412-04 Control Components: Description And Operation (page 16 of 21)

 
To protect the heater elements, the ATCM disables front seat heating if battery voltage exceeds 16.5 ± 0.3 volts for more 
than 5 seconds. Front seat heating is re-enabled when battery voltage decreases to 16.2 ± 0.3 volts. 
 
The ATCM monitors the power feeds to the heater elements and disables the applicable front seat heating if a short or 
open circuit is detected. The ATCM also disables seat heating if the seat temperature rises significantly above the target 
temperature setting. 
 
The plausibility of the temperature sensor inputs is also monitored by the ATCM. When seat heating is selected, if one of 
the temperature sensor inputs is within 5 °C (9 °F) below the target temperature, the ATCM monitors the sensor input for 
a temperature increase and checks that it is between the minimum and maximum working temperatures. If a temperature 
sensor input is at the high end of the working range, while the ambient air temperature and the engine temperature are 
within 10 °C (18 °F) of each other, the ATCM disables front seat heating until the input decreases below the target 
temperature setting. The ATCM interprets a temperature sensor input value of -45 °C (-49 °F) or below as an open circuit, 
and temperature sensor input value of 100 °C (212 °F) or more as a short circuit. 
 

Rear Window Heater 

The ATCM controls operation of the rear window heater using medium speed CAN messages to operate the rear window 
heater relay in the Central Junction Box (CJB). The control module in the CJB interprets the CAN messages and switches 
the ground connection of the relay coil to operate the rear window heater. While the rear window heater relay is 
energized, a battery power feed is connected to the rear window heater elements. Rear window heater operation is only 
enabled when the engine is running. 
 
The ATCM operates the rear window heater in heating cycles of varying power and time. The heating cycle used depends 
on the ambient air temperature and whether it is the initial or subsequent operation during the current ignition cycle. 
 
When the rear window heater switch is pressed, the ATCM illuminates an LED in the switch and initiates the appropriate 
heating cycle. The LED remains illuminated until the rear window heater is selected off, the heating cycle is completed or 
the engine stops. If the engine stalls or the ignition is turned off, rear window heating resumes if the engine is re-started 
within 20 seconds. 
 
On the initial selection of rear window heating, the ATCM uses a short or long defrost phase at full power, followed by a 
low power phase. The defrost phase used depends on the ambient temperature. During the low power phase, the rear 
window heater relay is cycled off for 80 seconds and on for 40 seconds. 
 
On subsequent operations, during the same ignition cycle, the ATCM operates the rear window heater at full power for a 
fixed time period. 
 

Rear Window Heating Phases 

Windshield Heater 

The ATCM controls operation of the windshield heater using the windshield heater relay in the BJB. The ATCM switches 
the ground connection of the relay coil to operate the windshield heater. While the windshield heater relay is energized, a 
battery power feed is connected to each of the two windshield heater elements. Windshield heater operation is only 
enabled when the engine is running. 
 
The ATCM operates the windshield heater in heating cycles of varying power and time. The heating cycle used depends 
on the ambient air temperature and whether it is the initial or subsequent operation during the current ignition cycle. 
 
When the windshield heater switch is pressed, the ATCM illuminates an LED in the switch and initiates the appropriate 
heating cycle. The LED remains illuminated until the windshield heater is selected off, the heating cycle is completed or 
the engine stops. If the engine stalls or the ignition is turned off, windshield heating resumes if the engine is re-started 

manually de-selected 

Phase 

Time, minutes

Short defrost (-5°C (23°F) and above) 

10 

Long defrost (less than -5°C (23°F)) 

15 

Low power 

20 

Subsequent operation 

10