ory. The seat heaters are controlled by the Memory
Heated Seat and Mirror Module (MHSMM). The
heaters will only operate when the ignition switch is
in the ON position, and the surface temperature at
the from seat heating element sensors is below the
designed temperature set points of the system.

The heater switch is a three position (HI, LO, and

neutral position OFF) momentary rocker switch.
Green LED’s illuminate the switch for night visibil-
ity. Amber LED’s are located in the switch adjacent
to the HI and LO graphics, which indicate the
selected operating mode of the seat heater, The LO
heat position set point is about 98° F and the HI set
point is about 107° F.

The heaters are solid high side driven, and are

controlled by an NTC circuit. When either HI or LO
is activated, the temperature in the seat rises the
Negative Temperature Coefficient (NTC) resistance
decreases, and the voltage reading detected by the
MHSMM increases. The seat heater output is deacti-
vated once the voltage reading reaches its upper
threshold for either of the settings, HI or LO.

As the temperature decreases the voltage reading

decreases until the lower threshold is reached for
either setting, HI or LO. At this point the seat heat
output is activated once again. This cycle repeats
itself as long as the seat heat request remains on.
The thresholds for high and low settings are pre-pro-
grammed into the MHSMM.

The system will automatically turn off when the

ignition is turned to the OFF position. For complete
circuit descriptions and diagrams, refer to Group 8W,
Wiring Diagrams.

DESCRIPTION AND OPERATION

HEATED SEAT SWITCH

The heated seat switch is located in the outboard

cushion side shield of the driver and passenger seats
(Fig. 1). The switch is a three position (HI, LO, and
OFF) momentary rocker switch. Green LED’s illumi-
nate the switch for night visibility. Amber LED’s are
located in the switch adjacent to the HI and LO
graphics, which indicate whether low or high has
been selected (Fig. 2). The heated seat switches and
their LED’s are not repairable. If either the switch or
LED is faulty, the switch will have to be replaced.

Fig. 1 Heated Seat Switch Location

300M

ELECTRICALLY HEATED SYSTEMS

8N - 1

MEMORY HEATED SEAT / MIRROR MODULE
(MHSMM)

The Memory Heated Seat and Mirror Module con-

trols the seat heaters, and is mounted unto the bot-
tom of the driver seat (Fig. 3). The module controls
the seat heaters and is connected to the seat harness
by four connectors. There is only one module per
memory and heated system. The module cannot be
repaired, and if faulty or damaged will have to be
replaced.

HEATED SEAT ELEMENT AND SENSOR

Two heated seat elements are used in both of the

front seats, one for the seat cushion and the other for
the seat back. The two elements for each seat are
connected in series to the MHSMM.

The temperature sensor is a Negative Temperature

Coefficient (NTC) thermistor. One temperature sen-
sor is used for each set, and it is integrated into the
seat cushion heating element.

The heating elements are sewn into the seat cush-

ion cover and seat back cover assemblies, which are
serviced individually. The heating elements and tem-
perature sensor cannot be repaired, and if faulty or
damaged, the affected seat cover must be replaced.
Refer to Group 23 Body, for the Seat Cushion Cover
and Seat Back Cover Removal and Installation.

DIAGNOSIS AND TESTING

HEATED SEAT SYSTEM

For circuit descriptions and diagrams, refer to

Group 8W, Wiring Diagrams.

Before testing the individual components in the

heated seat system, cheek the following:

• If the heated seat switch LED doesn’t light up

with the ignition in the ON position, and the switch
in the LO or HI position, check the battery voltage, If
the battery is below 10 volts, recharge the battery. If
the problem persists, then check the fuse in the junc-
tion block. If the fuse is OK, test the heated set
switch as described in this group. if not OK, repair
the shorted circuit or component failure and replace
the faulty fuse.

• If the heated seat LED’s light up, but the heat-

ing elements don’t heat, check the circuit breaker in
the junction block. If the circuit is OK, test the
heated set elements as described in this group, oth-
erwise replace the faulty circuit breaker.

HEATED SEAT SWITCH

For circuit description and diagrams, refer to

Group 8W Wiring Diagrams.

Inspect the Heated Seat Switches for apparent

damage or sticking/binding and replace as required.
Refer to Heated Seat Switch Removal and Installa-
tion.

