Seat Temperature Element: Description and Operation
HEATED SEAT SYSTEMIndividually controlled, electrically heated front seats are available factory-installed optional equipment on this model. Vehicles with this option can be visually identified by the two separate heated seat switches mounted in a single switch bezel located on the center floor console, directly behind the transmission gearshift selector lever opening. The heated seat system allows the front seat driver and passenger to select from two different levels of supplemental 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:
- Heated Seat Element - The heated seat elements are integral to the individual front seat cushion and front seat back trim covers.
- Heated Seat Module - A single heated seat module, also referred to as the Seat Heat Interface Module (SHIM), is mounted to the power seat track front bracket under the forward edge of the right front seat cushion.
- Heated Seat Relay - A single heated seat relay is mounted to the power seat track front bracket under the forward edge of the right front seat cushion.
- Heated Seat Sensor - A heated seat sensor is integral to each heated seat cushion trim cover.
- Heated Seat Switch - Two heated seat switches, one each for the driver side and passenger side front seats, are mounted in a single switch bezel located on the center floor console behind the gear- shift selector lever.
Hard wired circuitry connects the heated seat system components to each other through the electrical system of the vehicle. These hard wired circuits are integral to several wire harnesses, which are routed throughout the vehicle and retained by many different methods. These circuits may be connected to each other, to the vehicle electrical system and to the heated seat system components through the use of a combination of soldered splices, splice block connectors and many different types of wire harness terminal connectors and insulators.
The heated seat system is designed to provide individually controlled, supplemental heat to the cushion and back surfaces of both front seats. Because this system converts electrical current to heat, the heated seat system can provide a measure of warm comfort almost immediately upon entering a cold vehicle, rather than having to wait for the engine coolant to reach sufficient temperature to deliver heat through the conventional heater system. This system allows each front seat occupant to individually select one of two comfort levels, Hi or Lo, or to turn the heater for their seat OFF.
The heated seat system components operate on battery current received through a fuse in the Junction Block (JB) on a fused ignition switch output (run-acc) circuit so that the system will only operate when the ignition switch is in the ON or Accessory positions. The heated seat system will be turned OFF automatically whenever the ignition switch is turned to any position except ON or Accessory. Also, the heated seat system will not operate when the surface temperature of the seat cushion cover at either heated seat sensor is above the designed temperature set points of the system. The heated seat system has a self-diagnostic capability. When certain failures are detected within the heated seat system, the system will provide a visual indication of the failure by flashing the Light Emitting Diode (LED) indicator lamps located in the heated seat switches. 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 Switch:
LEFT HEATED SEAT SWITCH
Two heated seat switches, one each for the driver side and passenger side front seats, are mounted in a single switch bezel located on the center floor console behind the gearshift selector lever. On models with the heated seat system option, the heated seat switch bezel replaces the standard equipment ash receiver on the center floor console, and an available optional ash receiver is installed in one of the cup holders. The two switches are snapped into the mounting holes of the heated seat switch bezel, and the switch and bezel assembly snaps into the mounting hole of the center floor console. The two switches are identical in appearance and construction, but the power seat wire harness connector insulators for the heated seat switches are color-coded (red is for the left seat control, and blue is for the right) to ensure proper electrical connection.
Each heated seat switch is a single pole, double throw, resistor-multiplexed, rocker-type switch with a center neutral position, and momentary Lo and Hi positions. Each switch has two amber Light-Emitting Diode (LED) indicator lamps, which illuminate to indicate the selected Lo or Hi heating mode. Each switch also has LED illumination lamps that illuminate a small heated seat icon in the center of the switch rocker and the Lo/Hi nomenclature on the opposite ends of the switch rocker anytime the ignition switch is in the ON position. The switch illumination lamps provide improved switch identification, visibility, and ease of use. The heated seat switches cannot be adjusted or repaired. If either switch or any of its LED lamps is damaged or faulty, the individual switch unit must be replaced.
