Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage |
|---|---|---|---|
Outside Air Temperature (OAT) Signal Circuit | B0158 1 | B0158 1 | B0158 1 |
Outside Air Temperature (OAT) Ground Reference Circuit | 1 | B0158 1 | 1 |
1. Outside Air Temperature (OAT) Display Inaccurate or Inoperative | |||
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 2 discrete 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°C (-40°F) in the DIC and an OAT sensor shorted to ground displays 87.5°C (189.5°F).
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 to +87.5°C ( -40 to +189.5°F). The default readout is 87.5°C (190°F). |
| • | 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. |
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
Scan Tool Reference
Circuit/System Verification
The DIC should display approximately 87.5°C (189.5°F) or -40°C (-40°F) .
Circuit/System Testing
- Ignition OFF disconnect the harness connector at the OAT sensor.
- Ignition OFF, test for less than 1 ohm of resistance between the ground circuit terminal and ground.
- Ignition ON, verify the driver information center (DIC) displays approximately -40°F (-40°C).
- Ignition ON, perform an outside air temperature Instant Update to by pass filtering by simultaneously pressing the following switches on the HVAC control module:
- Ignition ON, install a 3-amp fused jumper wire between the signal circuit and the ground circuit of the OAT sensor. Verify the DIC displays approximately 189.5°F (+87.5°C).
- If all circuits test normal, test or replace the OAT sensor.
| ⇒ | If greater than 1 ohm, test the ground circuit for an open/high resistance. |
| ⇒ | If the DIC does not display approximately -40°C (-40°F), test the OAT sensor signal circuit for a short to ground. If the circuit tests normal, replace the HVAC module. |
| • | AUTO |
| • | A/C OFF |
| • | REAR DEFOGGER |
| ⇒ | If the DIC does not display approximately 87.5°C (189.5°F), test the signal circuit of the OAT sensor for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC module. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for the HVAC module replacement, setup and programming |
