Seat Temperature Element: Description and Operation
HEATED SEAT SYSTEMIndividually controlled electrically heated front seats are available as an factory-installed option on this model. Vehicles with this option can be visually identified by the heated seat switch mounted in the center console. The heated seat system allows the front seat driver and passenger to select from six different levels of supplemental seat heating, or no seat heating to suit their individual comfort requirements. The heated seat system for this vehicle includes the following major components:
- Heated Seat Switch - One heated seat switch is used per vehicle. The switch is mounted in the center console, behind the gear selector and has two rotary knobs, one for the driver and one for the passenger front seats. Each rotary knob has six different detents (0-6), to indicate the desired level of seat heating. Zero (0) being off and 6 for the highest level of seat heating. There are three Light Emitting Diodes (LED's) in the heated seat switch. The first LED illuminates the heated seat symbol on the switch anytime the ignition switch is in the ON position. The remaining two LED's illuminate the selected numbers on the switch, indicating that the heated seat system is on and the level of seat heating selected.
- Heated Seat Module - Two heated seat modules are used per vehicle, these modules contain the solid state electronic control and diagnostic logic circuitry for the entire heated seat system. Each heated seat module is mounted under each of the front seat cushions. Refer to the Electronic Control Modules section of the service manual for additional heated seat module information.
- Heated Seat Elements - Four heated seat elements are used per vehicle, one in each front seat cushion and one in each front seat back. These elements, which are integral to the individual front seat cushion and front seat back trim covers are connected in series with the heated seat module.
- Heated Seat Sensor - Two heated seat sensors are used per vehicle, one for each front seat. The heated seat sensors are integrated into each of the seat cushion heating elements. The temperature sensor is a negative temperature coefficient thermistor.
- Low Voltage Cut-Out Relay - The 12 volt power to the heated seat elements is supplied through a relay that removes power from the heating elements when a low battery voltage is sensed by the powertrain control module (PCM). This micro-relay is located in cavity #8 in the Power Distribution Center (PDC).
HEATED SEAT MODULE
The heated seat module receives fused battery current through the low voltage cut-out relay in the Power Distribution Center (PDC). This relay is controlled by the Body Control Module (BCM) only when the ignition switch is in the run position. The heated seat switches receive battery current through a fused ignition switch output (run) circuit also when the ignition switch is in the Run position. The heated seat module shares a common ground circuit with each of the heated seat elements. The heated seat elements will only operate when the surface temperature of the seat cushion cover at the heated seat sensors is below the designed temperature set points of the system and the system voltage is above the cut-out level.
The heated seat module will automatically turn off the heated seat elements if it detects a short in the heated seat element circuit or a heated seat sensor value that is out of range. The heated seat system will remain ON anytime the ignition switch is turned to the ON position and the heated seat switch is at any of the #1 through #6 positions.
The heated seat module monitors inputs from the heated seat sensors and the heated seat switches. In response to these inputs the heated seat module uses its internal programming to control outputs to the heated seat elements in both front seats and to control the heated seat LED indicator lamps located in both of the heated seat switches.
HEATED SEAT ELEMENT
Vehicles equipped with the optional heated seat system have two sets of electrically operated heating element grids located in each outboard seating position of the front seat, one set for the seat cushion and the other set for the seat back. Each of the heated seat element grids consists of a single length of resistor wire that is routed in a zigzag pattern and captured between the leather trim cover and the foam rubber backing on the underside of its respective seat cushion trim cover and seat back trim cover assembly. Short pigtail wires with connectors are soldered to each end of each resistor wire element grid, which connect all of the element grids for each seating position to each other in series with the heated seat module through the seat wire harness.
One temperature sensor is used for each outboard seating position of the front seat, and it is located in the center insert area of the seat cushion cover. The heated seat sensors and their pigtail wires are also captured between the leather trim cover and the foam rubber backing on the underside of their respective seat cushion trim cover assemblies. The heated seat sensors are Negative Thermal Coefficient (NTC) thermistors. The sensors for both front seats receive a voltage feed from a single output of the heated seat module, but the module receives individual sensor inputs from the driver side and passenger side sensors.
The heated seat elements and sensors cannot be repaired. If damaged or faulty, the elements and sensors must be replaced.
One end of the heated seat element resistor wire is connected to a ground feed at all times through a splice in the heated seat module ground circuit. Battery current is directed to the other end of the heated seat element resistor wire by the energized N-channel Field Effect Transistor (N-FET) located within the heated seat module. The heated seat module will energize the N-FET only when the heated seat switch is in one of the six positions and the heated seat sensor indicates that the seat cushion surface temperature is below the selected (Low or High) temperature set point. As electrical current passes through the heating element grid, the resistance of the wire used in the element disperses some of that electrical current in the form of heat. The heat produced by the heated seat element grid then radiates through the underside of the seat cushion and seat back trim covers, warming the seat cover and its occupant.
HEATED SEAT SENSOR
The resistance of the heated seat sensor increases and decreases as the surface temperature of the seat cushion cover changes. The heated seat module supplies each sensor with a voltage feed, then detects the sensor resistance by monitoring the voltage of the separate sensor return circuits. The heated seat module compares the heated seat sensor resistance (seat cushion surface temperature) with the heated seat switch resistance (Low or High set point) to determine when the heated seat element grids need to be cycled on or off in order to maintain the selected temperature set point.
LOW VOLTAGE CUT-OUT RELAY
A low voltage cut-out relay controls the 12v battery feed to the heated seat elements. When the system voltage is above 12.2 volts the relay will supply voltage to the elements. When the PCM senses low system voltage, below 11.7 volts, a message will be sent to the BCM to turn off the low voltage relay to prevent battery discharge during normal operation. The power will be restored when the system voltage rises above 12.2 volts for 14 seconds and stays there. The cycle will repeat if the system voltage again drops below 11.7 volts.