HVAC System - Automatic Chevrolet Cab & Chassis Silverado 3500 HD

Fastener Tightening Specifications

Sensor Resistance Table

Scheme 33

Scheme 34

Scheme 35

Scheme 36

Scheme 37

Scheme 38

DIAGNOSTIC CODE INDEX

DIAGNOSTIC CODE INDEX

Diagnostic Instructions

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

Diagnostic Fault Information

Air Temperature Sensors

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

1. Ambient Air Temperature Sensor
2. Inside Air Temperature Sensor Assembly
3. Upper Left Air Temperature Sensor
4. Upper Right Air Temperature Sensor
5. Lower Left Air Temperature Sensor
6. Lower Right Air 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 and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. 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

Ambient Air Temperature Display Update

Conditions for Running the DTC

Ignition ON.

Conditions for Setting the DTC

The HVAC control module detects the temperature sensor signal circuit is less than -35°C (-39°F) or greater than 85°C (185°F) for greater than 15 seconds.

Action Taken When the DTC Sets

The system operates using a default value.

Conditions for Clearing the DTC

1. The DTC will become history if the HVAC control module no longer detects the condition that set the DTC.
2. The history DTC will clear after 100 fault-free ignition cycles.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)
2. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__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»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__ambientinside-air-temperature-sensor-circuit-malfunction) .
2. Measure actual inside air temperature using a thermometer. Compare this value to the scan tool Inside Air Temp. Sensor parameter and Aux. Inside Air Temp. parameter. The measured temperature should be within 5 degrees of parameter. If not within the specified range, refer to «Duct Temperature Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__duct-temperature-sensor-circuit-malfunction) .
3. Measure actual temperature using a thermometer at each of the duct temperature sensors. Compare this value to the appropriate scan tool Duct Actual parameter. The measured temperature should be within 5 degrees of appropriate Duct Actual parameter. If not within the specified range, refer to «Duct Temperature Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__duct-temperature-sensor-circuit-malfunction) .

Ambient/Inside Air Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate temperature sensor.
2. Test for less than 1 ohm 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 parameter is greater than 85°C (185°F). 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 parameter is less than less than -35°C (-39°F). 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 temperature sensor.

Duct Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate temperature sensor.
2. Test for less than 1.0 ohm 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 Duct Temp Sensor Actual parameter is greater than 85°C (185°F). 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 Duct Temp Sensor Actual parameter is less than -35°C (-39°F) 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 temperature sensor.

Component Testing

1. A thermometer can be used to test the sensor off the vehicle in warn and or cold water. Refer to «Sensor Resistance Table»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__sensor-resistance-table) for this test.
2. Test the evaporator air temperature sensor by varying the sensor temperature while monitoring the sensor resistance.
3. Compare the readings with the Temperature VS Resistance table and verify that the resistance is within 5 percent of the specification. If not within the specified range, replace the sensor.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Duct Air Temperature Sensor Replacement - Lower Left Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
2. «Duct Air Temperature Sensor Replacement - Lower Right Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
3. «Ambient Air Temperature Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
4. «Inside Air Temperature Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
5. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for HVAC control module replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

Circuit Description

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are integrated into the sunload sensor assembly along with the ambient light sensor. Ground and signal circuits enable the sensors to operate. As the sunload increases, the sensor signal decreases. The sensor signal varies between 0 and 5 volts. The HVAC control module converts the signal to a range between 0 and 255 counts. The sunload sensor provides the HVAC control module with a measurement of the amount of light shining on the vehicle. Bright or high intensity light causes the vehicles interior temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.

The following DTCs are for the left and right sunload sensors.

1. B0183 is for the left sunload sensor.
2. B0188 is for the right sunload sensor.

Ignition ON.

The HVAC control module detects the signal circuit is less than 5 counts or greater than 250 counts.

1. The driver information center (DIC) will display SERVICE A/C SYSTEM.
2. The system will operate as if no sunload condition exists.

1. The DTC will become history if the HVAC control module no longer detects the condition that set the DTC.
2. The history DTC will clear after 100 fault-free ignition cycles.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)
2. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

Ignition ON, observe the appropriate scan tool Sunload Sensor parameter. The reading should be between 5 and 250 counts and change with sunlight intensity.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the sunload sensor.
2. Test for less than 10 ohms between the ground circuit terminal C and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, test for 4.8-5.2 volts between the ground circuit terminal C and the appropriate signal circuit terminal listed below: Left solar load sensor circuit terminal A. Right solar load sensor circuit terminal B. If less than the specified range, test the signal circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the HVAC control module. If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the HVAC control module.
4. If all circuits test normal, test or replace the sunload sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Sun Load Temperature Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
2. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for HVAC control module replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

Circuit/System Description

The HVAC control module controls the HVAC door actuators to regulate the airflow through the HVAC system. Each actuator consists of an electric motor and a potentiometer. The module supplies a low reference and 5-volt reference source voltage to the potentiometer. The HVAC control module monitors the voltage drop across the potentiometer on the door position signal circuit. When the actuator shaft rotates, the voltage on the door position signal circuit changes. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions.

1. B0228 is for the recirculate actuator.
2. B0413 is for the drivers air temperature actuator.
3. B0423 is for the passenger air temperature actuator.
4. B3779 is for the mode actuator.

1. Ignition ON.
2. The HVAC module is ON.

The actual door position differs from the commanded door position by more than 4 counts or the HVAC control module detects the door position signal circuit is less than 3 counts or greater than 253 counts.

The control circuit is deactivated for the appropriate actuator.

1. The DTC becomes history when the HVAC control module no longer detects the condition that set the DTC.
2. The history DTC will clear after 100 fault-free ignition cycles.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)
2. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

Ignition ON, command the appropriate actuator in both directions with a scan tool. The scan tool door position parameter reading should be between 3 and 253 counts.

