIRRORS
For circuit descriptions and diagrams, (Refer to
Appropriate Wiring Information).
(1) If both mirror heaters are inoperative, check
for proper operation of the Rear Window Defogger
System. (Refer to 8 - ELECTRICAL/HEATED GLASS
- DIAGNOSIS AND TESTING - REAR WINDOW
DEFOGGER SYSTEM). If Rear Window Defogger
System operates correctly, or if only one mirror
heater is inoperative, go to Step 2.
(2) Disconnect and isolate the battery negative
cable. Remove the front door trim panel on the side
of the inoperative mirror heater. Go to Step 3.
(3) Disconnect the door wire harness connector
from the door module connector receptacle. Check for
continuity between the mirror heater 12 volt supply,
and the mirror heater ground. There should be con-
tinuity. If OK, go to Step 4. If not OK, check for con-
tinuity of the individual circuits between the power
mirror and the door module, and of the mirror heater
grid right at the power mirror.
(4) Use a DRB IIItand (Refer to Appropriate
Diagnostic Information) to test the door module and
the PCI data bus.
8G - 8 HEATED MIRRORSWJ

these electronic modules or of the PCI data bus net-
work, the use of a DRBtscan tool and the proper
Diagnostic Procedures manual are recommended.
The electronic modules that may affect heated seat
system operation are as follows:
²Body Control Module (BCM)- Refer toBody
Control Modulein Electronic Control Modules for
more information.
²Heated Seat Module (HSM)- Refer toHeated
Seat Modulein Electronic Control Modules for more
information.
²Memory Heated Seat Module (MHSM)-If
the vehicle is equipped with the Memory System,
refer toMemory Seat Modulein Electronic Control
Modules for more information.
Refer toPower Seats Premium I/IIIin the Con-
tents of Wiring Diagrams for complete circuit dia-
grams. Following are general descriptions of the
major components in the heated seat system.
OPERATION
The heated seat system will only operate when the
ignition switch is in the On position, and the surface
temperature at the front seat heating element sen-
sors is below the designed temperature set points of
the system. The heated seat system will not operate
in ambient temperatures greater than about 41É C
(105É F). The front seat heating elements and sensors
are hard wired to the Heated Seat Module (HSM) or
the Memory Heated Seat Module (MHSM).
The heated seat switches are hard wired to the
Body Control Module (BCM). The BCM monitors the
heated seat switch inputs, then sends heated seat
switch status messages to the HSM or MHSM over
the Programmable Communications Interface (PCI)
data bus. The HSM or MHSM contains the control
logic for the heated seat system. The HSM or MHSM
responds to the heated seat switch status messages,
ignition switch status messages, and the front seat
heating element sensor inputs by controlling the out-
put to the front seat heating elements through inte-
gral solid-state relays.
When a seat heater is turned on, the sensor
located on the seat cushion electric heater element
provides the HSM or MHSM with an input indicating
the surface temperature of the seat cushion. If the
surface temperature input is below the temperature
set point for the selected Low or High heated seat
switch position, the HSM or MHSM energizes the
integral solid-state relay, which supplies battery cur-
rent to the heating elements in the seat cushion and
back. When the sensor input indicates the correct
temperature set point has been achieved, the HSM or
MHSM de-energizes the solid-state relay. The HSM
or MHSM will continue to cycle the solid-state relay
as needed to maintain the temperature set point.The HSM or MHSM and the seat heater elements
operate on non-switched battery current supplied
through the power seat circuit breaker in the junc-
tion block. However, the HSM or MHSM will auto-
matically turn off the heating elements if it detects
an open or short in the sensor circuit, a short or open
in the heating element circuit causing an excessive
current draw, or when the ignition switch is turned
to the Off position.
See the owner's manual in the vehicle glove box for
more information on the features, use and operation
of the heated seat system.