Inspect the continuity between pin 4 and pin 6 of

the switch. Measure the resistance between the two
pins. Refer to (Fig. 4) and the Heated Seat Switch
Pin Call-Out table. If the readings don’t correspond
to those in the Heated Seat Switch Continuity table,
replace the heated seat switch.

MEMORY HEATED SEAT / MIRROR MODULE
(MHSMM)

Before testing the Memory Heated Seat / Mirror

Module (MHSMM), test the heated seat switches, the
heated seat elements, and the heated seat sensors. If
there are no problems, proceed as follows:

• Replace the current MHSMM with a known

good module, and test the operation of the heated
seats. If OK, discard the faulty MHSMM. If the prob-
lem persists, proceed.

Fig. 2 Memory / Heated Seat Switch

Fig. 3 Memory Heated Seat / Mirror Module

(MHSMM)

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ELECTRICALLY HEATED SYSTEMS

300M

DESCRIPTION AND OPERATION (Continued)

• Test each of the circuits from the heated seat

switch, elements, and sensors to the MHSMM.
Repair any shorts or open circuits as required.

HEATED SEAT ELEMENT

The wire harness connectors for the seat cushion

and seat back heating elements are located under the
seat, near the rear edge of the seat cushion frame.

SEAT CUSHION

(1) Unplug the 4–way heated seat cushion wire

harness connector.

(2) Check the continuity between the two heated

seat driver circuit cavities of the seat cushion cover
half of the heated seat cushion wire harness connec-

tor. If there is continuity, go to Step 3, otherwise
replace the faulty seat cushion cover.

(3) Check the continuity between one of the heated

seat driver circuit cavities of the seat cushion cover
half of the heated seat cushion wire harness connec-
tor and the seat cushion frame. If there isn’t any con-
tinuity, go to the Seat Back test, otherwise replace
the faulty seat cushion cover.

SEAT BACK

(1) Unplug the 2–way heated seat back wire har-

ness connector.

(2) Check the continuity between the heated seat

driver circuit cavity and the ground cavity of the seat
back cover half of the heated seat cushion wire har-
ness connector. If there is continuity go to Step 3,
otherwise replace the faulty seat cushion cover.

(3) Check the continuity between the heated seat

driver circuit cavity of the seat back cover half of the
heated seat cushion wire harness connector and the
seat back frame. If there isn’t any continuity, test the
heated seat sensor as described in this section, oth-
erwise replace the faulty seat cushion cover.

HEATED SEAT SENSOR

The wire harness connector for the seat cushion

heating element and sensor are located under the
seat, near the rear edge of the seat cushion frame.
For circuit descriptions and diagrams, refer to Group
8W, Wiring Diagrams.

(1) Unplug the 4–way heated seat cushion wire

harness connector.

(2) Using an ohmmeter, measure the resistance

between the heated seat switch output circuit cavity
and the ground circuit cavity of the seat cushion
cover half of the heated seat cushion wire harness
connector. The sensor resistance should be between 2
Kohms and 200 Kohms. If OK, test the MHSMM as
described in this section, otherwise replace the faulty
seat cushion cover.

REMOVAL AND INSTALLATION

HEATED SEAT SWITCH

REMOVAL

(1) Disconnect and isolate the negative battery

cable remote terminal from the remote battery post
(Fig. 5).

(2) Unscrew the side shield on the seat, either

driver or passenger side (Fig. 6).

(3) From the bottom, disengage the switch and

gently rock through the opening.

INSTALLATION

For installation, reverse the above procedures.

Fig. 4 Heated Seat Switch (Rear View)

HEATED SEAT SWITCH PIN CALL-OUT

FUNCTION

LO LED

OPEN

GROUND

IGNITION FEED

HI LED

REQUEST LINE

HEATED SEAT SWITCH CONTINUITY

CONTINUITY

BETWEEN

SWITCH

POSITION

READING

PIN 4 AND 6

OFF

2200 OHMS

PIN 4 AND 6

415 OHMS

PIN 4 AND 6

33 OHMS

300M

ELECTRICALLY HEATED SYSTEMS

8N - 3

DIAGNOSIS AND TESTING (Continued)

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