1. Ignition OFF, disconnect the harness connector at the appropriate actuator.
2. Test for less than 1 ohm between the low reference circuit terminal 7 and ground. If greater than the specified range, test the low reference circuit for an open/high resistance.
3. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit terminal 10 and ground. If less than the specified range, test the 5-volt reference circuit for a short to ground or an open/high resistance. If greater than the specified range, test the 5-volt reference circuit for a short to voltage.
4. Verify the appropriate scan tool parameter is less than 4 counts. If greater than the specified range, test the signal circuit terminal 9 for a short to ground.
5. Install a 3-amp fused jumper wire between the signal circuit terminal 9 and the low reference circuit terminal. Verify the appropriate scan tool parameter is greater than 250 counts. If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance.
6. Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal 5 and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance.
7. Install a DMM between the actuator control circuit terminal 6 and ground. While monitoring the DMM, use the scan tool to command the appropriate actuator from one direction to the other. The voltage on the control circuit should move from 0 volts in one direction to 5 volts in the other direction and 2.5 volts in the at rest state. If the voltage does not change, test the control circuit for an open, short to voltage or short to ground. If circuit tests normal replace the HVAC control module.
8. If all circuits test normal, replace the actuator.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Air Inlet Valve Actuator Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
2. «Mode Valve Actuator Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
3. «Temperature Valve Actuator Replacement - Right Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
4. «Temperature Valve Actuator Replacement - Left Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

This DTC is not associated with any wiring. It is an internal fault inside the HVAC control module for a stuck or shorted control button.

Battery voltage is between 9-16 volts and data link communications operate normally.

The HVAC control module has internal fault inside the HVAC control module for a stuck or shorted control button.

1. A current DTC clears when the malfunction is no longer present.
2. A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

1. Reprogram the HVAC control module that set DTC B1023. The module should successfully reprogram. If the module will not reprogram, replace the HVAC control module that set the DTC.
2. With a scan tool, clear the DTC.
3. Ignition OFF.
4. Wait 60 seconds, then recheck for DTCs. DTC B1023 should not set. If DTC B1023 sets, replace the module.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for control module replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

The engine control module (ECM) monitors the high side refrigerant pressure through the A/C refrigerant pressure sensor. The ECM supplies a 5-volt reference and a low reference to the sensor. Changes in the A/C refrigerant pressure cause the sensor signal to the ECM to vary. When the pressure is high, the signal voltage is high. When the pressure is low, the signal voltage is low. When pressure is high, the ECM commands the cooling fans on. When pressure is too high or too low, the ECM will not allow the A/C compressor clutch to engage.

1. Engine is running.
2. Any of the conditions for setting the DTC are met for 15 seconds.
3. Battery voltage is between 11-18 volts.

1. The ECM detects that the A/C pressure is less than 1 psi (0.01 volt) sets DTC P0532.
2. The ECM detects that the A/C pressure is more than 428 psi (4.92 volts) sets DTC P0533.

1. The ECM will not illuminate the malfunction indicator lamp (MIL).
2. The ECM stores the Failure Records.
3. The A/C compressor clutch is disabled.

1. The history DTC will clear after 40 consecutive ignition cycles have occurred without a malfunction.
2. The DTC will become history if the ECM no longer detects a failure.

A malfunction within the refrigerant system causing high pressure can cause this DTC to set.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)
2. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

Ignition ON, observe the scan tool A/C Pressure Sensor parameter. The reading should be between 0.1 volts (1 psi) and 4.90 volts (425 psi) and change with A/C refrigerant pressure changes.

1. Ignition OFF, disconnect the harness connector at the A/C refrigerant pressure sensor.
2. Ignition OFF, test for less than 10 ohms between the low reference circuit terminal 1 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 ECM.
3. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit terminal 2 and ground. If less than the specified range, test the 5-volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM. If greater than the specified range, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the ECM.
4. Verify the scan tool A/C High Side Pressure Sensor parameter is less than 0.1 volts (1 psi). If greater than the specified range, test the signal circuit terminal 3 for a short to voltage. If the circuit tests normal, replace the ECM.
5. Install a 3A fused jumper wire between the signal circuit terminal 3 and the 5-volt reference circuit terminal 2. Verify the scan tool A/C High Side Pressure Sensor parameter is greater than 4.90 volts (425 psi). If less than the specified range, test the signal circuit for short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.
6. If all circuits test normal, test or replace the A/C refrigerant pressure sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement (Non-HP2)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
2. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for the ECM replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

Circuit System Description

Ignition voltage is supplied directly to the A/C compressor clutch relay. The engine control module (ECM) controls the relay by grounding the A/C clutch relay control circuit. When the ECM is commanding a component ON, the voltage of the control circuit should be near 0 volt. When the ECM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage.

1. The ignition voltage is between 11-18 volts.
2. The engine speed is greater than 600 RPM.
3. An A/C request is made.

Conditions for Setting the DTCs

The ECM detects an open on the control circuit of the A/C compressor clutch relay when commanded OFF with the engine in crank or run status.

The ECM will store conditions which were present when the DTC set as Failure Records data only. This information will not be stored as Freeze Frame data.

A history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)
2. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

Ignition ON, command the A/C Relay ON and OFF with a scan tool. Feel/listen for the A/C relay to click.

1. Ignition OFF, disconnect the A/C relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 and ground. If the test lamp illuminates, test the control circuit for a short to voltage.
3. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or a open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test all components connected to the ignition circuit and replace as necessary.
4. Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or a open/high resistance. If the circuit tests normal and B+ circuit fuse is open, replace the A/C clutch relay.
5. Disconnect the harness connector at the A/C clutch coil.
6. Test for less than 1 ohm between the ground circuit terminal 2 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
7. Connect the harness connector at the A/C clutch coil.
8. Connect a 10-amp fused jumper wire between the B+ circuit terminal 30 and control circuit terminal 87. Verify the A/C clutch is activated. If the A/C clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C clutch.
9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
10. Command the A/C relay output function ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states. If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM. If the test lamp is always OFF, test the control circuit for a short to voltage, or an open/high resistance. If the circuit tests normal, replace the ECM.
11. If all circuits test normal, test or replace the A/C clutch relay.