DIAGNOSIS AND TESTING - HEATED SEAT
SYSTEM
Following are tests that will help to diagnose the
components and circuits that are hard wired inputs
or outputs of the heated seat system. However, these
tests may not prove conclusive in the diagnosis of
this system. In order to obtain conclusive testing of
the heated seat system, the Programmable Commu-
nications Interface (PCI) data bus network and all of
the electronic modules that provide inputs to, or
receive outputs from the heated seat system compo-
nents must be checked.
The most reliable, efficient, and accurate means to
diagnose the heated seat system requires the use of a
DRB scan tool and the proper Diagnostic Procedures
manual. The DRBtscan tool can provide confirma-
tion that the PCI data bus is functional, that all of
the electronic modules are sending and receiving the
proper messages on the PCI data bus, and that the
Heated Seat Module (HSM) or Memory Heated Seat
Module (MHSM) is receiving the proper hard wired
inputs and relaying the proper hard wired outputs to
perform its heated seat system functions.
For complete circuit diagrams, refer toWiring
Diagrams.
NOTE: DO NOT ATTEMPT TO SWAP MEMORY OR
NON-MEMORY HEATED SEAT MODULES FROM
ONE VEHICLE TO ANOTHER. MOST OF THESE
MODULES ARE VEHICLE FEATURE SPECIFIC AND
THEREFORE NOT INTERCHANGEABLE. ALWAYS
USE THE CORRECT PART NUMBERED MODULE
WHEN DIAGNOSING OR REPLACING A MODULE.
WARNING: REFER TO THE RESTRAINTS SECTION
OF THIS MANUAL BEFORE ATTEMPTING ANY
STEERING WHEEL, STEERING COLUMN, OR
INSTRUMENT PANEL COMPONENT DIAGNOSIS OR
SERVICE. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
8G - 10 HEATED SEAT SYSTEMWJ
HEATED SEAT SYSTEM (Continued)

PRELIMINARY TEST
Before testing the individual components in the
heated seat system, check the following:
²If the heated seat switch LED indicators do not
light with the ignition switch in the On position and
the heated seat switch in the Low or High position,
check the fused ignition switch output (run) fuse in
the junction block. If OK, refer toHeated Seat
Switch Diagnosis and Testingin this section. If
not OK, repair the shorted circuit or component as
required and replace the faulty fuse.
²If the heated seat switch LED indicators light,
but the heating elements do not heat, check the
power seat circuit breaker in the junction block. If
OK, refer toHeated Seat Element Diagnosis and
Testingin this section of the manual. If not OK,
replace the faulty power seat circuit breaker.
DRIVER HEATED SEAT
SWITCH
DESCRIPTION
The heated seat switches are mounted in the
instrument panel center lower bezel (Fig. 2), which is
located near the bottom of the instrument panel cen-
ter stack. The two three-position rocker-type
switches, one switch for each front seat, provide a
resistor multiplexed signal to the Body Control Mod-
ule (BCM) through separate hard wired circuits.Each switch has an Off, Low, and High position so
that both the driver and the front seat passenger can
select a preferred seat heating mode. Each switch
has two Light-Emitting Diodes (LED), one each for
the Low position and the High position, which light
to indicate that the heater for the seat that the
switch controls is turned on. Each switch is also back
lit by a replaceable incandescent bulb.
The heated seat switches and their LEDs cannot
be repaired. If either switch or LED is faulty or dam-
aged, the entire switch unit must be replaced. The
incandescent switch illumination bulb and bulb
holder units are available for service replacement.
OPERATION
There are three positions that can be selected with
each of the heated seat switches: Off, Low, or High.
When the top of the switch rocker is fully depressed,
the High position is selected and the high position
LED indicator illuminates. When the bottom of the
switch rocker is fully depressed, the Low position is
selected and the low position LED indicator illumi-
nates. When the switch rocker is moved to its neutral
position, Off is selected and both LED indicators are
extinguished.
