Outside Air Temperature

The Ambient Air Temperature (AAT) sensor is hardwired to the PCM through separate input and return circuits. The PCM provides a reference voltage to the Ambient Air Temperature (AAT) sensor and monitors the change in voltage resulting from changes in resistance as determined by outside air temperature.

The PCM sends the ambient air temperature data to the GWM through the HS-CAN1. The GWM sends the ambient air temperature message to the HVAC over the MS-CAN. The HVAC module filters the data and sends the outside air temperature data to the GWM over the MS-CAN. The GWM sends the outside air temperature message to the IPC over the HS-CAN3.

The HVAC module is programmed to update the messaged outside 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 Ambient Air Temperature (AAT) sensor than the actual outside air temperature.

The outside air temperature display update strategy requires a starting temperature to update from. This starting temperature is controlled based on the length of time the engine is off and the engine temperature. When the engine has been off for longer than 4 hours, the update strategy begins with the unfiltered ambient air temperature input to the PCM. If the engine has been off for less than 4 hours, and the engine coolant temperature is less than 30 deg. C (86 deg. F), the update strategy begins with the filtered ambient air temperature equal to the unfiltered ambient air temperature. If the engine has been off for less than 4 hours, and the engine coolant temperature is greater than 30 deg. C (86 deg. F), the update strategy begins at the stored previous outside air temperature value.

When the sensed outside temperature rises and the vehicle speed is above 32 km/h (21 mph), the outside air temperature display updates after approximately 90 seconds. As the vehicle speed increases, the outside air temperature display updates at a faster rate that is proportional to the increase in vehicle speed. Once the vehicle speed exceeds 80 km/h (50 mph), the display updates without any delay. If the vehicle speed drops below 32 km/h (21 mph), the update delays reset. When the sensed outside temperature drops, the display updates more quickly following the drop experienced by the Ambient Air Temperature (AAT) sensor.