1. Ignition OFF, disconnect the A/C COMP relay.
2. Test for 60-180 ohms between terminals 85 and 86. If the resistance is not within the specified range, replace the relay.
3. Test for infinite resistance between the following terminals: 30 and 86 30 and 87 30 and 85 85 and 87 If not the specified value, replace the relay.
4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87. If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Relay Replacement (Attached to Wire Harness)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-introduction) or «Relay Replacement (Within an Electrical Center)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-introduction)
2. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for ECM replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

The engine control module (ECM)/powertrain control module (PCM) controls the A/C compressor clutch based on an A/C request serial data message from the HVAC control module. The ECM/PCM monitors the A/C refrigerant pressure sensor and if pressure is within a specified range, will engage the A/C compressor by grounding the A/C compressor clutch relay control circuit.

B+ is applied to the A/C relay in the RUN/START position and with the A/C compressor clutch relay control circuit grounded by the ECM/PCM, current flows through the coil side of the A/C relay. With current flowing through the coil side of the relay, a magnetic field is created which pulls the switch side of the relay closed, sending current to the A/C compressor clutch, allowing the A/C compressor to be engaged.

An A/C request is received from the HVAC module.

A short to voltage, short to ground, or open/high resistance is seen on the A/C compressor clutch relay control circuit.

DTC P0646 and DTC P0647 are Type C DTCs

DTC P0646 and DTC P0647 are Type C DTCs

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Air Temperature Description and Operation

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Engine running, command the A/C Relay On and OFF with a scan tool. The A/C compressor clutch should engage and disengage.

1. Ignition OFF, disconnect the A/C relay.
2. Ignition ON, verify a test lamp illuminates between the relay coil B+ circuit terminal and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
3. Connect a test lamp between the relay coil B+ circuit terminal and the A/C compressor clutch relay control circuit terminal.
4. Engine running, command the A/C relay ON and OFF. The test lamp should turn ON and OFF when changing between the commanded states. If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM/PCM. If the test lamp is always OFF< test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM/PCM.
5. If all circuits test normal, test or replace the A/C relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for ECM/PCM replacement, setup, and programming

Symptoms - HVAC Systems - Automatic

Visual/Physical Inspection

1. Inspect for aftermarket devices which could affect the operation of the HVAC System. Refer to «Checking Aftermarket Accessories»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__checking-aftermarket-accessories) .
2. Inspect the easily accessible or visible system components for obvious damage or conditions which could cause the symptom.
3. Verify the A/C compressor clutch turns freely and is not seized.
4. The A/C compressor will not operate in cold outside air temperatures. Refer to «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation) .
5. The following could cause window fogging: Wet carpet or mats High humidity Interior water leak Blocked A/C evaporator drain tube Maximum passenger capacity Blocked body pressure relief valves
6. Inspect the air distribution system for causes of reduced air flow: Obstructed or dirty passenger compartment air filter, if equipped Blocked or damaged air inlet or outlet vents

Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to Testing for Intermittent Conditions and Poor Connections .

Symptom List

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom

1. «Blower Motor Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
2. «Air Conditioning Compressor Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
3. «Air Temperature Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
4. «Leak Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
5. «Noise Diagnosis - HVAC Module»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning__noise-diagnosis-hvac-module)
6. «Noise Diagnosis - Air Conditioning (A/C) System»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning__noise-diagnosis-air-conditioning-ac)
7. «Noise Diagnosis - Blower Motor»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning__noise-diagnosis-blower-motor)
8. «Odor Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning__odor-diagnosis)

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

When the A/C switch is pressed, the HVAC control module grounds the A/C request signal circuit. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

The following conditions must be met in order for the ECM to turn on the compressor clutch

1. Battery voltage is between 9-18 volts
2. Engine coolant temperature (ECT) is less than 123°C (253°F)
3. Engine speed is greater than 600 RPM
4. Engine speed is less than 4,760 RPM
5. A/C high side pressure is between 2929-269 kPa (39-425 psi)
6. Throttle position is less than 100 percent
7. Evaporator temperature is greater than 0°C (32°F)
8. ECM does not detect excessive torque load
9. ECM does not detect insufficient idle quality
10. The ambient temperature is above 1°C (34°F)

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)
2. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, press the A/C request switch. The A/C compressor clutch should engage.
2. Place the mode switch in the defrost position. The A/C compressor clutch should engage.

1. Ignition OFF, disconnect the harness connector at the A/C low pressure switch.
2. Test for less than 5 ohms between the ground circuit terminal B and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify the scan tool Low Pressure Switch parameter is Open. If not the specified value, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the HVAC control module
4. Connect a 3A fused jumper wire between the signal circuit terminal A and ground. Verify the scan tool Low Pressure Switch parameter is Closed. If not the specified value, 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. Connect the harness connector at the A/C low pressure switch. Verify the scan tool Low Pressure Switch parameter is Closed. If not the specified value, test or replace the A/C low pressure switch.
6. Ignition OFF, disconnect the A/C relay.
7. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 and ground. If the test lamp illuminates, test the control circuit for a short to voltage.
8. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground. If the test lamp does not illuminate, test the ignition circuit for a short to ground or a open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test all components connected to the ignition circuit and replace as necessary.
9. Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or a open/high resistance. If the circuit tests normal and B+ circuit fuse is open, replace the A/C clutch relay.
10. Disconnect the harness connector at the A/C clutch coil.
11. Test for less than 1 ohm between the ground circuit terminal 2 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
12. Connect the harness connector at the A/C clutch coil.
13. Connect a 10A fused jumper wire between the B+ circuit terminal 30 and control circuit terminal 87. Verify the A/C clutch is activated. If the A/C clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C clutch.
14. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
15. Command the A/C relay output function ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states. If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM. If the test lamp is always OFF, test the control circuit for a short to voltage, or an open/high resistance. If the circuit tests normal, replace the ECM.
16. If all circuits test normal, test or replace the A/C clutch relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Air Conditioning Compressor Replacement (LLY/LMM)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning) , «Air Conditioning Compressor Replacement (LU3)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning) , «Air Conditioning Compressor Replacement (LY2, LY5, LH6, L76, L92 and LMG)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning) or «Air Conditioning Compressor Replacement (HP2)»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
2. «Compressor Clutch Plate/Hub Assembly Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
3. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for HVAC control module and ECM replacement, setup, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

