HVAC Systems - Automatic: Overview Chevrolet Silverado 2500

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 instrument Panel Integration 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.

Scheme 106

Scheme 107

Scheme 108

Scheme 109

Scheme 110

Scheme 111

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

1. 2) - The scan tool uses a default value. This will not give an accurate reading of the sensor value if an open or a short to ground is detected.

Scheme 112

Scheme 113

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

1. 2) - The scan tool uses a default value. This will not give an accurate reading of the sensor value if an open or a short to ground is detected.

Scheme 114

Scheme 115

The number(s) below refer to the step number(s) on the diagnostic table.

1. 3) - Tests for the proper operation of the circuit in the high voltage range.
2. 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.

Scheme 116

Scheme 117

The number(s) below refer to the step number(s) on the diagnostic table.

1. 3) - Tests for the proper operation of the circuit in the high voltage range.
2. 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.

Scheme 118

Scheme 119

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

1. 2) - The scan tool uses a default value. This will not give an accurate reading of the sensor value if an open or a short to ground is detected.

Scheme 120

Scheme 121

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

1. 2) - This step determines if the air temperature door position is within the specified range of the commanded value.
2. 4) - Tests for the proper operation of the circuit in the low voltage range.
3. 5) - Tests for the proper operation of the circuit in the high voltage range. If the jumper fuse opens when you perform this test, then the signal circuit contains a short to ground.
4. 6) - Tests for a short to voltage in the 5-volt reference circuit.

Scheme 122

Scheme 123

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

1. 3) - This step determines if the air temperature door position is within the specified range of the commanded value.
2. 6) - Tests for a short to voltage in the 5-volt reference circuit.

Scheme 124

Scheme 125

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

1. 2) - This step determines if the air temperature door is within a known calibration range that is stored in the HVAC control module.

Scheme 126

Scheme 127

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

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) - These actions will enable the A/C compressor to operate.
3. 4) - 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.
4. 5) - If the HVAC control module does not respond to requests, the module is inoperative.
5. 6) - This action ensures that the powertrain control module (PCM) is getting an A/C request from the HVAC control module.
6. 7) - If the A/C request LED does not illuminate the HVAC control module is not grounding the A/C request signal circuit.
7. 8) - Since the A/C system pressure has been verified in Step 4 this step will verify the integrity of the A/C request signal circuits, if the A/C Request Signal parameter is Yes then there is a concern with the A/C high pressure switch.
8. 10) - Since the A/C system pressure has been verified in Step 4 this step will verify the integrity of the A/C low pressure signal circuit and the ground circuit of the A/C low pressure switch, if the A/C Compressor Cycling Switch parameter is Normal then there is a concern with the A/C low pressure switch.
9. 14) - If the test lamp remains illuminated the A/C compressor clutch relay control circuit is shorted to ground.
10. 18) - If the fused jumper opens during this step the A/C compressor clutch supply voltage circuit is short to ground.
11. 26) - Ensure that both A/C request signal circuits of the A/C high pressure switch are tested for listed conditions.

Scheme 128

Scheme 129

Scheme 130

Scheme 131

Scheme 132

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

1. 2) - These actions will disable the HVAC control module output to powertrain control module (PCM).
2. 3) - This step checks the A/C Request Signal parameter to see if the PCM is interpreting an A/C request.
3. 4) - Ensure that both A/C request signal circuits of the A/C high pressure switch are tested for listed conditions.
4. 5) - The A/C Request Signal parameter is Yes. The A/C request signal circuits both are not shorted to ground. This step verifies that the A/C high pressure switch is not shorted internally to ground.

Scheme 133

Scheme 134

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

1. 2) - This step tests for blower motor operation in the OFF setting.
2. 4) - This step tests for a possible short to ground on the blower motor control circuit.
3. 5) - This step tests for a possible short to voltage in the blower motor supply circuit.

