Outside Air Temperature Display Inaccurate or Inoperative

OUTSIDE AIR TEMPERATURE DISPLAY INACCURATE OR INOPERATIVE

CIRCUIT DESCRIPTION
The outside air temperature sensor is a variable resistor interfaced to the body control module (BCM) an a log input. The BCM provides the logic for reading the sensor, filtering the data and transmitting this information via the GMLAN 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. 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 BCM also provides the sensor out-of-range detection and compensates for engine temperature by utilizing vehicle speed and engine off time duration. 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 to 70°C (-40 to +158°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 is accurate. 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 produces a false ambient temperature reading attributable to engine heat. A falling outside air temperature sensor reading is always accepted and filtered into the display.
- 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.

TEST DESCRIPTION

Steps 1 - 9:

Step 10:

The number below refers to the step number on the diagnostic table.
3. This step tests for resistance in the signal and ground circuit of the ambient air temperature sensor.