The blower motor control module is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive and ground circuits enable the control module to operate. The HVAC control module provides a variable voltage to the blower motor control module to request the selected blower speed. The blower motor control module provides a pulse width modulation (PWM) signal to the blower motor in order to control the blower motor speed. The module supplies 12 volts to the blower motor through the blower motor voltage supply circuit.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/manual-hvac-system/#hvac-system-manual__air-delivery-description-and-operation)
2. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/manual-hvac-system/#hvac-system-manual__air-temperature-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

1. Ignition OFF, disconnect the X2 harness connector at the blower motor control module.
2. Test for less than 1.0 ohm between the ground circuit terminal 5 and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 6 and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse (HVAC BLWR 40-amp) in the underhood fuse block is open, test all components connected to the B+ circuit and replace as necessary.
4. Connect a DMM between the control circuit terminal 3 and ground.
5. Cycle the blower motor from OFF to HIGH. The voltage should increase while changing between the commanded states. If voltage remains greater than 11.5 volts while changing between the commanded states, test the control circuit for a short to voltage. If the circuit test normal, replace the HVAC control module. If voltage remains less than 1.0 volts while changing between the commanded states, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the HVAC control module. If voltage is between 1.0-11.5 volts and does not increase when changing between the commanded states, replace the HVAC control module.
6. Ignition OFF, connect the X2 harness connector at the blower motor control module. Disconnect the harness connector at the blower motor.
7. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal A and ground. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the blower motor control module.
8. Connect a test lamp between the control circuit terminal B and the B+ circuit terminal A.
9. Turn the blower motor on high speed then to low speed. The test lamp intensity should be bright when in high speed and very dim or off when the blower is on low speed. If the test lamp intensity does not vary, test the control circuit for a short to voltage, short to ground, or an open/high resistance. If the circuit tests normal, replace the blower motor control module.
10. If all circuits test normal, test or replace the blower motor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Blower Motor Control Module Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
2. «Blower Motor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#heating-ventilation-and-air-conditioning)
3. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references) for ECM, and HVAC control module setup, replacement, and programming

1. Perform the «Diagnostic System Check - Vehicle»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-system-check-vehicle) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__strategy-based-diagnosis) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/oem-general-information/#vehicle-diagnostic-information__diagnostic-procedure-instructions) provides an overview of each diagnostic category.

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

1. Ambient Air Temperature Sensor
2. Inside Air Temperature Sensor Assembly
3. Upper Left Air Temperature Sensor
4. Upper Right Air Temperature Sensor
5. Lower Left Air Temperature Sensor
6. Lower Right Air 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 and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

Sun Load Sensor Assembly

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are integrated into the sunload sensor assembly along with the ambient light sensor. Ground and signal circuits enable the sensors to operate. As the sunload increases, the sensor signal decreases. The sensor signal varies between 0 and 5 volts. The HVAC control module converts the signal to a range between 0 and 255 counts. The sunload sensor provides the HVAC control module with a measurement of the amount of light shining on the vehicle. Bright or high intensity light causes the vehicles interior temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.

Ambient Air Temperature Display Update

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

1. «Air Delivery Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-delivery-description-and-operation)
2. «Air Temperature Description and Operation»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__air-temperature-description-and-operation)

Electrical Information Reference

1. «Circuit Testing»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__circuit-testing)
2. «Connector Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__connector-repairs)
3. «Testing for Intermittent Conditions and Poor Connections»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__testing-for-intermittent-conditions-and-poor)
4. «Wiring Repairs»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/ignition-switchsteering-lock/#wiring-systems-and-power-management-service-troubleshooting-diagnosis__wiring-repairs)

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, cover the sunload sensor assembly with a shop towel or other suitable item. Verify the scan tool Right Sunload Sensor and Left Sunload Sensor parameters are within 3 counts of each other. If not within the specified range, refer to «Sun Load Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__sun-load-sensor-circuit-malfunction) .
2. 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 Air Temperature Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__ambient-air-temperature-sensor-circuit-malfunction) .
3. Measure actual inside air temperature using a thermometer. Compare this value to the scan tool Inside Air Temp. Sensor parameter and Aux. Inside Air Temp. parameter. The measured temperature should be within 5 degrees of parameter. If not within the specified range, refer to «Inside Air Temperature Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__inside-air-temperature-sensor-circuit-malfunction) .
4. Measure actual temperature using a thermometer at each of the duct temperature sensors. Compare this value to the appropriate scan tool Duct Actual parameter. The measured temperature should be within 5 degrees of appropriate Duct Actual parameter. If not within the specified range, refer to «Duct Temperature Sensor Circuit Malfunction»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic) .

Sun Load Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the sunload sensor.
2. Test for less than 10 ohms between the ground circuit terminal C and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, test for 4.8-5.2 volts between the ground circuit terminal C and the appropriate signal circuit terminal listed below: Left solar load sensor circuit terminal A. Right solar load sensor circuit terminal B. If less than the specified range, test the signal circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the HVAC control module. If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the HVAC control module.
4. If all circuits test normal, test or replace the sunload sensor.

Ambient Air Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the ambient temperature sensor.
2. Test for less than 1 ohm 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 scan tool Outside Air Temp. Raw parameter is greater than 85°C (185°F). 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 Outside Air Temp. Raw parameter is less than -35°C (-39°F). 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 ambient temperature sensor.

