Jeep Grand Cherokee WJ. Manual - part 145

HEATED SEAT SYSTEM

TABLE OF CONTENTS

page

DESCRIPTION AND OPERATION

HEATED SEAT ELEMENT AND SENSOR . . . . . . . 10
HEATED SEAT MODULE . . . . . . . . . . . . . . . . . . . . 9
HEATED SEAT SWITCH. . . . . . . . . . . . . . . . . . . . . 9
HEATED SEAT SYSTEM . . . . . . . . . . . . . . . . . . . . 8

DIAGNOSIS AND TESTING

HEATED SEAT ELEMENT . . . . . . . . . . . . . . . . . . 12

HEATED SEAT MODULE . . . . . . . . . . . . . . . . . . . 12
HEATED SEAT SENSOR . . . . . . . . . . . . . . . . . . . 14
HEATED SEAT SWITCH . . . . . . . . . . . . . . . . . . . . 10
HEATED SEAT SYSTEM . . . . . . . . . . . . . . . . . . . 10

REMOVAL AND INSTALLATION

HEATED SEAT MODULE . . . . . . . . . . . . . . . . . . . 14
HEATED SEAT SWITCH . . . . . . . . . . . . . . . . . . . . 14

DESCRIPTION AND OPERATION

HEATED SEAT SYSTEM

DESCRIPTION

Individually controlled driver and passenger side

electrically heated front seats are available factory-
installed optional equipment on this model, when it
is also equipped with the power seat option. The
heated seat system allows both the driver and the
front seat passenger the option to select one of two
seat heating ranges, Low or High, or to turn the indi-
vidual seat heaters Off using the heated seat
switches located in the center lower bezel near the
bottom of the instrument panel center stack. The
heated seat switch circuit operates on ignition
switched battery current supplied through a fuse in
the junction block, only when the ignition switch is
in the On position.

This group covers the following components of the

heated seat system:

• Heated seat elements and sensors

• Heated seat module (or memory heated seat

module)

• Heated seat switches.
The heated seat system also relies upon resources

shared with other electronic modules in the vehicle
over the Programmable Communications Interface
(PCI) data bus network. The PCI data bus network
allows the sharing of sensor information. This helps
to reduce wire harness complexity, internal controller
hardware, and component sensor current loads. At
the same time, this system provides increased reli-
ability, enhanced diagnostics, and allows the addition
of many new feature capabilities. For diagnosis of
these electronic modules or of the PCI data bus net-
work, the use of a DRB scan tool and the proper
Diagnostic Procedures manual are recommended.

The electronic modules that may affect heated seat

system operation are as follows:

• Body Control Module (BCM) - Refer to Body

Control Module in the Description and Operation
section of Group 8E - Instrument Panel Systems for
more information.

• Heated Seat Module (HSM) - Refer to Heated

Seat Module in the Description and Operation sec-
tion of this group for more information.

• Memory Heated Seat Module (MHSM) - If

the vehicle is equipped with the Memory System,
refer to Memory Seat Module in the Memory Sys-
tem section of Group 8R - Power Seat Systems for
more information.

Refer to Power Seats Premium I/III in the Con-

tents of Group 8W - Wiring Diagrams for complete
circuit diagrams. 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 41° C
(105° 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

8N - 8

ELECTRICALLY HEATED SYSTEMS

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 in the sensor circuit, a short in the heating
element circuit causing an excessive current draw, or
when the ignition switch is turned to the Off posi-
tion.

See the owner’s manual in the vehicle glove box for

more information on the features, use and operation
of the heated seat system.

HEATED SEAT SWITCH

DESCRIPTION

The heated seat switches are mounted in the cen-

ter lower bezel, which is located near the bottom of
the instrument panel center stack. The two three-po-
sition rocker-type switches, one switch for each front
seat, provide a resistor multiplexed signal to the
Body Control Module (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 36° C (97° F),
and the High heat position set point is about 41° C
(105° F).

HEATED SEAT MODULE

DESCRIPTION

There are two different electronic modules that can

be used in the optional heated seat system. The
Heated Seat Module (HSM) is used on vehicles that
are not equipped with the optional Memory System.
The Memory Heated Seat Module (MHSM) is used on
vehicles that are equipped with the optional Memory
System and the optional heated seat system. A third
electronic module, the Memory Seat Module (MSM),
is used on vehicles equipped with the Memory Sys-
tem without the optional heated seat system. Refer
to Memory System in the Memory System section
of Group 8R - Power Seat Systems for more informa-
tion on the memory system option.

All three modules are packaged in an identical

molded plastic housing which is mounted on a
bracket that is located between the power seat track
and the seat cushion frame under the forward edge
of the driver side front seat cushion. The HSM or
MHSM is used to control all of the heated seat sys-
tem functions for both front seats. The HSM or
MHSM contains a central processing unit and inter-
faces with other electronic modules in the vehicle on
the Programmable Communications Interface (PCI)
data bus network.

For diagnosis of the HSM, MHSM or the PCI data

bus, a DRB scan tool and the proper Diagnostic Pro-
cedures manual are recommended. The HSM or
MHSM cannot be repaired and, if faulty or damaged,
it must be replaced.

OPERATION

The HSM or MHSM receives message inputs from

the BCM over the PCI data bus and hard wired
inputs from the front seat heating element sensors
through the element return circuits. The program-

ELECTRICALLY HEATED SYSTEMS

8N - 9

DESCRIPTION AND OPERATION (Continued)

ming in the HSM or MHSM allows it to process the
information from these inputs and send control out-
puts to the integral solid state relays to regulate the
flow of battery current and the temperature of the
front seat heating elements.

