Seat Temperature Element: Description and Operation

Heated Seat System Switches:

HEATED SEAT SYSTEM
Individually controlled electrically heated front seats are available factory-installed optional equipment on the Ram quad cab models that are also equipped with the optional SLT Plus (leather) trim package. Vehicles with this option can be visually identified by the two separate heated seat switches mounted in a bezel located in the lower right corner of the instrument cluster bezel, next to the radio receiver. 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:
- The heated seat switches, including two heated seat Light-Emitting Diode (LED) indicator lamps and an incandescent back lighting bulb for each switch.
- The heated seat module, also referred to as the Seat Heat Interface Module (SHIM), which contains the solid state electronic control and diagnostic logic circuitry for the heated seat system.
- The heated seat elements and sensors, which are integral to the individual front seat cushion and front seat back trim covers.
- The heated seat relay, which controls the availability of battery current to the heated seat module or SHIM.

Following are general descriptions 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.

The heated seat module receives fused battery current through the energized heated seat relay in the Junction Block (JB) only when the engine is running. The heated seat switches receive battery current through a fused ignition switch output (run) circuit only when the ignition switch is in the ON 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.

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 also be turned off automatically whenever the ignition switch is turned to any position except On or if the engine quits running. If the ignition switch is turned to the OFF position or if the engine quits running while a heated seat is turned ON, the heated seat will remain Off after the engine is restarted until a heated seat switch is depressed again.

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. The heated seat module is also programmed to provide a self-diagnostic capability. When the module detects certain failures within the heated seat system, it will provide a visual indication of the failure by flashing the indicator lamps in the heated seat switches.

Heated Switch Switches:

HEATED SEAT SWITCH
The heated seat switches used on vehicles with this option are both mounted in a heated seat switch bezel, which replaces the standard equipment cubby bin located in the lower right corner of the instrument cluster bezel next to the radio receiver. The two switches are snapped into the mounting holes of the heated seat switch bezel, and the heated seat switch bezel is secured with three screws to the instrument panel. The mounts for the heated seat switch bezel are concealed behind the instrument cluster bezel. The two heated seat switches are identical in appearance and construction, except for the location of a keyway in the single connector receptacle on the back of each switch. The instrument panel wire harness connectors for the heated seat switches are keyed to match the connector receptacles on the switches so that the two heated seat switches can only be connected to the proper heated seat.

The momentary, bidirectional rocker-type heated seat switch provides a resistor-multiplexed signal to the heated seat module. 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 seat heating mode. Each heated seat switch has two Light-Emitting Diode (LED) indicator lamps, which indicate the selected mode (Low or High) of the seat heater for each seat and to provide diagnostic feedback for the heated seat system. Each switch also has an incandescent bulb, which provides panel lamps dimmer controlled back lighting of the switch nomenclature when the headlamps or park lamps are turned on.

The two LED indicator lamps and the incandescent bulb in each heated seat switch cannot be repaired. If the indicator lamps or back lighting bulb are faulty or damaged, the individual heated seat switch unit 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 hard- wired resistor multiplexed voltage request signal to the heated seat module to power 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 (Low or High) than the currently selected state, the heated seat module 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 heated seat module interprets the second input as a request to turn the seat heater off. The heated seat module will then turn the heated seat elements for that seat off.

The 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 a separate (high or low/driver or passenger) indicator lamp driver circuit by the heated seat module. The heated seat module control of the switch indicator lamps also allows the module to provide diagnostic feedback to the vehicle operator to indicate monitored heated seat system faults by flashing the indicator lamps ON and OFF. One side of the incandescent back lighting bulb in each heated seat switch 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.

Heated Seat Module:

HEATED SEAT MODULE
The heated seat module is also known as the Seat Heat Interface Module or SHIM. The heated seat module is located under the front seat center cushion, where it is secured to a mounting bracket that also serves as the support for the slide-out rear seat cup holder unit. Two of the three mounting tabs that are molded into the plastic housing of the heated seat module are inserted through two holes in the mounting bracket and then secured by two push on retainers. The heated seat module has a single connector receptacle that allows the module to be connected to all of the required inputs and outputs through the seat wire harness.

