Electrically Heated Seats

The heated seat feature is available with or without the memory system. The module is located under the driver's seat and is known as the Heated Seat Module (HSM) or the Memory Heated Seat Module (MHSM).

The control switches for the heated seats are located on the instrument panel and are wired to the Body Control Module (BCM). The BCM then sends the message to the HSM via the PCI bus. The system offers two seat temperature settings of approximately 96.8°F (36°C) (LO) and approximately 107.6°F (41°C) (HI). As the temperature in the seat rises, the Negative Temperature Coefficient (NTC) resistance decreases and the voltage reading detected by the HSM decreases. The seat heater output is deactivated once the voltage reading reaches it's threshold for either setting, high or low.

As the temperature decreases, the voltage reading increases until the upper threshold is reached for either setting high or low. At this point the seat heat output is activated once again and this cycle repeats itself as long as the seat heat request is on. The thresholds for low and high settings are pre-programmed into the module memory. The heated seat switch contain resistors pulled up to 5 volts which are processed by the body control module as the voltage readings indicating desired heat setting high or low. The heater elements inside the seat back and cushion are different for the Limited and Laredo models. The Laredo seat heater elements are wired in series, the Limited seat elements are wired in parallel. This means that if an element is open in a Laredo model there will be no heater output to either the cushion or the seat back. If an element is open in a Limited model, the other elements will operate as usual. However if one of the elements is open, the seat cushion will have to be replaced. There are three heater elements in the seat cushions for both models. The Laredo uses a resistor wire for its center and both side bolsters (raised outer side panels) and the Limited uses a carbon fiber element for its center and resistor wire for the bolsters.