Outside Air Temperature Display

The AAT sensor is hardwired to the PCM through separate input and return circuits. The PCM provides a reference voltage to the AAT sensor and monitors the change in voltage resulting from changes in resistance as determined by outside air temperature. The PCM messages the outside air temperature data to the HVAC control module through the BCM. The BCM messages the outside air temperature in degrees Celsius (metric) to the IPC. When the Fahrenheit (English) display is selected by the driver, the IPC converts the Celsius to Fahrenheit and displays the temperature in the message center. The outside air temperature is displayed in the base IPC only  . For all others, the IPC sends the data over the I-CAN to the FDIM to display the temperature.

The HVAC control module is programmed to update the messaged outside air temperature data at different rates depending on several criteria to prevent false temperature displays due to a condition known as heat soaking. Heat soaking is where the outside air temperature is hotter in the location of the AAT sensor than the actual outside air temperature. When the sensed outside temperature rises, the display updates slowly at varying rates based on vehicle speed. When the sensed outside temperature drops, the display updates more quickly following the drop experienced by the AAT sensor. While the actual external temperature can vary continuously, the value that is displayed updates at a specific rate depending on whether or not the engine is "hot" and whether or not the vehicle is moving.

When the external temperature is rising, the display updates slowly. The updates (when the actual external temperature is higher than currently displayed value) are limited to 0.6 °C (1 °F) every 20 seconds while the vehicle is moving at greater than 32 km/h (20 mph). For more than 60 seconds or 0.6 °C (1 °F), the updates are made every 20 minutes if the vehicle is not moving at this speed. This is to prevent the heat from the engine compartment from affecting the accuracy of the display.

When the external temperature is dropping, the display updates quickly. The updates (when the actual external temperature is lower than currently displayed value) are limited to 0.6 °C (1 °F) every 2 seconds (regardless of vehicle speed). Consequently the display essentially follows the drop experienced by the external temperature thermistor.