Inside Air Temperature Sensor Circuit Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate inside air temperature sensor.
2. Test for less than 1 ohm between the ground circuit terminal C and ground. If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify a scan tool illuminates between the control circuit terminal B and ground. If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance.
4. Ignition OFF, test for less than 1 ohm 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.
5. Ignition ON, verify the appropriate scan tool Inside Temp Sensor parameter is greater than 85°C (185°F). 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.
6. 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 -35°C (-39°F). 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.
7. If all circuits test normal, test or replace the appropriate temperature sensor.

1. Ignition OFF, disconnect the harness connector at the appropriate temperature sensor.
2. Test for less than 1.0 ohm 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 Duct Temp Sensor Actual parameter is greater than 85°C (185°F). 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 Duct Temp Sensor Actual parameter is less than -35°C (-39°F). 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 temperature sensor.

1. Ignition OFF, disconnect the harness connector at the appropriate temperature 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»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic__sensor-resistance-table) for resistance values.

1. If not within the specified range, replace the appropriate sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

1. «Duct Air Temperature Sensor Replacement - Lower Left Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
2. «Duct Air Temperature Sensor Replacement - Lower Right Side»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
3. «Ambient Air Temperature Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
4. «Sun Load Temperature Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/automatic-hvac-system/#hvac-system-automatic)
5. «Ambient Light Sensor Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exterior-lights/#lighting-systems)
6. «Control Module References»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/communication-devices/#programming-and-setup-all-systems__control-module-references)

Afterblow Enabling

The afterblow mode can be enabled using the scan tool. The afterblow mode allows the blower motor to operate after the engine has been turned off. This operation of the blower motor dries the evaporator core, which reduces the amount of microbial growth which can create undesirable odors.

Use the following procedure in order to enable the afterblow mode

1. Connect the Scan Tool.
2. Turn ON the ignition, with the engine OFF.
3. Build the vehicle with the scan tool.
4. Select HVAC Control Module
5. Select Module Setup.
6. Select Afterblow Option.
7. Upon selecting the afterblow option a asterix will appear next to the word afterblow meaning the function is enabled.
8. Select the save option in the lower left of the screen.

When afterblow has been enabled by the scan tool the blower motor will operate at 68 percent blower speed for 2 minutes 30 seconds to 4 minutes after the engine has been in Key OFF for 30 minutes.

The following conditions must be met for the HVAC module to operate the afterblow

1. The outside air temperature must be at least 21°C (70°F).
2. The A/C compressor must have operated during the latest key cycle.
3. The engine must be turned off for at least 30 minutes.
4. The system voltage must be at least 12 volts.

Actuator Recalibration

When replacing the HVAC control module it will be necessary to allow the HVAC control module to perform a calibration process. When installing the HVAC control module be sure to perform the following

1. Place the ignition switch to the OFF position.
2. Disconnect the scan tool.
3. Install the HVAC control module.
4. Connect all previously disconnected components.
5. Start the vehicle.
6. Wait 40 seconds for the HVAC control module to self-calibrate.
7. Verify that no DTCs have set as current DTCs.

When replacing the HVAC actuator it will be necessary to allow the HVAC control module to perform a calibration process. When installing the HVAC actuator be sure to perform one of the following

Preferred Method (w/Scan Tool)

1. Clear all DTCs.
2. Place the ignition switch in the OFF position.
3. Install the HVAC actuator.
4. Connect all previously disconnected components.
5. Start the vehicle.
6. With the scan tool, initiate the Motor Re-calibration feature of the Heating and Air Conditioning Special Functions menu.
7. Verify that no DTCs have set as current DTCs.

Alternate Method (w/o Scan Tool)

1. Clear all DTCs.
2. Place the ignition switch to the OFF position.
3. Install the HVAC actuator.
4. Connect all previously disconnected components.
5. Remove the HVAC/ECAS fuse for a minimum of 10 seconds.
6. Install the HVAC/ECAS fuse.
7. Start the vehicle.
8. Wait 40 seconds for the HVAC control module to self-calibrate.
9. Verify that no DTCs have set as current DTCs.

Scheme 39

Scheme 40

Scheme 41

Scheme 42

Scheme 43

Scheme 44

Scheme 45

Scheme 46

Removal Procedure

1. Remove the left windshield garnish molding. Refer to «Windshield Pillar Garnish Molding Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
2. Remove the left center pillar assist handle. Refer to «Roof Rail Rear Assist Handle Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
3. Remove the center pillar garnish molding. Refer to «Center Pillar Upper Garnish Molding Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
4. Remove the left sunshade. Refer to «Sunshade Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
5. Gently pull down the headliner.
6. Disconnect the electrical connector from the inside air temperature sensor.
7. Partially remove the inside air temperature sensor from the headliner.
8. Remove the sensor grille from the inside air temperature sensor.
9. Remove the inside air temperature sensor from the headliner.

Installation Procedure

1. Install the new sensor grille to the headliner.
2. Install the inside air temperature sensor to the headliner and sensor grille.
3. Connect the electrical connector to the inside air temperature sensor.
4. Install the left sunshade. Refer to «Sunshade Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
5. Install the left center pillar garnish molding. Refer to «Center Pillar Upper Garnish Molding Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
6. Install the left center assist handle. Refer to «Roof Rail Rear Assist Handle Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .
7. Install the left windshield garnish molding. Refer to «Windshield Pillar Garnish Molding Replacement»(/chevrolet/cab-chassis-silverado-3500-hd/2008-2011/remont/exteriorinterior-trim/#interior-trim-and-paneling) .

Scheme 47

Air Delivery Description and Operation

The air delivery description and operation is divided into 7 areas

1. HVAC Control Components
2. Air Speed
3. Auxiliary Air Speed
4. Air Delivery
5. Auxiliary Air Delivery
6. Recirculation Operation
7. Automatic Operation

HVAC Control Module

The HVAC control module is a GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features

Auxiliary HVAC Control Module

The auxiliary HVAC control module is a GMLAN device that receives analog Inputs from the front HVAC control assembly and GMLAN message inputs from the rear seat audio (RSA) control module to maintain auxiliary air temperature and auxiliary air distribution settings. The battery positive voltage circuit provides power that the control module uses for KAM. If the battery positive voltage circuit loses power, all auxiliary HVAC DTCs and settings will be erased from KAM. The auxiliary HVAC control module will perform a recalibration of the electric actuators when commanded with a scan tool or if KAM is lost. This will ensure the actuators are moving with in the calibrated range.

