Seat Temperature Element: Description and Operation

INTRODUCTION
Heated seats only available with leather and memory. The seat heaters are controlled by the Memory Heated Seat and Mirror Module (MHSMM). The heaters will only operate when the ignition switch is in the ON position, and the surface temperature at the from seat heating element sensors is below the designed temperature set points of the system.

The heater switch is a three position (HI, LO, and neutral position OFF) momentary rocker switch. Green LED's illuminate the switch for night visibility. Amber LED's are located in the switch adjacent to the HI and LO graphics, which indicate the selected operating mode of the seat heater, The LO heat position set point is about 98° F and the HI set point is about 107° F.

The heaters are solid high side driven, and are controlled by an NTC circuit. When either HI or LO is activated, the temperature in the seat rises the Negative Temperature Coefficient (NTC) resistance decreases, and the voltage reading detected by the MHSMM increases. The seat heater output is deactivated once the voltage reading reaches its upper threshold for either of the settings, HI or LO.

As the temperature decreases the voltage reading decreases until the lower threshold is reached for either setting, HI or LO. At this point the seat heat output is activated once again. This cycle repeats itself as long as the seat heat request remains on. The thresholds for high and low settings are pre-programmed into the MHSMM.

The system will automatically turn off when the ignition is turned to the OFF position.

SYSTEM OPERATION
HEATED SEAT ELEMENT AND SENSOR
Two heated seat elements are used in both of the front seats, one for the seat cushion and the other for the seat back. The two elements for each seat are connected in series to the MHSMM.

The temperature sensor is a Negative Temperature Coefficient (NTC) thermistor. One temperature sensor is used for each set, and it is integrated into the seat cushion heating element

The heating elements are sewn into the seat cushion cover and seat back cover assemblies, which are serviced individually. The heating elements and temperature sensor cannot be repaired, and if faulty or damaged, the affected seat cover must be replaced.