HVAC Systems - Automatic: Overview Chevrolet Silverado 1500 HD

Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 3. Lack of communication may be due to a partial malfunction of the class 2 serial data circuit or due to a total malfunction of the class 2 serial data circuit. The specified procedure will determine the particular condition.
2. 5. Determine if the HVAC Control Module, Body Control Module, or Powertrain Control Module have set DTC's which may affect HVAC operation are present.
3. 6. The presence of DTCs which begin with "U" indicate some other module is not communicating. The specified procedure will compile all the available information before tests are performed.
4. 7. Answer Yes if the first three characters of the DTC name begins with B10; regardless of the last two characters.

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The number below refers to the step number on the diagnostic table.

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1. 2. Verifies that the condition that set the DTC is present.

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The numbers below refer to the step numbers on the diagnostic table.

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1. 2. Verifies that the temperature displayed is not within the calibrated range.
2. 3. Tests for the proper operation of the circuit in the high voltage range.
3. 4. Tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.

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The numbers below refer to the step numbers on the diagnostic table.

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1. 2. Verifies that the value displayed is within the specified range.
2. 3. Tests for the proper operation of the circuit in the high voltage range.
3. 4. Tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.

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The numbers below refer to the step numbers on the diagnostic table.

1. 3. Tests for the proper operation of the circuit in the low voltage range.
2. 4. Tests for the proper operation of the circuit in the high voltage range. If the jumper fuse opens when you perform this test, the circuit is shorted to ground.
3. 5. Tests for a short to voltage in the 5-volt reference circuit.
4. 6. Tests for a high resistance or for an open in the low reference circuit.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2. Determines if the HVAC door actuator is stalled.
2. 3. Applies control circuit voltage to the input of the HVAC door actuator.

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The numbers below refer to the step numbers on the diagnostic table.

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1. 3. Tests for the proper operation of the circuit in the low voltage range.
2. 4. Tests for the proper operation of the circuit in the high voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to ground.
3. 5. Tests for a short to voltage in the 5-volt reference circuit.
4. 6. Tests for a high resistance or an open in the low reference circuit.

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Symptoms - HVAC Systems-Automatic

1. Perform the «Diagnostic System Check - HVAC System - Automatic»(ref-188963-S07260196812005091100000) before using the Symptom Tables in order to verify that all of the following are true: There are no DTCs set. The control module(s) can communicate via the serial data link.
2. Review the system operation in order to familiarize yourself with the system function. Refer to the following information: «Air Delivery Description and Operation»(ref-188963-S29619719582005091100000) «Air Temperature Description and Operation»(ref-188963-S36666053962005091100000)

The numbers below refer to the step numbers on the diagnostic table.

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1. 2. The A/C compressor relay output is disabled if engine coolant temperature is above 121°C (250°F). The engine coolant indicator will illuminate at this temperature.
2. 3. This step ensure that the HVAC control module is receiving an input from the A/C switch.
3. 4. These actions will enable the A/C compressor to operate.
4. 5. This test ensures that there is sufficient refrigerant in the A/C system. The specific values come from the A/C System Performance Test in Heating, Ventilation and Air Conditioning.
5. 7. The A/C low pressure switch parameter is out of range when the HVAC control module interprets the signal being below 151 kPa (22 psi).
6. 8. This action will simulate a closed switch condition. If the Pressure Cycle Switch parameter reads Low Pressure than there is a circuit condition or a condition with the HVAC control module.
7. 11. The A/C compressor relay output from the powertrain control module (PCM) is disabled if the A/C high side system pressure is interpreted to be higher than 2958 kPa (429 psi).

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The number below refers to the step number on the diagnostic table.

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1. These action will disable the HVAC control module output to powertrain control module (PCM) or engine control module (ECM).

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The numbers below refer to the step numbers on the diagnostic table.

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1. 6. The ambient air temperature must be above 5°C (40°F) in order for this A/C compressor test to be run.
2. 8. This step ensures that the lower air temperature sensors operate properly. The LH and RH Heater Duct Actual parameter count values should lower as the air temperature of the ducts get warmer. If there is high resistance in either of the air temperature sensors circuits or the sensor itself then the count value of the effected sensor would remain high.
3. 9. This step ensures that the upper air temperature sensors operate properly. The LH and RH AC Duct Actual parameter count values should lower as the air temperature of the ducts get warmer. If there is a high resistance in either of the air temperature sensors circuits or the sensor itself then the count value of the effected sensor would remain high.
4. 11. This action ensures that the internal fan of the inside air temperature sensor assembly is providing air flow across the internal sensor.
5. 14. Use the following table to determine that the recorded scan tool values are within range for the recorded probe temperatures of the J 43600.

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The numbers below refer to the step numbers in the diagnostic table.

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1. 8. This step ensures that the lower air temperature sensors operate properly. The LH and RH Heater Duct Actual parameter count values should lower as the air temperature of the ducts get warmer. If there is a high resistance in either of the air temperature sensors circuits or the sensor itself then the count value of the effected sensor would remain high.
2. 9. This step ensures that the upper air temperature sensors operate properly. The LH and RH AC Duct Actual parameter count values should lower as the air temperature of the ducts get warmer. If there is a high resistance in either of the air temperature sensors circuits or the sensor itself then the count value of the effected sensor would remain high.
3. 11. This action ensures that the internal fan of the inside air temperature sensor assembly is providing air flow across the internal sensor.
4. 15. Use the following table to determine that the recorded scan tool values are within range for the recorded probe temperatures.

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1. The condition may be intermittent. Refer to «TESTING WIRING SYSTEMS FOR INTERMITTENT & POOR CONNECTIONS»(ref-170169) .
2. Inspect the air delivery system for the following conditions: A dirty HVAC air filter, if applicable An obstruction to the airflow Air leaks Misaligned air ducts Broken or binding linkages or doors

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Re-Calibrating Actuators

When replacing the HVAC control module it will be necessary to allow the HVAC control module to perform a re-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. Re-connect all previously disconnected components.
5. Start the vehicle.
6. Wait 40 seconds for the HVAC control module to re-calibrate.
7. Verify that no DTCs have set as current DTCs.

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

Air Delivery Description and Operation

The air delivery description and operation is divided into 5 areas

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

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 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

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

Air Temperature Description and Operation

The air temperature controls are divided into 5 areas

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

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 effect 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

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.
3. 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: Sunload Duct temperatures Ambient temperature 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 class 2 message to the 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

1. The ambient air temperature is above 4°C (40°F).
2. The A/C low pressure switch signal circuit is grounded.
3. The A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi).
4. The PCM receives an A/C request from the HVAC control module.
5. The 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%.

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. The ambient air temperature is less than 4°C (40°F).
2. The throttle position is 100%.
3. The A/C low pressure switch is open.
4. The A/C high side pressure is more than 2957 kPa (429 psi).
5. The A/C low side pressure is less than 151 kPa (22 psi).
6. The engine coolant temperature (ECT) is more than 121°C (250°F).
7. The engine speed is more than 5500 RPM.
8. Transmission shift
9. The PCM detects excessive torque load.
10. The PCM detects insufficient idle quality.
11. The 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

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