Both switches provide separate resistor multi-
plexed hard wire inputs to the BCM to indicate the
selected switch position. The BCM monitors the
switch inputs and sends heated seat switch status
messages to the Heated Seat Module (HSM) or the
Memory Heated Seat Module (MHSM) over the Pro-
grammable Communications Interface (PCI) data
bus. The HSM or MHSM responds to the heated seat
switch status messages by controlling the output to
the seat heater elements of the selected seat. The
Low heat position set point is about 36É C (97É F),
and the High heat position set point is about 41É C
(105É F).
DIAGNOSIS AND TESTING - DRIVER HEATED
SEAT SWITCH
For complete circuit diagrams, refer toWiring
Diagrams.
WARNING: REFER TO THE RESTRAINTS SECTION
OF THIS MANUAL BEFORE ATTEMPTING ANY
STEERING WHEEL, STEERING COLUMN, OR
INSTRUMENT PANEL COMPONENT DIAGNOSIS OR
SERVICE. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
(1) Check the fused ignition switch output (run)
fuse in the junction block. If OK, go to Step 2. If not
Fig. 2 HEATED SEAT SWITCHES
1 - HEATED SEAT SWITCHES
WJHEATED SEAT SYSTEM 8G - 11
HEATED SEAT SYSTEM (Continued)

OPERATION
There are three positions that can be selected with
each of the heated seat switches: Off, Low, or High.
When the top of the switch rocker is fully depressed,
the High position is selected and the high position
LED indicator illuminates. When the bottom of the
switch rocker is fully depressed, the Low position is
selected and the low position LED indicator illumi-
nates. When the switch rocker is moved to its neutral
position, Off is selected and both LED indicators are
extinguished.
Both switches provide separate resistor multi-
plexed hard wire inputs to the BCM to indicate the
selected switch position. The BCM monitors the
switch inputs and sends heated seat switch status
messages to the Heated Seat Module (HSM) or the
Memory Heated Seat Module (MHSM) over the Pro-
grammable Communications Interface (PCI) data
bus. The HSM or MHSM responds to the heated seat
switch status messages by controlling the output to
the seat heater elements of the selected seat. The
Low heat position set point is about 36É C (97É F),
and the High heat position set point is about 41É C
(105É F).
DIAGNOSIS AND TESTING - PASSENGER
HEATED SEAT SWITCH
For complete circuit diagrams, refer toWiring
Diagrams.
WARNING: REFER TO THE RESTRAINTS SECTION
OF THIS MANUAL BEFORE ATTEMPTING ANY
STEERING WHEEL, STEERING COLUMN, OR
INSTRUMENT PANEL COMPONENT DIAGNOSIS OR
SERVICE. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
(1) Check the fused ignition switch output (run)
fuse in the junction block. If OK, go to Step 2. If not
OK, repair the shorted circuit or component as
required and replace the faulty fuse.
(2) Turn the ignition switch to the On position.
Check for battery voltage at the fused ignition switch
output (run) fuse in the junction block. If OK, go to
Step 3. If not OK, repair the open fused ignition
switch output (run) circuit to the ignition switch as
required.
(3) Disconnect and isolate the battery negative
cable. Remove the lower center bezel from the instru-
ment panel and disconnect the instrument panel wire
harness connectors from both heated seat switch con-
nector receptacles. Check for continuity between the
ground circuit cavity of the instrument panel wire
harness connector for the inoperative heated seatswitch(es) and a good ground. There should be conti-
nuity. If OK, go to Step 4. If not OK, repair the open
ground circuit to ground as required.
(4) Reconnect the battery negative cable. Turn the
ignition switch to the On position. Check for battery
voltage at the fused ignition switch output (run) cir-
cuit cavity of the instrument panel wire harness con-
nector for the inoperative heated seat switch(es). If
OK, turn the ignition switch to the Off position, dis-
connect and isolate the battery negative cable, and go
to Step 5. If not OK, repair the open fused ignition
switch output (run) circuit to the junction block fuse
as required.
(5) Test the heated seat switch(es) (Fig. 8) as
shown in the Heated Seat Switch Test chart. If OK,
go to Step 6. If not OK, replace the faulty heated seat
switch(es).
