Seat Temperature Element: Description and Operation
HEATED SEAT SYSTEMIndividually controlled, electrically heated front seats are available as a factory-installed option on some LH models. Vehicles with this option can be visually identified by the leather trim seats and the two separate heated seat switches mounted in the seat cushion side shields. The heated seat system allows the front seat driver and passenger to select from two different levels of electrical seat heating, or no seat heating to suit their individual comfort requirements. The heated seat system for this vehicle includes the following major components, which are described in further detail later in this section:
Heated Seat Elements - Four heated seat elements are used per vehicle, one for each front seat back and one for each front seat cushion. The heating elements are integral to the individual front seat and seat back trim covers and cannot be removed once installed at the factory. On some models, replacement seat heating elements/sensor are available, without having to replace the entire seat cushion or trim cover. Refer to heated seat element later in this section for additional information.
Memory Heated Seat/Mirror Module - also referred to as the MHSMM, this module contains the solid state electronic control and diagnostic logic circuitry for the heated seat and memory seat/mirror systems. One module is used per vehicle and is mounted under the drivers front seat cushion. Refer to the Electronic Control Modules section for additional memory heated seat/mirror module information.
Heated Seat Sensors - Two heated seat sensors are used per vehicle, one for each front seat cushion. The sensors are integral to the individual front seat cushion heating elements and cannot be removed once installed at the factory. Refer to heated seat sensor later in this section for additional information.
Heated Seat Switches - Two heated seat switches are used per vehicle. One switch is mounted in each of the front seat cushion side shields. Each switch includes two Light-Emitting Diode (LED) HI/LO indicator lamps and an incandescent back lighting bulb. Refer to the appropriate heated seat switch later in this section for additional information.
WARNING: SOME VEHICLES ARE EQUIPPED WITH SEATBACK MOUNTED AIRBAGS. BEFORE ATTEMPTING TO DIAGNOSE OR SERVICE ANY SEAT OR POWER SEAT SYSTEM COMPONENT YOU MUST FIRST DISCONNECT AND ISOLATE THE BATTERY NEGATIVE CABLE. THEN WAIT TWO MINUTES FOR THE SYSTEM CAPACITOR TO DISCHARGE BEFORE FURTHER SYSTEM SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO DO SO COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.
The heated seat system also relies upon resources shared with other components 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 reliability, enhanced diagnostics, and allows the addition of many new feature capabilities. For diagnosis of the heated seat system or of the PCI data bus network, the use of a DRB III scan tool and the proper Diagnostic Procedures manual are recommended.
Following are general descriptions and operations of the major components in the heated seat system. See the owner's manual in the vehicle glove box for more information on the features, use and operation of the heated seat system. Refer to Wiring for the location of complete heated seat system wiring diagrams.
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 are 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 heating elements, sensors and the heated seat switches are hard wired to the Memory Heated Seat/Mirror Module (MHSMM).
The MHSMM contains the control logic for the heated seat system. The module responds to the heated seat switch status, ignition switch status, and the 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 heated seat sensor located in the seat cushion heating element provides the module with an input (low-voltage) indicating the surface temperature of the seat cushion. If the surface temperature is below the temperature set point for the selected Low or High heated seat switch position, the module energizes the integral solid-state relay, which supplies battery current to the heating elements in the seat cushion and seat back. When the sensor input indicates the correct temperature set point has been achieved, the module de-energizes the solid-state relay The module will continue to cycle the solid-state relay as needed to maintain the temperature set point. 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).
The module and the heated seat elements operate on non-switched battery current supplied through the power seat circuit breaker in the junction block. The module will automatically turn off the heating elements if it detects an open or a HI/LOW short in the sensor circuit, or a open or LOW short in the heating element circuit.
DRIVER HEATED SEAT SWITCH
The heated seat switches are located on the outboard seat cushion side shield of the driver and passenger front seats.
Fig.1 Heated Seat Switch:
The momentary, bidirectional rocker-type heated seat switch provides a resistor-multiplexed signal to the Memory Heated Seat/Mirror Module (MHSMM) via a hard-wired mux circuit. Each switch has a center neutral position and momentary Low and High positions so that both the driver and the front seat passenger can select a preferred level of seat heating. Each heated seat switch contains two Light-Emitting Diode (LED) indicator lamps, which indicate the selected mode of the seat heater (HI/LO). Each switch also contains a incandescent bulb, which provides dimmer controlled back lighting of the switch when the headlamps or park lamps are on.
The two LED indicator lamps and the incandescent bulb in each heated seat switch cannot be repaired or replaced. If the LED indicator lamps or back lighting bulb are faulty or damaged, the complete heated seat switch must be replaced.
The heated seat switches receive battery current through a fused ignition switch output (run) circuit when the ignition switch is in the On position. Depressing the heated seat switch rocker to its momentary High or Low position provides a hardwired resistance signal to the Memory Heated Seat/Mirror Module (MHSMM). This signal tells the module to energize the heated seat element of the selected seat and maintain the requested temperature setting. If the heated seat switch is depressed to a different position (LO or HI) than the currently selected state, the module will change states to support the new selection. If a heated seat switch is depressed a second time, the module interprets the second input as a request to turn the seat heater "OFF".