RSA Control Module

The RSA Control Module is a GMLAN device that interfaces between the rear seat passengers and the auxiliary HVAC control module. The battery positive voltage circuit provides power that the control module uses for KAM. Air temperature, blower motor and air delivery rocker type switches are located on the face of the module.

These switches enable the module to send a GMLAN serial data message to the auxiliary HVAC control module for the change request. The module does not store auxiliary HVAC DTCs.

Mode Actuator

The mode actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Auxiliary Mode Actuator

The auxiliary mode actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts.

The auxiliary HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module.

Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Blower Motor Control Processor

The blower motor control module is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive and ground circuits enable the control module to operate. The HVAC control module provides a variable voltage to the blower motor control module to request the selected blower speed. The blower motor control module provides a pulse width modulation (PWM) signal to the blower motor in order to control the blower motor speed. The module supplies 12 volts to the blower motor through the blower motor voltage supply circuit.

Auxiliary Blower Motor Control Processor

The auxiliary blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the auxiliary blower motor. The auxiliary HVAC control module or rear seat audio module (RSA) provides a voltage signal to the auxiliary blower motor control processor over the auxiliary blower motor speed control circuit. As the requested blower speed increases, the auxiliary HVAC control module or rear seat audio module (RSA) increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the auxiliary HVAC control module or rear seat audio module (RSA) decreases the amount of time that the signal is modulated to ground.

Air Speed - Front Control

The blower control switch is integrated into the HVAC control module. The 2 rocker type switches provide the vehicle operator the ability to select several blower speeds. The HVAC control module uses a bar graph type display to indicate the selected blower speed. The blower motor control module is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive and ground circuits enable the control module to operate. The HVAC control module provides a variable voltage to the blower motor control module to request the selected blower speed. The blower motor control module provides a pulse width modulation (PWM) signal to the blower motor in order to control the blower motor speed. The module supplies 12 volts to the blower motor through the blower motor voltage supply circuit. When the HVAC control module is operating in AUTO mode, the system automatically controls the blower speed. If an OnStar® call is initiated while the blower is in the high speed position the blower speed will be lowered to reduce background noise.

Air Speed - Auxiliary

A rocker type switch on the RSA control module provides the operator the ability to select several blower speeds. The RSA control module uses a bar graph to indicate the selected blower speed. The auxiliary blower motor control processor supplies a 5-volt reference to the auxiliary HVAC control module on the auxiliary blower motor speed control circuit. The auxiliary HVAC control module pulls the 5-volt reference low towards ground to provide a blower motor speed signal. As the voltage decreases the blower speed increases. Power and ground are provided to the auxiliary blower motor control processor through the battery positive voltage and ground circuits. The auxiliary HVAC control module can not request A/C operation from the powertrain control module (PCM).

Air Distribution

The HVAC control module controls the distribution of air by the use of a mode actuator. The modes that may be selected are

1. Defrost
2. Defog
3. Panel
4. Bi-Level
5. Floor

The mode actuator is connected to the mode door by a cam type linkage system. Depending on the position of the door, air is directed through the HVAC module and distributed through various ducts leading to the outlets in the dash. If the HVAC control module detects a fault with the mode door the HVAC control module will try to drive the actuator for a predetermined amount of time, to defrost, which is the defaulted position for the mode door actuator. When the mode switch is placed in the defrost or defog positions the A/C is commanded on and the recirculation door is moved to the outside air position to help reduce window fogging. A/C is available in all modes and recirculation is only available in the panel and bi-level modes.

The mode actuator is an electronic stepper motor with feedback potentiometers. The HVAC control module sends signals to the mode door actuator through the mode door control circuit. Zero volts drive the actuator in one direction while 5 volts move the actuator in the opposite direction. When the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the mode actuator. When you select a desired mode setting, logic determines the value of the mode actuator signals. The HVAC control modules software uses this reference voltage in order to determine the position of the mode actuator through the mode door position signal circuit. The motor moves the mode door to the desired position.

Front Defrost

When defrost is selected, the A/C compressor is activated. The A/C compressor clutch will engage when ambient temperatures are above 3°C (38 F). The blower motor will be activated, regardless of the coolant temperature. The HVAC control module will override the auxiliary HVAC control module so a high volume of air is delivered to the front defrost vents. The rear window defogger does not affect the HVAC system.

Air Distribution - Auxiliary Control

The auxiliary HVAC system provides ventilation for the rear seat occupants. The rear seat occupants will exercise control of the auxiliary air delivery modes, air speed and air temperature setting.

When the auxiliary mode switch is toggled, a signal is sent to the auxiliary mode actuator through the auxiliary mode door control circuit. Power and ground are supplied to the auxiliary mode actuator through the ignition 3 voltage and ground circuits.

A rocker type switch on the RSA control module provides the operator the ability to select several blower speeds. The RSA control module uses a bar graph to indicate the selected blower speed.

Recirculation Operation

The HVAC control module controls the air intake through the recirculation actuator. The recirculation switch closes the recirculation door in order to circulate the air within the vehicle. The outside air switch opens the recirculation door in order to route outside air into the vehicle. Regardless of the blower motor switch position, recirculation is available only in the panel and bi-level mode switch positions. Including the OFF position. The mode switch must be placed in either the panel or bi-level position before the blower motor switch is placed in the OFF position. In order to reduce windshield fogging, outside air is circulated when the mode switch is in the defrost or defog positions. If the recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, then the recirculation switch LED will flash 3 times. If the HVAC control module detects a fault with the recirc door the HVAC control module will try to drive the actuator for a predetermined amount of time, to outside air, which is the defaulted position for the recirculation actuator.