Scheme 135

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

1. 2) - This step tests for blower motor operation in any speed setting.
2. 3) - This step identifies if the HVAC control module is commanding the blower motor to operate at HI speed.
3. 5) - This step verifies that power is being supplied to the blower motor.
4. 6) - This step tests for a possible open in the blower motor ground circuit or for a poor motor.

Scheme 136

Scheme 137

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

1. 2) - This step will determine if one or more blower speeds are inoperative.
2. 4) - This step verifies if the HVAC control switch is attempting to control the blower motor speed.
3. 5) - This step tests for a voltage change in the blower motor control circuit.

Scheme 138

Scheme 139

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

1. 2) - The DTCs listed may set, if there is a concern with the actuator, when a calibration of the actuators is performed.
2. 3) - Ambient air temperature must be above 3°C (38°F) in order for this A/C compressor test to be run.
3. 4) - Misaligned air baffles will cause poor A/C performance at idle and low speed operation.
4. 6) - The specified values are from the A/C System Performance Test.
5. 8) - This action ensures that the internal fan of the inside air temperature sensor assembly is providing air flow across the internal sensor.

Scheme 140

Scheme 141

Scheme 142

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

1. 2) - The DTCs listed may set, if there is a concern with the actuator, when a calibration of the actuators is performed.
2. 10) - This action ensures that the internal fan of the inside air temperature sensor assembly is providing air flow across the internal sensor.

Scheme 143

Scheme 144

Scheme 145

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

1. 2) - This step tests for blower motor operation in the off setting.
2. 3) - This step test for blower motor operation in any speed setting.
3. 4) - This step test for proper blower motor speed in each setting.
4. 5) - This step test for proper operation of the recirculation actuator.
5. 8) - This step checks for an internal fault in the HVAC control module.
6. 10) - This step checks for any physical and mechanical problems.

Scheme 146

Scheme 147

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

1. 2) - This step tests for a inoperative mode actuator in one mode positions or in the wrong mode positions.
2. 3) - This step tests for a inoperative recirculation actuator in one mode positions or in the wrong mode positions.
3. 4) - This step tests A/C system performance that is correct for the current conditions. that would mandate a state of automatic recirculation mode being active. The automatic recirculation mode becomes active when the A/C high side pressure reaches 2206-2620 kPa (320-380 psi). The mode remains ON until the high side pressure drops below 1447-1861 kPa (210-270 psi). In automatic recirculation mode the HVAC controller does not indicate that the mode is active. The outside air (OSA) and the recirculation (RECIRC) switches and indicators are inoperative on the HVAC controller.
4. 8) - This step tests for an A/C high pressure recirculation switch, which is internally shorted and which causes the automatic recirculation mode to be active.

Scheme 148

Scheme 149

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

1. 2) - If the vehicle has not been driven for at least 90 minutes you do not need to drive vehicle to update the outside air temperature display.

Scheme 150

Scheme 151

Air Delivery Description & Operation

The air delivery controls are divided into two primary areas

1. Air speed is dependent upon blower motor speed.
2. Air distribution is related to various single or multiple air outlets.

Air Temperature Description & Operation

The air temperature controls are divided into 6 areas

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

Heating Operation

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by turning the air temperature switch, located on the HVAC control module, to any setting. The air temperature switch can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C.

When the air temperature switch is placed to the 82° position the HVAC control module will not regulates the air temperature. The HVAC control module will move the air temperature actuator to divert all air flow through the heater core. The blower speed and mode selection will be set at the last known manual input to the HVAC control module. Unless the blower motor switch is in the Auto position then the blower motor is at full speed. The HVAC control module will not monitor the sensors as the module will place the air temperature actuator to its full warm position.

When the air temperature switch is placed to any position other than 82° the air temperature actuator position is regulated to achieve desired temperature. This action does not place the entire HVAC system to automatic mode. When a warm air temperature is needed the HVAC control module will monitor the lower air temperature sensor, the inside air temperature assembly and the solar sensor to correctly position the air temperature door. The HVAC control module will only change the position of air temperature actuator to get the desired temperature, blower speed and mode are not regulated.