The HI and LO LED indicator lamps in the heated seat switches receive battery current through a fused ignition switch output (run) circuit when the ignition switch is in the On position. The ground side of each indicator lamp is controlled by the memory heated seat/mirror module. One side of the incandescent back lighting bulb is connected to ground at all times. The other side of the incandescent bulb is connected to the fused panel lamps dimmer switch signal circuit. These bulbs are energized when the park lamps or headlamps are turned on, and their illumination intensity is controlled by the panel lamps dimmer switch.
PASSENGER HEATED SEAT SWITCH
The heated seat switches are located on the outboard seat cushion side shield of the driver and passenger front seats.
Fig.4 Heated Seat Switch:
The momentary, bidirectional rocker-type heated seat switch provides a resistor-multiplexed signal to the Memory Heated Seat/Mirror Module (MHSMM) via a hard-wired mux circuit. Each switch has a center neutral position and momentary Low and High positions so that both the driver and the front seat passenger can select a preferred level of seat heating. Each heated seat switch contains two Light-Emitting Diode (LED) indicator lamps, which indicate the selected mode of the seat heater (HI/LO). Each switch also contains a incandescent bulb, which provides dimmer controlled back lighting of the switch when the headlamps or park lamps are on.
The two LED indicator lamps and the incandescent bulb in each heated seat switch cannot be repaired or replaced. If the LED indicator lamps or back lighting bulb are faulty or damaged, the complete heated seat switch must be replaced.
The heated seat switches receive battery current through a fused ignition switch output (run) circuit when the ignition switch is in the On position. Depressing the heated seat switch rocker to its momentary High or Low position provides a hardwired resistance signal to the Memory Heated Seat/Mirror Module (MHSMM). This signal tells the module to energize the heated seat element of the selected seat and maintain the requested temperature setting. If the heated seat switch is depressed to a different position (LO or HI) than the currently selected state, the module will change states to support the new selection. If a heated seat switch is depressed a second time, the module interprets the second input as a request to turn the seat heater "OFF".
The HI and LO LED indicator lamps in the heated seat switches receive battery current through a fused ignition switch output (run) circuit when the ignition switch is in the On position. The ground side of each indicator lamp is controlled by the memory heated seat/mirror module. One side of the incandescent back lighting bulb is connected to ground at all times. The other side of the incandescent bulb is connected to the fused panel lamps dimmer switch signal circuit. These bulbs are energized when the park lamps or headlamps are turned on, and their illumination intensity is controlled by the panel lamps dimmer switch.
Fig.7 Heated Seat Element - Typical:
HEATED SEAT ELEMENT
The heated seat element is the component which produces the heat felt in the front seats The heated seat system includes four seat heating elements. Two elements are located in each front seat, one for the seat cushion and the other for the seat back.
These individual heating elements are electrically connected in series with the Memory Heated Seat/ Mirror Module. With the elements electrically connected in this way if a break anywhere in the heating element circuit occurs, neither the seat cushion or seat back heating element will function. The heating elements are made of carbon fiber, which provides a strong and dependable design. Refer to Heated Seat Element Diagnosing and Testing for detailed diagnostic procedures.
A heated seat sensor is integral to the individual heated seat cushion elements. The heated seat sensor cannot be replaced separately If a problem is found with the sensor the complete heated seat element must be replaced. On some models, this can also include the leather trim cover. Refer to Heated Seat Sensor in this section for more information.
Some LH models have the heating elements stuck to the seat cushion foam, while others have the element permanently sewn into the seat cushion/back leather trim covers. The heated seat elements and temperature sensor cannot be adjusted or repaired and, if faulty or damaged, the heated seat element must be replaced.
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 is released in the form of heat. The heated seat temperature sensor is a NTC thermistor. When the temperature of the seat cushion cover rises, the resistance of the sensor decreases. The Memory Heated Seat/Mirror Module supplies five volts to one side of each sensor, and monitors the voltage drop through the sensor on a signal/return circuit. The Memory Heated Seat/Mirror Module uses this temperature sensor input to monitor the temperature of the seat, and regulates the current flow to the heated seat elements accordingly.
HEATED SEAT SENSOR
Two heated seat sensors are used per vehicle. One is located in each front seat cushion heating element. Refer to Heated Seat Element in this section for more information. The heated seat temperature sensors are Negative Temperature Coefficient (NTC) thermistors that are permanently attached to the individual cushion heating elements.
The heated seat sensors cannot be repaired or adjusted and if found to be faulty, the complete heated seat cushion element must be replaced.
When the temperature of the heated seat element rises, the resistance of the heated seat sensor decreases. The memory heated seat/mirror module supplies five volts to one side of each sensor, and monitors the voltage drop through the sensor on a return circuit. The module uses this temperature sensor input to monitor the temperature of the seat, and regulates the current flow to the seat heating elements accordingly.