Grand Cherokee 4WD Laredo V8-4.7L (2002)
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 current 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
junction block. However, the HSM or MHSM will automatically 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.
Fig.1 Heated Seat Switches
DRIVER HEATED SEAT SWITCH
The heated seat switches are mounted in the instrument panel center lower bezel, which is located near the bottom of the instrument panel center
stack. The two three-position 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 damaged, the entire switch unit must be replaced.
The incandescent switch illumination bulb and bulb holder units are available for service replacement.
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 illuminates. When the switch rocker is moved to its neutral position, OFF is
selected and both LED indicators are extinguished.
Both switches provide separate resistor multiplexed 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 Programmable 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).
