Heated Seats

WARNING: This page is about a different car, the 2008 Mercury Mariner and 2008 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

The HVAC module contains the heated seat control buttons and indicators. The heated seat system functions independently from the climate control system.

A heated seat relay is located underneath each heated seat cushion frame. There are 2 voltage supplies to the heated seat relay. Voltage is supplied from the smart junction box (SJB) to each heated seat relay coil when the ignition is ON. Voltage is also supplied to the relay contacts from the SJB to supply voltage to the heater mats. The HVAC module supplies ground to the relay coil when a heated seat switch button is pressed to energize the relay. This closes the relay contact to supply battery voltage from the SJB to the cushion heater mat. The cushion and backrest heater mats are wired in series and the backrest heater mats are directly connected to ground to complete the electrical circuit to heat a seat.

Each cushion heater mat contains a non-serviceable thermostatic switch that closes when cold to complete the heater grid circuit and heat the seat. When hot, the thermostatic switch opens the heater grid circuit and allows the seat to cool. During normal operation, several minutes may be required before seat temperature is increased and several more minutes of cooling before the heater mats continue with heating. The heated seat indicator illuminates continuously while the thermostatic switch cycles open and closed.

The seat should be occupied to correctly test for normal operation. Temperature measuring devices do not provide an accurate method for testing heated seat operation as surface temperatures are subject to human and environmental characteristics. Correct testing requires heater mat resistance measurements which have tight tolerances affecting heat cycle time length.

Heated seats in vehicles built before April 11, 2007 automatically turn off after 10 minutes use. Heated seats in vehicles built after April 10, 2007 operate continuously without system time-out. The heated seat system time-out is a characteristic of the HVAC module. Installation of a new HVAC module in a vehicle built prior to April 11, 2007 will change the heated seat system operation to no longer time-out.