Heated Seat System

Individually 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:

- 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.

- 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.

- 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.

- 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.

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 (PACE) data bus network. The PACE data bus network allows the sharing of sensor information. This helps to reduce wire harness complexity, internal controller hard- ware, 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 PACE data bus network, the use of a DRB III(R) scan tool and the proper Diagnostic Procedures are recommended.

Following are general descriptions and operations of the major components in 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.