HEATED SEAT SWITCH TEST
SWITCH
POSITIONRESISTANCE
BETWEENRESISTANCE
(OHMS)
Off Pin1&6 55
Low Pin1&61430
High Pin1&6 365
All resistance values are  5%.
Fig. 8 Rear of Heated Seat Switch
1 - LEFT SHOWN (RIGHT TYPICAL)
2 - ILLUMINATION LAMP
3 - CONNECTOR RECEPTACLE
4 - HEATED SEAT SWITCH
8G - 16 HEATED SEAT SYSTEMWJ
PASSENGER HEATED SEAT SWITCH (Continued)

connector pin-out information and location views for
the various wire harness connectors, splices and
grounds. Following are general descriptions of the
remaining major components in the horn system.
OPERATION
The horn system is activated by a horn switch con-
cealed beneath the driver side airbag module trim
cover in the center of the steering wheel. Depressing
the center of the driver side airbag module trim cover
closes the horn switch. Closing the horn switch acti-
vates the horn relay. The activated horn relay then
switches the battery current needed to energize the
horns.
The BCM can also activate the horn system by
energizing the horn relay through a single hard
wired output circuit. The BCM energizes and de-en-
ergizes the horn relay in response to internal pro-
gramming as well as message inputs received over
the Programmable Communications Interface (PCI)
data bus network. The BCM can energize the horn
relay for a single chirp (RKE lock request), or for
extended operation (RKE panic mode and VTSS
alarm mode).
Refer to the owner's manual in the vehicle glove
box for more information on the features, use and
operation of the horn system.
DIAGNOSIS AND TESTING - HORN SYSTEM
In most cases, any problem involving continually
sounding horns can be quickly alleviated by removing
the horn relay from the Power Distribution Center
(PDC). Refer to Horn Relay for the proper removal
procedure. Refer to the appropriate wiring informa-
tion. The wiring information includes wiring dia-
grams, proper wire and connector repair procedures,
further details on wire harness routing and reten-
tion, as well as pin-out and location views for the
various wire harness connectors, splices and grounds.
WARNING: ON VEHICLES EQUIPPED WITH AIR-
BAGS, DISABLE THE AIRBAG SYSTEM BEFORE
ATTEMPTING ANY STEERING WHEEL, STEERING
COLUMN, OR INSTRUMENT PANEL COMPONENT
DIAGNOSIS OR SERVICE. DISCONNECT AND ISO-
LATE THE BATTERY NEGATIVE (GROUND) CABLE,
THEN WAIT TWO MINUTES FOR THE AIRBAG SYS-
TEM CAPACITOR TO DISCHARGE BEFORE PER-
FORMING FURTHER DIAGNOSIS OR SERVICE. THIS
IS THE ONLY SURE WAY TO DISABLE THE AIRBAG
SYSTEM. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
HORN SYSTEM DIAGNOSIS
CONDITION POSSIBLE CAUSES CORRECTION
BOTH HORNS
INOPERATIVE1. Faulty fuse. 1. Check the fuses in the Power Distribution
Center (PDC) and the Junction Block (JB).
Replace the fuse and repair the shorted circuit or
component, if required.
2. Faulty horn relay. 2. Refer to Horn Relay for the proper horn relay
diagnosis and testing procedures. Replace the
horn relay or repair the open horn relay circuit, if
required.
3. Faulty horn switch. 3. Refer to Horn Switch for the proper horn switch
diagnosis and testing procedures. Replace the
horn switch or repair the open horn switch circuit,
if required.
4. Faulty horns. 4. Refer to Horn for the proper horn diagnosis
and testing procedures. Replace the horns or
repair the open horn circuit, if required.
ONE HORN INOPERATIVE 1. Faulty horn. 1. Refer to Horn for the proper horn diagnosis
and testing procedures. Replace the horn or
repair the open horn circuit, if required.
