2003 JEEP GRAND CHEROKEE BCM wiring

repaired and, if faulty or damaged, the entire a/c
heater control must be replaced. (Refer to 24 - HEAT-
ING & AIR CONDITIONING/CONTROLS/A/C
HEATER CONTROL - REMOVAL)
OPERATION
When the rear window defogger switch push but-
ton is depressed, it momentarily closes the rear win-
dow defogger switch sense circuit for the BCM to
ground. The BCM monitors the rear window defogger
switch sense circuit. Each time the BCM rear win-
dow defogger timer and logic circuitry sees another
input from the switch, it toggles a control output to
the rear window defogger relay. Energizing the rear
window defogger relay provides electrical current to
the rear window defogger grid and to the LED indi-
cator in the switch, which lights to indicate when the
defogger system is turned on. A dedicated fuse in the
junction block protects the rear window defogger
relay output circuit to the LED indicator.
DIAGNOSIS AND TESTING - REAR WINDOW
DEFOGGER SWITCH
For complete circuit diagrams, refer to the Appro-
priate Wiring Information.
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 AN ACCIDENTAL
AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
(1) Disconnect and isolate the battery negative
cable. Remove the a/c heater control from the instru-
ment panel and disconnect the 11-way (manual tem-
perature control) or 16-way (automatic zone control)
instrument panel wire harness connector from the a/c
heater control receptacle.
(2) Check for continuity between the ground cir-
cuit cavity of the 11-way or 16-way instrument panel
wire harness connector for the a/c heater control and
a good ground. There should be continuity. If OK, go
to Step 3. If not OK, repair the open ground circuit to
ground as required.
(3) Connect two jumper wires to the a/c heater
control 11-way or 16-way connector receptacle. Con-
nect one jumper from the ground circuit terminal inthe 11-way or 16-way a/c heater control connector
receptacle to a good ground. Connect the other
jumper from the fused rear window defogger relay
output circuit terminal of the 11-way or 16-way con-
nector receptacle to a 12-volt battery feed. The rear
window defogger switch LED indicator should light.
If OK, go to Step 4. If not OK, replace the faulty a/c
heater control.
(4) Check for continuity between the ground cir-
cuit and rear window defogger switch sense circuit
terminals of the 11-way or 16-way a/c heater control
connector receptacle. There should be momentary
continuity as the rear window defogger switch push
button is depressed, and then no continuity. If OK, go
to Step 5. If not OK, replace the faulty a/c heater
control.
(5) Disconnect the 22-way instrument panel wire
harness connector from the Body Control Module
(BCM) connector receptacle. Check for continuity
between the rear window defogger switch sense cir-
cuit cavity of the 11-way or 16-way instrument panel
wire harness connector for the a/c heater control and
a good ground. There should be no continuity. If OK,
go to Step 6. If not OK, repair the shorted rear win-
dow defogger switch sense circuit as required.
(6) Check for continuity between the rear window
defogger switch sense circuit cavities of the 11-way or
16-way instrument panel wire harness connector for
the a/c heater control and the 22-way instrument
panel wire harness connector for the BCM. There
should be continuity. If OK, refer to (Refer to 8 -
ELECTRICAL/HEATED GLASS/REAR WINDOW
DEFOGGER RELAY - DIAGNOSIS AND TESTING).
REMOVAL
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 AN ACCIDENTAL
AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
(Refer to 24 - HEATING & AIR CONDITIONING/
CONTROLS/A/C HEATER CONTROL - REMOVAL)
WJHEATED GLASS 8G - 7
REAR WINDOW DEFOGGER SWITCH (Continued)

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)