Automatic Operation

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and circulation.

To place the HVAC system in Automatic mode, the following is required

1. The Auto switch must be activated
2. The air temperature switch must be in any other position other than full hot or full cold position

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature

Monitor the following sensors

1. Inside Air Temperature Sensor
2. Ambient Air Temperature Sensor
3. Lower Left Air Temperature Sensor
4. Lower Right Air Temperature Sensor
5. Upper Left Air Temperature Sensor
6. Upper Right Air Temperature Sensor Regulate blower motor speed Position the air temperature actuator Position the mode actuator Position the recirculation actuator Request A/C operation

Remote Start Activation

The following describes the HVAC control head functionality upon receiving the remote start active serial data message and a power mode status is set to off/awake. The outside air temperature (OAT) sensor on automatic systems, is utilized to determine the moding of the following features: Blower Mode, Bi-level, Floor, Defrost, Recirc and Rear Defrost. Calibratable temperatures are utilized to provide low, mid and high temperature ranges for the selectable feature. Blower and mode have the range to select any valid feature positions. When the remote start active serial data is received, ignition-OFF loop HVAC algorithms will act the same as if a run power mode was received, i.e. re-calibration, afterblow, ignition-OFF motor positioning etc. The OFF timer and the ignition ON timer have the functionality as in the Run mode.

The remote start system operates in one of three zones of operation

1. Cold ambient temperature that is below 5°C (40°F)
2. Hot ambient temperature that is above 30°C (86°F)
3. Mid ambient temperature that is anything between the cold and hot values

1. In cold ambient, the system uses 74°F setpoint, auto blower, defrost mode, rear defrost and/or heated mirrors on, heated seats on, rear in mimic.
2. In mid ambient, the system uses 74°F setpoint, auto blower, auto mode, rear in mimic.
3. In hot ambient, the system uses 74°F setpoint (60°F setpoint in 2008), auto blower, auto mode, rear in mimic.

Remote Start De-activation

When the remote start is exited, GMLAN power mode equals run mode or remote start engine. In the case of automatic settings, the displays will revert to the actual state of operation of the features, on the basis of their normal control algorithms.

Air Temperature Description and Operation

The air temperature controls are divided into 7 areas

1. HVAC Control Components
2. Heating and A/C Operation
3. Auxiliary Heating and A/C Operation
4. Automatic Operation
5. Auxiliary Automatic Operation
6. Engine Coolant A/C Cycle

HVAC Control Components

The HVAC control module is a GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features

The auxiliary HVAC control module is a GMLAN device that receives analog inputs from the front HVAC control assembly and GMLAN message inputs from the rear seat audio control module to maintain auxiliary air temperature and auxiliary air distribution settings. The battery positive voltage circuit provides power that the control module uses for KAM. If the battery positive voltage circuit loses power, all auxiliary HVAC DTCs and settings will be erased from KAM. The auxiliary HVAC control module will perform a recalibration of the electric actuators when commanded with a scan tool or if KAM is lost. This will ensure the actuators are moving with in the calibrated range.

Rear Seat Audio Control Module

The rear seat audio control module is a GMLAN device that interfaces between the rear seat passengers and the auxiliary HVAC control module. The battery positive voltage circuit provides power that the control module uses for KAM. Air temperature, blower motor and air delivery rocker type switches are located on the face of the module. Pressing a switch enables the module to send a GMLAN serial data message to the auxiliary HVAC control module for the change request. The module does not store auxiliary HVAC DTCs.

Air Temperature Actuator

The air temperature actuators are a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts. The auxiliary HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Air Temperature Sensor

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

1. Ambient Air Temperature Sensor
2. Inside Air Temperature Sensor Assembly
3. Upper Left Air Temperature Sensor
4. Upper Right Air Temperature Sensor
5. Lower Left Air Temperature Sensor
6. Lower Right Air Temperature Sensor
7. Upper Air Temperature Sensor - Auxiliary
8. Lower Air Temperature Sensor - Auxiliary

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 input of the duct air temperature sensors are different from the ambient and inside air temperature sensors. 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 and inside air temperature sensors will be displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is corrected.

The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display. The ambient air temperature value is updated under the following conditions

The scan tool has the ability to update the displayed ambient air temperature. To update the ambient air temperature display on the HVAC control module, perform the following procedure: Turn ON the ignition. Simultaneously press the A/C and Recirc switches for 5 seconds.

Sunload Sensor Assembly

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The 2 sensors are integrated into the sunload sensor assembly. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor resistance increases. The sensor signal decreases as the resistance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts. The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload value. This value will not 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 fixed. The scan tool parameter for the sunload sensor is the actual state of the signal circuit.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts. The powertrain control module (PCM) converts the voltage signal to a pressure value.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high pressure condition exists. The PCM disables the compressor clutch if the A/C pressure is more than 2957 kPa (429 psi). The clutch will be enabled after the pressure decreases to less than 1578 kPa (229 psi).

The A/C low pressure switch protects the A/C system from a low pressure condition that could damage the A/C compressor or cause evaporator icing. The HVAC control module applies 12 volts to the A/C low pressure switch signal circuit. The switch will open when the A/C low side pressure reaches 151 kPa (22 psi). This prevents the A/C compressor from operating. The switch will then close when A/C low pressure side reaches 275 kPa (40 psi). This enables the A/C compressor to turn back ON.

Heating and A/C Operation

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. The vehicle passenger can offset the passenger temperature as much as 16.7°C (30°F). Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature

1. Recirculation actuator setting
2. Difference between inside and desired temperature
3. Difference between ambient and desired temperature
4. Blower motor speed setting
5. Mode setting
6. Auxiliary HVAC settings

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected

1. When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.
2. When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature

1. Sunload
2. Duct temperatures
3. Ambient temperature
4. Inside temperature

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a GMLAN message to the powertrain control module (PCM) for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn ON if they are not all met and an A/C request is made the LED on the A/C switch will flash

1. Ambient air temperature above 7°C (45°F)
2. A/C low pressure switch signal circuit is grounded
3. A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)
4. PCM receives an A/C request from the HVAC control module
5. Engine coolant temperature (ECT) is less than 121°C (250°F)
6. The engine RPM is more than 550 RPM
7. The throttle position is less than 100 percent

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa (429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229 psi), the PCM enables the compressor to operate.