8H - 2 HORNWJ
HORN SYSTEM (Continued)

OPERATION
The ElectroMechanical Instrument Cluster (EMIC)
is designed to allow the vehicle operator to monitor
the conditions of many of the vehicle components and
operating systems. The gauges and indicators in the
EMIC provide valuable information about the various
standard and optional powertrains, fuel and emis-
sions systems, cooling systems, lighting systems,
safety systems and many other convenience items.
The EMIC is installed in the instrument panel so
that all of these monitors can be easily viewed by the
vehicle operator when driving, while still allowing
relative ease of access for service. The microproces-
sor-based EMIC hardware and software uses various
inputs to control the gauges and indicators visible on
the face of the cluster. Some of these inputs are hard
wired, but most are in the form of electronic mes-
sages that are transmitted by other electronic mod-ules over the Programmable Communications
Interface (PCI) data bus network. (Refer to 8 -
ELECTRICAL/ELECTRONIC CONTROL MOD-
ULES/COMMUNICATION - OPERATION).
The EMIC microprocessor smooths the input data
using algorithms to provide gauge readings that are
accurate, stable and responsive to operating condi-
tions. These algorithms are designed to provide
gauge readings during normal operation that are con-
sistent with customer expectations. However, when
abnormal conditions exist, such as low or high bat-
tery voltage, low oil pressure or high coolant temper-
ature, the algorithm can drive the gauge pointer to
an extreme position and the microprocessor turns on
the Check Gauges indicator to provide a distinct
visual indication of a problem to the vehicle operator.
The instrument cluster circuitry also sends electronic
chime tone request messages over the PCI data bus
to the Body Control Module (BCM) when it monitors
Fig. 2 EMIC Gauges & Indicators
1 - BRAKE INDICATOR 15 - TRANSMISSION OVERTEMP INDICATOR
2 - REAR FOG LAMP INDICATOR 16 - PART TIME 4WD INDICATOR
3 - WATER-IN-FUEL INDICATOR 17 - CHECK GAUGES INDICATOR
4 - VOLTAGE GAUGE 18 - ENGINE TEMPERATURE GAUGE
5 - LEFT TURN INDICATOR 19 - ODOMETER/TRIP ODOMETER SWITCH BUTTON
6 - TACHOMETER 20 - ODOMETER/TRIP ODOMETER DISPLAY
7 - HIGH BEAM INDICATOR 21 - WAIT-TO-START INDICATOR
8 - AIRBAG INDICATOR 22 - OVERDRIVE-OFF INDICATOR
9 - SPEEDOMETER 23 - SEATBELT INDICATOR
10 - RIGHT TURN INDICATOR 24 - ABS INDICATOR
11 - OIL PRESSURE GAUGE 25 - FUEL GAUGE
12 - SKIS INDICATOR 26 - FRONT FOG LAMP INDICATOR
13 - MALFUNCTION INDICATOR LAMP (MIL) 27 - LOW FUEL INDICATOR
14 - CRUISE INDICATOR 28 - COOLANT LOW INDICATOR
8J - 4 INSTRUMENT CLUSTERWJ
INSTRUMENT CLUSTER (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)

The VFD is diagnosed using the EMIC self-diag-
nostic actuator test. (Refer to 8 - ELECTRICAL/IN-
STRUMENT CLUSTER - DIAGNOSIS AND
TESTING). Proper testing of the PCI data bus and
the data bus message inputs to the EMIC that con-
trol the VFD functions requires the use of a DRBIIIt
scan tool. Refer to the appropriate diagnostic infor-
mation. Specific operation details for the odometer
and trip odometer functions of the VFD may be found
elsewhere in this service information.
INDICATORS
Indicators are located in various positions within
the EMIC and are all connected to the EMIC circuit
board. The turn signal indicators are hard wired. The
brake indicator is controlled by PCI data bus mes-
sages from the Controller Antilock Brake (CAB) as
well as by hard wired park brake switch and brake
fluid level switch inputs to the EMIC. The Malfunc-
tion Indicator Lamp (MIL) is normally controlled by
PCI data bus messages from the Powertrain Control
Module (PCM); however, if the EMIC loses PCI data
bus communication, the EMIC circuitry will automat-
ically turn the MIL on until PCI data bus communi-
cation is restored. The EMIC uses PCI data bus
messages from the Airbag Control Module (ACM), the
BCM, the PCM, the CAB, the Sentry Key Immobi-
lizer Module (SKIM), and the Transmission Control
Module (TCM) to control all of the remaining indica-
tors.
