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 | B0158 | B0158 |
Outside Air Temperature (OAT) Ground Reference Circuit | -- | B0158 | -- |
Circuit/System Description
The ambient air temperature sensor is a variable resistor interfaced to the body control module (BCM). The BCM monitors the ambient air temperature sensor. The ambient air temperature sensor is a 2-wire negative temperature co-efficient thermistor. The BCM applies 5 volts to an internal input resistor connected to the signal circuit of the ambient air temperature sensor. The BCM also provides the low reference circuit to the ambient air temperature sensor. When the ambient air temperature is cold, the sensor resistance is high and the signal voltage is high. When the ambient air temperature is hot, the sensor resistance is low and the signal voltage is low. The BCM provides the logic for reading the ambient air temperature sensor, filtering the data and transmitting the information via the serial data circuit to the driver information center (DIC) display in the radio. The temperature is sent to the DIC in Degrees Celsius. The radio is responsible for displaying the temperature and converting to Degrees Fahrenheit. Two serial data signals are ultimately generated. An un-filtered value and a filtered value for use by the DIC display in the radio. The BCM also provides the sensor out-of-range detection and compensates for engine temperature by utilizing vehicle speed and engine off time duration.
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
Scan Tool Reference
Diagnostic Aids
| • | The following variable represents the latest temperature value to be displayed by the DIC. This variable can be degrees Celsius or Fahrenheit. Range: -40 to + 190°F (-40 to + 87.5°C). The default readout is 190°F (87.5°C). |
| • | After the ignition has been shut OFF and the engine is not running, the ambient air temperature sensor must cool down for a period of time before the ambient 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 ambient air temperature sensor produces a false ambient temperature reading attributable to engine heat. A falling ambient air temperature sensor reading is always accepted and filtered into the display. |
| • | Upon initial vehicle start-up, the ambient air temperature sensor speed counter estimates the amount of time the vehicle will need to be in motion before the ambient 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 ambient air temperature readings are filtered into the display. |
Circuit/System Testing
- Ignition OFF, disconnect the harness connector at the ambient air temperature sensor.
- Ignition OFF, test for less than 1 ohm of resistance between the ambient air temperature sensor low reference circuit terminal B and ground.
- Ignition ON, verify the scan tool Ambient Air Temperature Sensor parameter is greater than 4.7 volts.
- Install a 3-amp fused jumper wire between the ambient air temperature sensor signal circuit terminal A and the low reference circuit terminal B. Verify the scan tool Ambient Air Temperature Sensor parameter is less than 0.4 volt.
- If all circuits test normal, test or replace the ambient temperature sensor.
| ⇒ | If greater than the specified value, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the BCM. |
| ⇒ | If less than the specified value, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the BCM. |
| ⇒ | If greater than the specified value, test the signal circuit for an open/high resistance. If the circuit tests normal replace the BCM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for BCM replacement, setup, and programming |