The sensor information is used by the PCM to determine the following

1. The A/C high side pressure
2. An A/C system load on the engine
3. An excessive A/C high side pressure
4. The heat load at the A/C condenser

Once engaged, the compressor clutch will be disengaged for the following conditions

1. Ambient air temperature is less than 7°C (45°F)
2. Throttle position is 100 percent
3. The A/C low pressure switch is open
4. A/C high side pressure is more than 2957 kPa (429 psi)
5. A/C low side pressure is less than 151 kPa (22 psi)
6. ECT is more than 121°C (250°F)
7. Engine speed is more than 5500 RPM
8. Transmission shift
9. PCM detects excessive torque load
10. PCM detects insufficient idle quality
11. PCM detects a hard launch condition

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

To place the HVAC system in Automatic mode, the following is required

1. The Auto switch must be activated
2. The air temperature switch must be in any other position other than full hot or full cold position

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature

Monitor the following sensors

1. Inside Air Temperature Sensor
2. Ambient Air Temperature Sensor
3. Lower Left Air Temperature Sensor
4. Lower Right Air Temperature Sensor
5. Upper Left Air Temperature Sensor
6. Upper Right Air Temperature Sensor
7. Regulate blower motor speed.
8. Position the air temperature actuator.
9. Position the mode actuator.
10. Position the recirculation actuator.
11. Request A/C operation.

In automatic operation, the auxiliary HVAC control module will maintain the comfort level inside of the vehicle by controlling the auxiliary blower motor, the auxiliary air temperature actuator and the auxiliary mode actuator.

To place the auxiliary HVAC system in Automatic mode, the following is required

1. The auxiliary blower motor switch on the auxiliary HVAC control assembly must be in the Auto position.
2. The auxiliary mode switch on the auxiliary HVAC control assembly must be in the Auto position.
3. The auxiliary air temperature switch must be in any other position other than full hot or full cold position.

Once the desired temperature is reached, the auxiliary blower motor, auxiliary mode and auxiliary temperature actuator will automatically be adjusted to maintain the temperature selected. The auxiliary HVAC control module performs the following functions to maintain the desired air temperature

1. Upper Air Temperature Sensor - Auxiliary
2. Lower Air Temperature Sensor - Auxiliary
3. Regulate auxiliary blower motor speed
4. Position the auxiliary air temperature actuator
5. Position the auxiliary mode actuator

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal engine operating coolant temperature. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation. Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

The following describes the HVAC control head functionality upon receiving the remote start active serial data message and a power mode status is set to off/awake. The outside air temperature (OAT) sensor on automatic systems, is utilized to determine the moding of the following features: Blower Mode, Bi-level, Floor, Defrost, Recirc and Rear Defrost. Calibratable temperatures are utilized to provide low, mid and high temperature ranges for the selectable feature. Blower and mode have the range to select any valid feature positions. When the remote start active serial data is received, ignition-OFF loop HVAC algorithms will act the same as if a run power mode was received, i.e. re-calibration, afterblow, ignition-OFF motor positioning etc. The OFF timer and the ignition ON timer have the functionality as in the Run mode.

The remote start system operates in one of three zones of operation

1. Cold ambient temperature that is below 10°C (50°F)
2. Hot ambient temperature that is above 30°C (86°F)
3. Mid ambient temperature that is anything between the cold and hot values

1. In cold ambient, the system uses 74°F setpoint, auto blower, defrost mode, rear defrost and/or heated mirrors on, heated seats on, rear in mimic.
2. In mid ambient, the system uses 74°F setpoint, auto blower, auto mode, rear in mimic.
3. In hot ambient, the system uses 74°F setpoint in 2007, in 2008 and beyond a 60°F setpoint will be used, auto blower, auto mode, rear in mimic.

When the remote start is exited, GMLAN power mode equals run mode or remote start engine. In the case of automatic settings, the displays will revert to the actual state of operation of the features, on the basis of their normal control algorithms.

Special Tools

Illustration Tool Number/Description J 43600 ACR 2000 Air Conditioning Service Center

Scheme 48

See also:
• Diagnostic System Check - Vehicle
• Strategy Based Diagnosis
• Diagnostic Procedure Instructions
• Component Connector End Views
• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs
• Control Module References
• Diagnostic Repair Verification
• Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement (Non-HP2)
• Relay Replacement (Attached to Wire Harness)
• Powertrain Diagnostic Trouble Code (DTC) Type Definitions
• Checking Aftermarket Accessories
• Noise Diagnosis - HVAC Module
• Noise Diagnosis - Air Conditioning (A/C) System
• Noise Diagnosis - Blower Motor
• Odor Diagnosis
• Air Delivery Description and Operation
• Air Temperature Description and Operation
• Ambient Light Sensor Replacement
• Instrument Panel Accessory Trim Plate Replacement (with RPO SLT)
• Accessory Switch Replacement (with RPO SLT)
• Fastener Caution
• Instrument Panel Inflatable Restraint Module Replacement
• Actuator Recalibration
• Radio Front Floor Console Speaker Replacement
• Radiator Grille Replacement (Chevrolet)
• Windshield Pillar Garnish Molding Replacement
• DTC B0158, B0163, B0173, B0178, B0509, or B0514
• Air Temperature Description and Operation
• Air Delivery Description and Operation
• Ambient/Inside Air Temperature Sensor Circuit Malfunction
• Duct Temperature Sensor Circuit Malfunction
• Sensor Resistance Table
• Sun Load Sensor Circuit Malfunction
• Ambient Air Temperature Sensor Circuit Malfunction
• Inside Air Temperature Sensor Circuit Malfunction
• Actuator Recalibration