The various indicators are controlled by different
strategies; some receive fused ignition switch output
from the EMIC circuitry and have a switched ground,
others are grounded through the EMIC circuitry and
have a switched battery feed, while still others are
completely controlled by the EMIC microprocessor
based upon various hard wired and electronic mes-
sage inputs. Some indicators are illuminated at a
fixed intensity, while the illumination intensity of
others is synchronized with that of the EMIC general
illumination lamps.
The hard wired indicators are diagnosed using con-
ventional diagnostic methods. The EMIC and PCI
bus message controlled indicators are diagnosed
using the EMIC self-diagnostic actuator test. (Refer
to 8 - ELECTRICAL/INSTRUMENT CLUSTER -
DIAGNOSIS AND TESTING). Proper testing of the
PCI data bus and the electronic data bus message
inputs to the EMIC that control each indicator
require the use of a DRBIIItscan tool. Refer to the
appropriate diagnostic information. Specific details of
the operation for each indicator may be found else-
where in this service information.CLUSTER ILLUMINATION
Two types of general cluster illumination are avail-
able in this model. Base versions of the EMIC have
several incandescent illumination lamps, while pre-
mium versions of the EMIC have a single electro-lu-
minescent lamp. Both types of lamps provide cluster
back lighting whenever the exterior lighting is
turned On with the control knob on the left (lighting)
multi-function switch control stalk. The illumination
intensity of these lamps is adjusted by the EMIC
microprocessor based upon electronic dimming level
messages received from the Body Control Module
(BCM) over the PCI data bus. The BCM provides
electronic dimming level messages to the EMIC
based upon internal programming and inputs it
receives when the control ring on the left (lighting)
multi-function switch control stalk is rotated (down
to dim, up to brighten) to one of six available minor
detent positions.
The incandescent illumination lamps receive bat-
tery current at all times, while the ground for these
lamps is controlled by a 12-volt Pulse Width Modu-
lated (PWM) output of the EMIC electronic circuitry.
The illumination intensity of these bulbs and of the
vacuum-fluorescent electronic display are controlled
by the instrument cluster microprocessor based upon
dimming level messages received from the Body Con-
trol Module (BCM) over the PCI data bus. The BCM
uses inputs from the headlamp and panel dimmer
switches within the left (lighting) multi-function
switch control stalk and internal programming to
decide what dimming level message is required. The
BCM then sends the proper dimming level messages
to the EMIC over the PCI data bus.
The electro-luminescent lamp unit consists of lay-
ers of phosphor, carbon, idium tin oxide, and dielec-
tric applied by a silk-screen process between two
polyester membranes and includes a short pigtail
wire and connector. The lamp pigtail wire is con-
nected to a small connector receptacle on the EMIC
circuit board through a small clearance hole in the
cluster housing rear cover. The EMIC electronic cir-
cuitry also uses a PWM strategy to control the illu-
mination intensity of this lamp; however, the EMIC
powers this lamp with an Alternating Current (AC)
rated at 80 volts rms (root mean squared) and 415
Hertz, which excites the phosphor particles causing
them to luminesce.
The BCM also has several hard wired panel lamp
driver outputs and sends the proper panel lamps
dimming level messages over the PCI data bus to
coordinate the illumination intensity of all of the
instrument panel lighting and the VFDs of other
electronic modules on the PCI data bus. Vehicles
equipped with the Auto Headlamps option have an
automatic parade mode. In this mode, the BCM uses
8J - 6 INSTRUMENT CLUSTERWJ
INSTRUMENT CLUSTER (Continued)