Circuit Description: Notes

WARNING: This page is about a different car, the 2005 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.

The temperature sense and display system includes a vehicle mounted variable resistance outside air temperature sensor interfaced to the IBCM analog input. The IBCM provides the logic for reading the sensor, filtering the data and transmitting this information via the GM LAN Low Speed Bus to the instrument panel cluster (IPC) for display. The temperature is sent to the IPC in Degrees Celsius. The IPC is responsible for displaying the temperature and converting to Degrees Fahrenheit if necessary. The IBCM reads the Outside Air Temperature sensor and converts A-D value to a temperature via a look-up table. Two GM LAN Low Speed Bus signals are ultimately generated. An un-filtered value, and a filtered value for use by the IPC display. The IBCM provides sensor out-of-range detection and compensates for engine temperature by utilizing vehicle speed and engine off time duration. The IPC shall display the outside air temperature. The temperature information is provided by the Outside Air Temperature corrected value GM LAN Signal. This information is in Degrees Celsius. The IPC will convert to Degrees Fahrenheit if necessary. English or metric mode information is available via the Display Measurement System GM-LAN signal. The algorithm for all messages is executed at a rate of once every 1 second.

Diagnostic Aids

The following variable represents the latest temperature value to be displayed by the IPC. This variable can be Degrees Celsius or Degrees Fahrenheit. Range -40 -87.5°C (-40 - 190°F). The default readout is 87.5°C (190°F).
After the ignition has been shut off and the engine is not running, the outside air temperature sensor must cool down for a period of time before the outside air temperature sensor reading will be believable. This is because the engine produces considerable heat after it has been shutdown. Without air movement across the sensor such as when the vehicle is moving the outside air temperature sensor will produce false ambient temperature readings attributable to engine heat. A falling outside air temperature sensor reading is always accepted and filtered into the display. Falling temperature readings are always believable but need some light filtering.
Upon initial vehicle start-up the outside air temperature sensor speed counter estimates the amount of time the vehicle will need to be in motion before the outside air temperature sensor will be reading true ambient temperature. As soon as the counter reaches this point it is assumed the driver has been driving the given amount of time for the sensor to cool to ambient. At this point the outside air temperature readings are filtered into the display.