The heated seat switches operate on battery current received through a fuse in the Junction Block (JB) on a fused ignition switch output (run-acc) circuit so that the heated seats will only operate when the ignition switch is in the ON or Accessory positions. The switches receive a ground feed from a two wire take out and eyelet terminal connector of the power seat wire harness that is secured by a ground screw to the top of the seat crossmember under the right front seat. The three illumination LED lamps within each switch are connected to ground at all times, resulting in the switch being illuminated whenever the ignition switch is in the ON or Accessory positions. The switch also contains three resistor circuits that are connected in parallel with each other to the heated seat module (Seat Heat Interface Module or SHIM) through the driver or passenger seat heater MUX circuit. Each switch resistor circuit has a different resistance value, which allows the heated seat module to continually monitor the state of each switch.
One resistor circuit in each switch is connected in series between the battery current source and the SHIM, while the other two resistor circuits are normally open. Depressing the heated seat switch rocker to its Hi or Lo position momentarily closes one of these two normally open resistor circuits, which provides a voltage request signal to the SHIM to power the heated seat elements of the selected seat. If the heated seat switch is depressed again to a different position (Lo or Hi) than the currently selected state, the SHIM will change states to support the new selection. If a heated seat switch is depressed a second time to the same position as the currently selected state, the SHIM interprets the second input as a request to cancel. The SHIM Will then turn OFF the heated seat elements for that seat.
The indicator lamps in the heated seat switches also receive battery current through the fused ignition switch output (run-acc) circuit whenever the ignition switch is in the ON or Accessory positions. The ground side of each indicator lamp is controlled by a separate driver or passenger seat high or low heat LED driver circuit from the SHIM. When the SHIM sees a Hi or Lo heat request signal from a heated seat switch, it simultaneously pulls the proper LED driver circuit to ground to indicate the current state of that seat heater. Likewise, the SHIM will turn OFF the indicator lamps when a cancel request is received. The SHIM control of the switch indicator lamps also allows the module to provide self-diagnostic feedback to indicate monitored heated seat system faults by flashing the LED indicator lamps in a programmed routine. Refer to Heated Seat System Diagnosis and Testing for more information on this heated seat system self-diagnostic feature.
Fig.3 Heated Seat Switch:
RIGHT HEATED SEAT SWITCH
Two heated seat switches, one each for the driver side and passenger side front seats, are mounted in a single switch bezel located on the center floor console behind the gearshift selector lever. On models with the heated seat system option, the heated seat switch bezel replaces the standard equipment ash receiver on the center floor console, and an available optional ash receiver is installed in one of the cup holders. The two switches are snapped into the mounting holes of the heated seat switch bezel, and the switch and bezel assembly snaps into the mounting hole of the center floor console. The two switches are identical in appearance and construction, but the power seat wire harness connector insulators for the heated seat switches are color-coded (red is for the left seat control, and blue is for the right) to ensure proper electrical connection.
Each heated seat switch is a single pole, double throw, resistor-multiplexed, rocker-type switch with a center neutral position, and momentary Lo and Hi positions. Each switch has two amber Light-Emitting Diode (LED) indicator lamps, which illuminate to indicate the selected Lo or Hi heating mode. Each switch also has LED illumination lamps that illuminate a small heated seat icon in the center of the switch rocker and the Lo/Hi nomenclature on the opposite ends of the switch rocker anytime the ignition switch is in the ON position. The switch illumination lamps provide improved switch identification, visibility, and ease of use. The heated seat switches cannot be adjusted or repaired. If either switch or any of its LED lamps is damaged or faulty, the individual switch unit must be replaced.
