For 1990-2009 cars

### 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 | Signal Performance
--- | --- | --- | --- | ---
Evaporator Sensor Signal | B3933 07 | 1 | B3933 03 | --
Evaporator Low Reference Signal | 1 | 1 | 1 | --
Ambient Air Temperature Sensor Signal | B0158 02 | B0158 05 | B0158 05 | 1
1. Air Temperature Malfunction

### Circuit/System Description

#### Air Temperature Sensors

The air temperature sensors are a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:

• | Ambient Air Temperature Sensor

• | A/C Evaporator Temperature Sensor

A signal and low reference circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases. The sensor signal voltage decreases as the resistance decreases. The sensor operates within a temperature range between -40 to +101°C (-40 to +215°F). The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts. As the air temperature increases the count value will decrease. If the HVAC control module or auxiliary HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted air temperature value. The default value for the ambient temperature sensor will be displayed on the scan tool. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

### Diagnostic Aids

Condition | Display
--- | ---
Using Scan Tool Special Function | Updates temperature display instantly
When the HVAC module MODE, FRONT DEFROST, and REAR DEFROST buttons are pressed simultaneously. | Updates temperature display instantly
At start up with the engine off less than 2 hours | Displays last stored temperature unless temperature has decreased. The outside air temperature reading is always instantly updated if ambient air temperature has decreased.
At start up with the engine off more than 2 hours | Displays actual outside temperature
Vehicle speed above 32 km/h (20 mph) for a minimum of 80 seconds | Updates temperature display at a slow filtered rate
Vehicle speed above 72 km/h (45 mph) | Displays actual outside temperature
Sensor ambient temperature reading is less than the last displayed value | Displays actual outside temperature
When the ambient air decreases | Updates temperature display rapidly

### Reference Information

#### Schematic Reference

HVAC Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

• | Air Delivery Description and Operation

• | Air Temperature Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for Scan Tool Information

### Circuit/System Verification

1. Measure actual outside air temperature using a thermometer. Compare this value to the scan tool Outside Air Temp. Raw parameter. The measured temperature should be within 5 degrees of parameter.

⇒ | If not within the specified range, refer to Ambient / Inside Air Temperature Sensor Circuit Malfunction.

2. Measure actual inside air temperature using a thermometer. Compare this value to the scan tool Inside Air Temp. parameter. The measured temperature should be within 5 degrees of parameter.

⇒ | If not within the specified range, refer to Ambient Temperature Sensor Circuit Malfunction.

3. Measure actual temperature using a thermometer at the A/C evap. temperature sensor. Compare this value to the scan tool A/C Evap. Temperature Sensor parameter. The measured temperature should be within 5 degrees of the parameter.

⇒ | If not within the specified range, refer to Evaporator Temperature Sensor Circuit Malfunction.

#### Ambient Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate temp sensor.
2. Test for less than 1 ohm of resistance between the low reference circuit terminal B and ground.

⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the HVAC control module.

3. Ignition ON, verify the appropriate scan tool Temp Sensor Voltage parameter is greater than 4.9 volts or 250 counts.

⇒ | If less than the specified range, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the HVAC control module.

4. Install a 3-amp fused jumper wire between the signal circuit terminal A and the low reference circuit terminal B. Verify the scan tool Temp Sensor Voltage parameter is less than 0.1 volt or 5 counts.

⇒ | If greater than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

5. If all circuits test normal, test or replace the appropriate temp sensor.

#### Evaporator Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate temp sensor.
2. Test for less than 1.0 ohm of resistance between the low reference circuit terminal 2 and ground.

⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the HVAC control module.

3. Ignition ON, verify the appropriate temp sensor parameter is greater than 4.9 volts or 250 counts.

⇒ | If less than the specified range, test the signal circuit terminal 1 for a short to ground. If the circuit tests normal, replace the HVAC control module.

4. Install a 3-amp fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool temp sensor parameter is less than 0.1 volt or 5 counts.

⇒ | If greater than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

5. If all circuits test normal, test or replace the appropriate temp sensor.

### Component Testing

1. Ignition OFF, disconnect the harness connector at the appropriate temp sensor.
2. Test for the appropriate resistance between the signal terminal A or 1 and the low reference terminal B or 2. Refer to Sensor Resistance Table for resistance values.

⇒ | If not within the specified range, replace the appropriate sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Evaporator Temperature Sensor Replacement

• | Ambient Air Temperature Sensor Replacement