A/C Operation

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The vehicle operator can activate the A/C system by manually placing the mode switch in the Defrost position or by pressing the A/C button. The A/C system can operate regardless of the temperature setting, as long as outside ambient temperature is above 4°C (40°F). If the A/C is requested and temperatures are too low, the LED will flash 3 times to remind the vehicle operator that A/C is not available. If the blower motor switch is in the Off position the A/C button is inoperative. The A/C indicator lamp will not illuminate if system mode is selected to Defog or Defrost if it is off, or when the A/C compressor is on and in Automatic mode. The A/C indicator lamp will illuminate when the outside air temperature is above 40° and vehicle operator selects A/C on in manual mode.

When the air temperature switch is placed to the coldest position the HVAC control module will not regulate the air temperature. The HVAC control module will move the air temperature actuator to divert all air flow through the evaporator. The blower speed and mode selection will be set at the last known manual input to the HVAC control module. Unless the blower motor switch is in the Auto position then the blower motor is at full speed. The HVAC control module will not monitor the sensors as the module will place the air temperature actuator to its full cool position.

When the air temperature switch is placed to any position other than the coldest position the air temperature actuator position is regulated to achieve desired temperature. This action does not place the entire HVAC system to automatic mode. When a warm air temperature is needed the HVAC control module will monitor the upper air temperature sensor, the inside air temperature assembly and the solar sensor to correctly position the air temperature door. The HVAC control module will only change the position of air temperature actuator to get the desired temperature, blower speed and mode are not regulated.

When the A/C button is pressed, a request is made to the powertrain control module (PCM) to turn on the A/C compressor. The PCM turns on the A/C compressor by providing a path to ground through the A/C clutch relay control circuit for the A/C compressor clutch relay. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned off, the A/C compressor clutch diode provides a path for the voltage spike resulting from the collapsing magnetic field of the compressor clutch coil.

In order for the PCM to internally ground the A/C clutch relay control circuit, two separate request signals need to be grounded. A 12-volt reference signal is sent out over the A/C request signal circuits, through the A/C high pressure switch and grounded through the HVAC control module when the vehicle operator makes an A/C request. The A/C request signal circuit output from the PCM to the HVAC control module can not survive a short to power. A separate 12-volt reference signal is sent out over the A/C refrigerant low pressure cut-out switch signal circuit, through the A/C low pressure switch and the ground circuit. When both these reference signals are grounded, the PCM may activate the A/C compressor clutch.

The PCM will engage the A/C clutch any time the engine speed is below 5500 RPM and the A/C is requested unless any of the following conditions exist

1. Throttle angle is at 100 percent.
2. The A/C low pressure switch pressure is less than 145-172 kPa (21-25 psi).
3. The A/C high pressure switch pressure is more than 2896 kPa (420 psi).
4. Engine speed is more than 5500 RPM.
5. Engine coolant temperature (ECT) is more than 121°C (250°F).

Automatic Operation

In order to place the HVAC system in Automatic mode these requirements need to be meet

1. The blower motor switch must be in the Auto position.
2. The air temperature switch must be in any other position other than 66° or 82°.
3. The mode switch must be in the Auto position.

The HVAC control module will perform the following routines to maintain the desired air temperature

1. Monitor the following sensors: The inside air temperature sensor The ambient air temperature sensor The upper air temperature sensor if cool air is required The lower air temperature sensor if warm air required The sunload 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

When the temperature is placed in the 66° position, full cold, the HVAC control module will do the following

1. The automatic blower will go to maximum speed
2. The automatic mode will position the mode door to the Panel position

When the temperature switch is in the 82° position, full hot, the HVAC control module will do the following

1. The automatic blower will go to maximum speed
2. The automatic mode will position the mode door to the Floor position