HEATED SEAT SENSOR
DESCRIPTION
The heated seat temperature sensor is a Negative
Temperature Coefficient (NTC) thermistor. One tem-
perature sensor is used for each seat. This tempera-
ture sensor is located in the seat cushion heating
element on all models.
The heated seat temperature sensor cannot be
repaired or adjusted and must be replaced if defec-
tive. The heated seat cushion element must be
replaced if the temperature sensor is defective. Refer
to the procedure in this section of the service man-
ual.
DIAGNOSIS AND TESTING - HEATED SEAT
SENSOR
For complete circuit diagrams, refer toWiring
Diagrams.
(1) Disconnect the Heated Seat Module (HSM) or
Memory Heated Seat Module (MHSM) C1 connector.
Check for continuity between the SEAT SENSOR 5V
SUPPLY circuit cavity and the seat cushion frame.
There should beNOcontinuity. If OK, go to Step 2.
If not OK, repair the shorted seat sensor 5V supply
circuit as required.
(2) Check for continuity between the SEAT SEN-
SOR 5V SUPPLY circuit cavity of the 4-way power
seat wire harness connector and the C1 connector for
the HSM or MHSM. There should be continuity. If
OK, go to Step 3. If not OK, repair the open seat sen-
sor 5V supply circuit as required.
(3) Connect the module electrical connector. Turn
system ON, using a voltmeter, backprobe the HSM or
MHSM C1 connector on the SEAT TEMPERATURE
SENSOR INPUT circuit cavity. Voltage should be
present (2-4 volts). If OK, proceed with testing the
other components in the heated seat system. If not
OK, refer to Heated Seat Module Diagnosis and Test-
ing in the electronic control modules section of this
manual.
REMOVAL
(1) For heated seat sensor replacement procedure
(Refer to 8 - ELECTRICAL/HEATED SEATS/
HEATED SEAT ELEMENT - REMOVAL).
PASSENGER HEATED SEAT
SWITCH
DESCRIPTION
The heated seat switches are mounted in the
instrument panel center lower bezel (Fig. 7), 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.
Fig. 7 HEATED SEAT SWITCHES
1 - HEATED SEAT SWITCHES
WJHEATED SEAT SYSTEM 8G - 15

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)

HORN
TABLE OF CONTENTS
page page
HORN SYSTEM
DESCRIPTION..........................1
OPERATION............................2
DIAGNOSIS AND TESTING - HORN SYSTEM . . . 2
HORN
DESCRIPTION..........................3
OPERATION............................3
DIAGNOSIS AND TESTING - HORN..........3
REMOVAL.............................3
INSTALLATION..........................4
HORN RELAY
DESCRIPTION..........................4OPERATION............................4
DIAGNOSIS AND TESTING - HORN RELAY....4
REMOVAL.............................5
INSTALLATION..........................5
HORN SWITCH
DESCRIPTION..........................6
OPERATION............................6
DIAGNOSIS AND TESTING - HORN SWITCH . . . 6
REMOVAL.............................7
INSTALLATION..........................7
HORN SYSTEM
DESCRIPTION
A dual-note electric horn system is standard facto-
ry-installed equipment on this model. The standard
equipment horn system features one low-note horn
unit and one high-note horn unit. The horn system
allows the vehicle operator to provide an audible
warning of the presence or approach of the vehicle to
pedestrians and the drivers of other vehicles in near
proximity. The horn system uses a non-switched
source of battery current so that the system will
remain functional, regardless of the ignition switch
position.
The horn system can also be activated by the Body
Control Module (BCM). The BCM is programmed to
activate the horns in order to provide the following
features:
²Remote Keyless Entry (RKE) system lock
request audible verification (except export)
²RKE system panic mode audible alert
²Vehicle Theft Security System (VTSS) audible
alarm.
This vehicle also offers several customer program-
mable features, which allows the selection of several
optional electronic features to suit individual prefer-
ences. Refer to Overhead Console for more informa-
tion on the customer programmable feature options.
Customer programmable feature options affecting the
horn system include:
²Sound Horn on Lock- Allows the option of
having the horn sound a short chirp as an audible
verification that the RKE system received a valid
Lock request from the RKE transmitter, or having no
audible verification.The horn system includes the following compo-
nents:
²Clockspring
²Horns
²Horn relay
²Horn switch
Certain functions and features of the horn system
rely upon resources shared with other electronic
modules in the vehicle over the Programmable Com-
munications Interface (PCI) data bus network. The
PCI data bus network allows the sharing of sensor
information. This helps to reduce wire harness com-
plexity, internal controller hardware, and component
sensor current loads. At the same time, this system
provides increased reliability, enhanced diagnostics,
and allows the addition of many new feature capabil-
ities. For diagnosis of these electronic modules or of
the PCI data bus network, the use of a DRB scan
tool and the proper Diagnostic Procedures manual
are recommended.
The other electronic modules that may affect horn
system operation are as follows:
²Body Control Module (BCM)(Refer to 8 -
ELECTRICAL/ELECTRONIC CONTROL MOD-
ULES/BODY CONTROL/CENTRAL TIMER MODUL
- DESCRIPTION) for more information.
²Electronic Vehicle Information Center
(EVIC)(Refer to 8 - ELECTRICAL/OVERHEAD
CONSOLE/ELECTRONIC VEHICLE INFO CENTER
- DESCRIPTION) for more information.
(Refer to 8 - ELECTRICAL/RESTRAINTS/CLOCK-
SPRING - DESCRIPTION) for more information on
this component. Refer to the appropriate wiring
information. The wiring information includes wiring
diagrams, proper wire and connector repair proce-
dures, details of wire harness routing and retention,
WJHORN 8H - 1

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)