Seat Temperature Element: Description and Operation

Fig.1 Heated Seat Switches:

HEATED SEAT SYSTEM
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 individual seat heaters OFF using the heated seat switches located in the instrument panel center bezel near the bottom of the HVAC outlets. The heated seat switch circuit operates on ignition switched battery current supplied through a fuse in the intelligent power module, only when the ignition switch is in the ON position.

This covers the following components of the heated seat system:
- Heated seat elements
- Heated seat sensors
- Heated seat switches

The Heated seat module (or memory heated seat module) can be found in the Electronic Control Modules.

The heated seat system also relies upon resources shared with other electronic modules in the vehicle over the Programmable Communications Interface (PCI) J1850 data bus network. The J1850 (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 reliability, enhanced diagnostics, and allows the addition of many new feature capabilities. For diagnosis of these electronic modules or of the J1850 (PCI) data bus network, the use of a DRB III scan tool, or equivalent and the proper Diagnostic Procedures 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 Electronic Control Modules for more information.
- Remote Keyless Entry Module (RKE) - Refer to Remote Keyless Entry Module in Power Locks for more information.
- Electronic Vehicle Information Center (EVIC) - Refer to Electronic Vehicle Information Center in Overhead Console for more information.
- Heated Seat Module (HSM) - Refer to Heated Seat Module in Electronic Control Modules for more information.
- Memory Heated Seat Module (MHSM) - If the vehicle is equipped with the Memory System, refer to Memory Seat Mirror Module (MSMM) in Electronic Control Modules for more information.

Following are general descriptions of the major components in the heated seat system.

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 sensors 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 J1850 (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 output to the front seat heating elements through integral 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 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 fuse in the intelligent power module. However, the HSM or MHSM will automatically 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 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 bezel. The two three-position rocker-type switches, one switch for each front seat, are incorporated into one large switch assembly that also includes the hazzard, rear window wiper and washer switches. The heated seat switches 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) which light to indicate that the heater for the seat is turned ON.

The heated seat switches and their LEDs cannot be repaired. If either switch or LED is faulty or damaged, the entire switch assembly must be replaced.

There are three positions that can be selected with each of the heated seat switches: OFF, Low, or High. When the left side of the switch rocker is fully depressed, the Low position is selected and the high position LED indicator illuminates. When the right side of the switch rocker is fully depressed, the High position is selected and the low position LED indicator illuminates. When the switch rocker is moved to its neutral position (middle), 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).

HEATED SEAT ELEMENT
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. The two elements for each seat that are connected in series with the Heated Seat Module (HSM). The temperature sensor is a Negative Temperature Coefficient (NTC) thermistor. One temperature 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 sensor cannot be adjusted or repaired and, if faulty or damaged, the seat cushion or seat back assembly must be replaced. Refer to Seat Cushion or Seat Back in the Removal and Installation for the seat cushion and seat back service procedures.

The heated seat elements resist the flow of electrical 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 monitor the temperature of the seat, and regulates the current flow to the seat heating elements accordingly.

HEATED SEAT SENSOR
The heated seat temperature sensor is a Negative Temperature Coefficient (NTC) thermistor. One temperature sensor is used for each seat, and is integrated into the seat cushion heating element.

Fig.1 Heated Seat Switches:

PASSENGER HEATED SEAT SWITCH
The heated seat switches are mounted in the instrument panel center bezel. The two three-position rocker-type switches, one switch for each front seat, are incorporated into one large switch assembly that also includes the hazzard, rear window wiper and washer switches. The heated seat switches 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) which light to indicate that the heater for the seat is turned ON.

The heated seat switches and their LEDs cannot be repaired. If either switch or LED is faulty or damaged, the entire switch assembly must be replaced.

There are three positions that can be selected with each of the heated seat switches: OFF, Low, or High. When the left side of the. switch rocker is fully depressed, the Low position is selected and the high position LED indicator illuminates. When the right side of the switch rocker is fully depressed, the High position is selected and the low position LED indicator illuminates. when the switch rocker is moved to its neutral position (middle), 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).