The heated seat switches operate on battery current received through a fuse in the Junction Block (JB) on a fused ignition switch output (run-acc) circuit so that the heated seats will only operate when the ignition switch is in the ON or Accessory positions. The switches receive a ground feed from a two wire take out and eyelet terminal connector of the power seat wire harness that is secured by a ground screw to the top of the seat crossmember under the right front seat. The three illumination LED lamps within each switch are connected to ground at all times, resulting in the switch being illuminated whenever the ignition switch is in the ON or Accessory positions. The switch also contains three resistor circuits that are connected in parallel with each other to the heated seat module (Seat Heat Interface Module or SHIM) through the driver or passenger seat heater MUX circuit. Each switch resistor circuit has a different resistance value, which allows the heated seat module to continually monitor the state of each switch.
One resistor circuit in each switch is connected in series between the battery current source and the SHIM, while the other two resistor circuits are normally open. Depressing the heated seat switch rocker to its Hi or Lo position momentarily closes one of these two normally open resistor circuits, which provides a voltage request signal to the SHIM to power the heated seat elements of the selected seat. If the heated seat switch is depressed again to a different position (Lo or Hi) than the currently selected state, the SHIM will change states to support the new selection. If a heated seat switch is depressed a second time to the same position as the currently selected state, the SHIM interprets the second input as a request to cancel. The SHIM will then turn OFF the heated seat elements for that seat.
The indicator lamps in the heated seat switches also receive battery current through the fused ignition switch output (run-acc) circuit whenever the ignition switch is in the ON or Accessory positions. The ground side of each indicator lamp is controlled by a separate driver or passenger seat high or low heat LED driver circuit from the SHIM. When the SHIM sees a Hi or Lo heat request signal from a heated seat switch, it simultaneously pulls the proper LED driver circuit to ground to indicate the current state of that seat heater. Likewise, the SHIM will turn OFF the indicator lamps when a cancel request is received. The SHIM control of the switch indicator lamps also allows the module to provide self-diagnostic feedback to indicate monitored heated seat system faults by flashing the LED indicator lamps in a programmed routine. Refer to Heated Seat System Diagnosis and Testing for more information on this heated seat system self-diagnostic feature.
Fig.5 Heated Seat Cushion Trim Cover - Typical:
HEATED SEAT ELEMENT
Vehicles equipped with the optional heated seat system have two sets of electrically operated heating elements located in each front seat, one set for the seat cushion and the other set for the seat back. Each of the heated seat elements consists of a single length of resistor wire that is routed in a zigzag pattern and captured between the seat cushion or seat back trim cover and the foam rubber backing on the underside of the cover. Short pigtail wires with connectors are soldered to each end of each resistor wire element, which connect all of the elements for each seating position to each other.
The heated seat elements are integral to their respective seat cushion cover and seat back cover assemblies, which are serviced individually. The heated seat elements cannot be adjusted or repaired; however, their interconnecting pigtail wires and connectors can be repaired. Refer to the appropriate wiring information for the proper wire and connector repair procedures. If a heated seat element is damaged or faulty, the affected seat cushion or seat back trim cover unit must be replaced. Refer to the appropriate service information for the proper seat trim cover service procedures.
The heated seat elements for each seat are connected in series between ground and the heated seat module (Seat Heat Interface Module or SHIM). One end of each heated seat element receives a ground feed at all times from a two wire take out and eyelet terminal connector of the power seat wire harness that is secured by a ground screw to the top of the seat crossmember under the right front seat. Battery current is directed to the other end of each heated seat element through the driver or passenger heated seat driver circuit by the energized N-channel Field Effect Transistor (N-FET) located within the SHIM. Each heated seat element is a resistor wire. As electrical current passes through the heating element, the resistance of the wire disperses some of that electrical current in the form of heat. The heat produced by the heated seat element then radiates through the underside of the seat cushion and seat back trim covers, warming the seat cover and its occupant.