HEATED SEAT ELEMENT AND SENSOR

DESCRIPTION

The heated seat system includes two seat heating

elements in each front seat, one for the seat cushion
and the other for the seat back. Two types of heated
seat elements are offered. All Laredo models use two
resistor wire heating elements for each seat that are
connected in series with the Heated Seat Module
(HSM). All Limited models use two carbon fiber mesh
heating elements for each seat that are connected in
parallel with the Memory Heated Seat Module
(MHSM). The temperature sensor is a Negative Tem-
perature Coefficient (NTC) thermistor. One tempera-
ture sensor is used for each seat, and it is located on
the seat cushion heating element for all models.

The seat heating elements are sewn into the seat

cushion cover trim and seat back cover trim units.
The heated seat elements and the temperature sen-
sor cannot be adjusted or repaired and, if faulty or
damaged, the seat cushion cover trim unit or seat
back cover trim unit must be replaced. Refer to
Bucket Seat Cushion Cover or Bucket Seat
Back Cover in the Removal and Installation section
of Group 23 - Body for the seat cushion cover trim
and seat back cover trim service procedures.

OPERATION

The heated seat elements resist the flow of electri-

cal current. When battery current is passed through
the elements, the energy lost by the resistance of the
elements to the current flow is released in the form
of heat. The temperature sensor is a NTC thermistor.
When the temperature of the seat cushion cover
rises, the resistance of the sensor decreases. The
HSM or MHSM supplies a five-volt current to one
side of each sensor, and monitors the voltage drop
through the sensor on a return circuit. The MSM or
MHSM uses this temperature sensor input to moni-
tor the temperature of the seat, and regulates the
current flow to the seat heating elements accordingly.

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 DRB scan tool can provide confirmation
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 to Power

Seats Premium I/III in the Contents of Group 8W -
Wiring Diagrams.

WARNING: ON VEHICLES EQUIPPED WITH AIR-
BAGS,

REFER

GROUP

PASSIVE

RESTRAINT SYSTEMS BEFORE ATTEMPTING ANY
STEERING

WHEEL,

STEERING

COLUMN,

INSTRUMENT PANEL COMPONENT DIAGNOSIS OR
SERVICE. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.

PRELIMINARY TEST

Before testing the individual components in the

heated seat system, check the following:

• If 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 to Heated Seat
Switch in the Diagnosis and Testing section of this
group. If not OK, repair the shorted circuit or compo-
nent as required and replace the faulty fuse.

• If 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 to Heated Seat Element in the Diagnosis
and Testing section of this group. If not OK, replace
the faulty power seat circuit breaker.

HEATED SEAT SWITCH

For complete circuit diagrams, refer to Power

Seats Premium I/III in the Contents of Group 8W -
Wiring Diagrams.

8N - 10

ELECTRICALLY HEATED SYSTEMS

DESCRIPTION AND OPERATION (Continued)

WARNING: ON VEHICLES EQUIPPED WITH AIR-
BAGS,

REFER

GROUP

PASSIVE

RESTRAINT SYSTEMS BEFORE ATTEMPTING ANY
STEERING

WHEEL,

STEERING

COLUMN,

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 seat
switch(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. 1) 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).

(6) Disconnect the 22-way instrument panel wire

harness connector from the Body Control Module
(BCM) connector receptacle. Check for continuity
between the seat heater switch sensor ground circuit
cavity of the instrument panel wire harness connec-
tor for the inoperative heated seat switch(es) and a
good ground. There should be no continuity. If OK, go
to Step 7. If not OK, repair the shorted seat heater
switch sensor ground circuit as required.

(7) Check for continuity between the seat heater

switch sensor ground circuit cavities of the instru-
ment panel wire harness connector for the in opera-
tive

heated

seat

switch(es)

and

the

22-way

instrument panel wire harness connector for the
BCM. There should be continuity. If OK, go to Step 8.
If not OK, repair the open seat heater switch sensor
ground circuit as required.

(8) Check for continuity between the seat heater

switch mux circuit cavity of the instrument panel
wire harness connector for the inoperative heated
seat switch and a good ground. There should be no
continuity. If OK, go to Step 9. If not OK, repair the
shorted seat heater switch mux circuit as required.

(9) Check for continuity between the seat heater

switch mux circuit cavity of the instrument panel
wire harness connector for the inoperative heated
seat switch and the 22-way instrument panel wire
harness connector for the BCM. There should be con-
tinuity. If OK, use a DRB scan tool and the proper
Diagnostic Procedures manual to test the BCM. If
not OK, repair the open seat heater switch mux cir-
cuit as required.

Fig. 1 Heated Seat Switch

1 – LEFT SHOWN (RIGHT TYPICAL)
2 – ILLUMINATION LAMP
3 – CONNECTOR RECEPTACLE
4 – HEATED SEAT SWITCH

HEATED SEAT SWITCH TEST

SWITCH

POSITION

RESISTANCE

BETWEEN

RESISTANCE

(OHMS)

Off

Pin 1 & 6

Low

Pin 1 & 6

1430

High

Pin 1 & 6

365

All resistance values are

1%.

ELECTRICALLY HEATED SYSTEMS

8N - 11

DIAGNOSIS AND TESTING (Continued)

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