Outside Air Temperature Display Inaccurate or Inoperative

OUTSIDE AIR TEMPERATURE DISPLAY INACCURATE OR INOPERATIVE

DIAGNOSTIC FAULT INFORMATION

IMPORTANT: Always perform the Diagnostic System Check - Vehicle Testing and Inspection

CIRCUIT/SYSTEM DESCRIPTION
The HVAC module monitors the signal circuit of the outside air temperature sensor. The outside air temperature (OAT) sensor is a thermistor which varies in resistance as the temperature changes. When the resistance of OAT sensor increases, the outside air temperature is cold. When the resistance of the OAT sensor decreases, the outside air temperature is high. The HVAC module interfaces with the ambient air temperature via two discreet circuits. The HVAC module applies 5 volts to an internal input resistor that is connected to the signal circuit of the outside air temperature sensor. The HVAC module provides ground through the low reference circuit. The HVAC module uses the outside air temperature resistance and voltage for automatic control calculations and conversion to temperature values. The HVAC module sends the temperature value to the instrument panel cluster (IPC) via GMLAN serial data circuits for display in the driver information center (DIC). If the OAT sensor is opened or shorted, the temperature display is either a cold or hot extreme. An open OAT sensor will display -40°F (-40°C) in the DIC and an OAT sensor shorted to battery displays +189.5°F (+87.5°C).

DIAGNOSTIC AIDS
- The following variable represents the latest temperature value to be displayed by the DIC. This variable can be Degrees Fahrenheit or Celsius.Range: -40°F to +189.5°F (-40°C to 87.5°F). The default readout is 190°F (87.5°C).
- After the ignition has been shutoff and the engine is not running, the OAT sensor must cool down for a period of time before OAT sensor reading is accurate. This is because the engine produces considerable heat after it has been shutdown. Without air movement across the OAT sensor such as when the vehicle is moving, the OAT sensor provides a false outside temperature reading that can be attributed to engine heat. A falling OAT sensor reading is always accepted and filtered into the display.
- Upon initial vehicle startup, the OAT sensor speed counter estimates the amount of time the vehicle will need to be in motion before the OAT sensor will be reading true outside 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.

CIRCUIT/SYSTEM VERIFICATION
The DIC should display "SC" or "OC".

CIRCUIT/SYSTEM TESTING
1. With the ignition OFF disconnect the OAT sensor.
2. Turn ON the ignition with the engine OFF.
- If the DIC does not display "OC", test the OAT sensor signal circuit for a short to ground, an open or high resistance.
- If the OAT sensor signal circuit tests normal, replace the HVAC module.
3. With the ignition OFF, connect a 3-amp fused jumper wire between the signal circuit of the OAT sensor and a good ground.
4. Turn ON the ignition.
- If the DIC displays "SC", test the OAT sensor ground circuit for an open or a high resistance.
- If the OAT sensor ground circuit tests normal, replace the OAT sensor.
- If the DIC does not displays "SC", test the OAT sensor signal circuit for an open, a short to voltage or a high resistance.
- If the OAT sensor signal circuit tests normal, replace the HVAC module.

REPAIR INSTRUCTIONS

IMPORTANT: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

- Ambient Air Temperature Gage Sensor Replacement
- Control Module References