Fig.6 Heated Seat Relay:
HEATED SEAT RELAY
The heated seat relay is located on the power seat track front bracket under the front edge of the right front seat cushion, where it is secured with a push-in type plastic fastener. The heated seat relay is a conventional International Standards Organization (ISO) mini relay. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions. The relay is contained within a small, rectangular, molded plastic housing with an extended skirt that serves as the connector receptacle. The relay is connected to all of the required inputs and outputs through the power seat wire harness by five male spade-type terminals that extend from the bottom of the relay base. The ISO designation for each terminal is molded into the base adjacent to each terminal. The ISO terminal designations are as follows:
- 30 (Common Feed) - This terminal is connected to the movable contact point of the relay.
- 85 (Coil Ground) - This terminal is connected to the ground feed side of the relay control coil.
- 86 (Coil Battery) - This terminal is connected to the battery feed side of the relay control coil.
- 87 (Normally Open) - This terminal is connected to the normally open fixed contact point of the relay.
- 87A (Normally Closed) - This terminal is connected to the normally closed fixed contact point of the relay.
The heated seat relay cannot be adjusted or repaired. If the relay is damaged or faulty, it must be replaced.
The heated seat relay is an electromechanical switch that uses a low current input from the ignition switch to control the high current output to the heated seat module. The movable common feed contact point is held against the fixed normally closed contact point by spring pressure. When the relay coil is energized, an electromagnetic field is produced by the coil windings. This electromagnetic field draws the movable relay contact point away from the fixed normally closed contact point, and holds it against the fixed normally open contact point. When the relay coil is de-energized, spring pressure returns the movable contact point back against the fixed normally closed contact point. The resistor or diode is connected in parallel with the relay coil in the relay, and helps to dissipate voltage spikes and electromagnetic interference that can be generated as the electromagnetic field of the relay coil collapses.
The heated seat relay terminals are connected to the vehicle electrical system through a connector and take out of the power seat wire harness. The inputs and outputs of the heated seat relay include:
- The common feed terminal (30) receives a battery current input from the power seat circuit breaker in the Junction Block (JB) through a fused B(+) circuit at all times.
- The coil ground terminal (85) receives a ground feed input at all times from a two wire take out and eyelet terminal connector of the power seat wire harness that is secured by a ground screw to the top of the seat crossmember under the right front seat.
- The coil battery terminal (86) receives a battery current input from a fuse in the JB through a fused ignition switch output (run-acc) circuit only when the ignition switch is in the ON or Accessory positions.
- The normally open terminal (87) provides a battery current output to the heated seat module (also known as the Seat Heat Interface Module or SHIM) through the heated seat relay output circuit only when the heated seat relay control coil is energized.
- The normally closed terminal (87A) is not connected to any circuit in this application, but battery current should be present at this terminal whenever the heated seat relay control coil is de-energized.
HEATED SEAT SENSOR
The heated seat sensors are Negative Thermal Coefficient (NTC) thermistors. One temperature sensor is used for each 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 captured between the seat cushion trim cover and the foam rubber backing on the underside of the cover, like the heated seat elements. Short pigtail wires are soldered to each end of the sensor and they share a 4-way connector with the pigtail wires from the heated seat cushion elements.
The heated seat sensors are integral to their respective seat cushion cover assemblies, which are serviced individually. The heated seat sensors cannot be adjusted or repaired; however, their pigtail wires and connectors can be repaired. Refer to the appropriate wiring information for the proper wire and connector repair procedures. If a heated seat sensor is damaged or faulty, the affected seat cushion trim cover unit must be replaced. Refer to the appropriate service information for the proper seat trim cover service procedures.
The resistance of the heated seat sensor increases and decreases as the surface temperature of the seat cushion cover changes. A single output of the heated seat module (Seat Heat Interface Module or SHIM) supplies each sensor with a reference voltage through the sensor feed circuit. The SHIM then detects the sensor resistance by monitoring the return voltage on separate return inputs through the driver and passenger heat sense input circuits. The SHIM uses the heated seat sensor resistance (seat cushion surface temperature) input to determine when to energize or de-energize the N-FET so that the heated seat element grids are cycled On or OFF as needed to maintain the selected temperature set point.