The heated seat module is an electronic microprocessor controlled device designed and programmed to use inputs from the heated seat relay, the two heated seat switches and the two heated seat sensors to operate and control the heated seat elements in both front seats and the two heated seat indicator lamp Light-Emitting Diodes (LEDs) in each heated seat switch. The heated seat module is also programmed to perform self-diagnosis of certain heated seat system functions and provide feedback of that diagnosis through the heated seat switch indicator lamps.

The heated seat module cannot be repaired. If the heated seat module is damaged or faulty, the entire module must be replaced.

The heated seat module operates on fused battery current received from the heated seat relay. The module is grounded at all times through a ground screw located below the left rear speaker in the cab of the vehicle. Inputs to the module include a resistor multiplexed heated seat switch request circuit for each of the two heated seat switches and the heated seat sensor inputs from the seat cushions of each front seat. In response to those inputs the heated seat module controls battery current feeds to the heated seat elements and sensors, and controls the ground for the heated seat switch indicator lamps.

When a heated seat switch (Driver or Passenger) request signal is received by the heated seat module, the module energizes the proper indicator lamp (Low or High) in the switch by grounding the indicator lamp circuit to indicate that the heated seat system is operating. At the same time, the heated seat module energizes the selected heated seat sensor circuit and the sensor provides the module with an input indicating the surface temperature of the selected seat cushion.

The Low heat set point is about 36 °C (96.8 °F), and the High heat set point is about 42 °C (107.6 °F). If the seat cushion surface temperature input is below the temperature set point for the selected temperature setting, the heated seat module energizes an N-channel Field Effect Transistor (N-FET) within the module which energizes the heated seat elements in the selected seat cushion and back. When the sensor input to the module indicates the correct temperature set point has been achieved, the module de-energizes the N-FET which de-energizes the heated seat elements. The heated seat module will continue to cycle the N-FET as needed to maintain the selected temperature set point.

If the heated seat module detects a heated seat sensor value input that is out of range or a shorted or open heated seat element circuit, it will notify the vehicle operator or the repair technician of this condition by flashing the High and/or Low indicator lamps in the affected heated seat switch. Refer to Heated Seat System and Heated Seat Module diagnosis and testing procedures.

Heated Seat Cushion Trim Cover:

HEATED SEAT ELEMENT AND SENSOR
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 front seat cushion trim cover or front seat back trim cover assembly must be replaced. Refer to Front Seat Cushion Cover - Quad Cab or Front Seat Back Cover - Quad Cab removal and installation procedures.

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 the Low or High position 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.

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.

Heated Seat Relay:

HEATED SEAT RELAY
The heated seat relay is an electromechanical device that switches battery current to the heated seat module when the relay control coil is energized. The heated seat relay is located in the Junction Block (JB), on the left end of the instrument panel in the passenger compartment. The heated seat relay is a International Standards Organization (ISO) micro-relay. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions, The ISO micro-relay terminal functions are the same as a conventional ISO relay. However, the ISO micro- relay terminal pattern (or footprint) is different, the current capacity is lower, and the physical dimensions are smaller than those of the conventional ISO relay.

The heated seat relay cannot be repaired or adjusted and, if faulty or damaged, it must be replaced.

The ISO relay consists of an electromagnetic coil, a resistor or diode, and three (two fixed and one movable) electrical contacts. The movable (common feed) relay contact is held against one of the fixed contacts (normally closed) by spring pressure. When the electromagnetic coil is energized, it draws the movable contact away from the normally closed fixed contact, and holds it against the other (normally open) fixed contact. When the electromagnetic coil is de-energized, spring pressure returns the movable contact to the normally closed position. The resistor or diode is connected in parallel with the electromagnetic coil in the relay, and helps to dissipate voltage spikes that are produced when the coil is de-energized.

The heated seat relay is controlled by the premium version of the Central Timer Module (CTM), which controls the ground feed to the coil ground terminal of the relay to energize and de-energize the electromagnetic coil of the relay. The CTM monitors engine operation through messages it receives from the Powertrain Control Module (PCM) over the Chrysler Collision Detection (CCD) data bus network. The CTM is programmed to energize the relay only when the engine is running, and to de-energize the relay when the engine is not running. Refer to Central Timer Module for the location of more information on the premium CTM.