HVAC Systems - Automatic: Overview Chevrolet Tahoe II

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 Rear Seat Audio (RAS) Control Module, HVAC Control Module, Body Control Module or Powertrain Control Module have set DTCs which may affect HVAC operation are present.
3. 6: The presence of DTCs which begin with "U", except U1500, indicate some other module is not communicating. The specified procedure will compile all the available information before tests are performed. Be sure to diagnose all other "U" codes before diagnosing a U1500.
4. 7: Answer Yes if the first three characters of the DTC name begins with B10; regardless of the last two characters.

Diagnostic System Check - HVAC Systems - Automatic

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.

DTC B0159

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.

DTC B0164

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.

DTC B0174

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.

DTC B0179

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.

DTC B0184

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.

DTC B0229

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.

DTC B0408

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.

DTC B0414

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

1. 3: This step determines if the auxiliary 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.

DTC B0429

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.

DTC B3579

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.

DTC B3584

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

1. 5: This step verifies that the HVAC control module data signal circuit is functioning.
2. 6: This test verifies that the HVAC control module clock signal circuit is operating at the correct frequency.

DTC U1500

1. Perform the «Diagnostic System Check - HVAC Systems - Automatic»(ref-190277-S09288727582005091400000) before using the Symptom Tables in order to verify that all of the following are true: There are no DTCs set. The control modules can communicate via the serial data link.
2. Review the system operation in order to familiarize yourself with the system functions. Refer to: «Air Delivery Description and Operation»(ref-190277-S29231917002005091400000) «Air Temperature Description and Operation»(ref-190277-S37762598012005091400000)

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.

HVAC Compressor Clutch Does Not Engage

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.

HVAC Compressor Clutch Does Not Disengage

The number(s) below refer to the step number(s) 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.

Blower Motor Always On

The number(s) below refer to the step number(s) 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 a poor motor.

Blower Motor Inoperative

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.

Blower Motor Malfunction

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

1. 2: This step tests for auxiliary blower motor operation in the OFF setting.
2. 6: This step tests for a possible short to ground in the auxiliary blower motor control module control circuit.

Blower Motor Always On - Auxiliary (W/O Sunroof)

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

1. 2: This step checks for continuous operation of the auxiliary blower motor.
2. 3: This step determines if the malfunction is in the auxiliary HVAC control assembly.
3. 4: This step tests for a short to ground in the control side of the system. It is easiest to perform this step by disconnecting the auxiliary blower motor resistor assembly connector C1 and the auxiliary HVAC control assembly. Test each auxiliary blower motor speed control circuit for continuity to a known good ground.

Blower Motor Always On - Auxiliary (W/Sunroof or Heat Only)

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

1. 2: This step tests for auxiliary blower motor operation in any speed setting.
2. 5: This step identifies if the auxiliary HVAC control module is commanding the auxiliary blower motor to operate at HI speed.
3. 8: This step verifies that power is being supplied to the auxiliary blower motor.
4. 9: This step tests for a possible open in the auxiliary blower motor ground circuit or a poor motor.

Blower Motor Inoperative - Auxiliary (W/O Sunroof)

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

1. 2: This step tests for auxiliary blower motor operation in any speed setting.
2. 8: This step determines if the malfunction is in the control side of the system.
3. 9: Test both the auxiliary HVAC control assembly chassis ground circuit and ground loop circuit.

Blower Motor Inoperative - Auxiliary (W/Sunroof or Heat Only)

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

1. 2: This step will determine if one or more blower speeds are inoperative.
2. 3: This step verifies if the rear HVAC control switch is attempting to control the auxiliary blower motor speed.
3. 4: This step verifies if the front HVAC control switch is attempting to control the auxiliary blower motor speed.

Blower Motor Malfunction - Auxiliary (W/O Sunroof)

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

1. 2: This step will determine if one or more blower speeds are inoperative.
2. 3: This step tests for a voltage change in the auxiliary blower motor feed circuit.
3. 4: This step tests for an open in one of the blower motor speed control circuits and the auxiliary HVAC control switch ground circuits.

Blower Motor Malfunction - Auxiliary (W/Sunroof or Heat Only)

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.

Too Hot in Vehicle

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.

Too Cold in Vehicle

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

1. 2: This step ensures that the primary HVAC system is functioning properly.
2. 3: The DTC's listed could set if a calibration of the actuators is performed.
3. 6: If the condition does not occur when an A/C request has been made then you must bypass the A/C performance test procedures.
4. 7: Performing the Air Conditioning System Performance Test ensures that the A/C system is operating properly for further testing.
5. 8: This step ensures that the rear auxiliary HVAC control module is able to move the auxiliary air temperature actuator.
6. 9: This step ensures that the front auxiliary HVAC control assembly is able to move the auxiliary air temperature actuator.
7. 11: The infrared sensor is located between the auxiliary blower motor switch and the auxiliary temperature switch on rear auxiliary HVAC control module. It has a glass cover protecting the sensor which is integral to the rear auxiliary HVAC control module.

Too Hot in Vehicle - Auxiliary (w/o Sunroof)

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

1. 2: This step ensures that the primary HVAC system is functioning properly.
2. 5: If the condition does not occur when an A/C request has been made then you must bypass the A/C performance test procedures.
3. 6: Performing the Air Conditioning System Performance Test ensures that the A/C system is operating properly for further testing.

Too Hot in Vehicle - Auxiliary (w/ Sunroof)

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

1. 2: This step ensures that the primary HVAC system is functioning properly.
2. 3: The DTC's listed could set if a calibration of the actuators is performed.
3. 6: If the condition does not occur when a heating request has been made then you must bypass the A/C compressor and cooling system diagnostic information.
4. 9: This step ensures that the rear auxiliary HVAC control module is able to move the auxiliary air temperature actuator.
5. 10: This step ensures that the front auxiliary HVAC control assembly is able to move the auxiliary air temperature actuator.
6. 11: The infrared sensor is located between the auxiliary blower motor switch and the auxiliary temperature switch on rear auxiliary HVAC control module. It has a glass cover protecting the sensor which is integral to the rear auxiliary HVAC control module.

Too Cold in Vehicle - Auxiliary (w/o sunroof)

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

1. 2: This step ensures that the primary HVAC system is functioning properly.
2. 5: If the condition does not occur when a heating request has been made then you must bypass the A/C compressor and cooling system diagnostic information.

Too Cold in Vehicle - Auxiliary (w/ sunroof)

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.

1. 3: This step test for blower motor operation in any speed setting.

1. 4: This step test for proper blower motor speed in each setting.

1. 5: This step test for proper operation of the recirculation actuator.

1. 8: This step checks for an internal fault in the HVAC control module.

Air Delivery Improper

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

1. 3: This step tests for auxiliary blower motor that does not operate from the front auxiliary HVAC control assembly regardless of the desired blower motor speed.
2. 4: This step tests for a auxiliary blower motor that operates, yet does not accurately respond to the desired blower motor speed from the front auxiliary HVAC control assembly.
3. 5: This step tests for an inoperative auxiliary mode actuator in one mode position(s) or in the wrong mode position(s) from the front auxiliary HVAC control module.
4. 6: This step tests for an inoperative auxiliary mode actuator in one mode position(s) or in the wrong mode position(s) from the rear auxiliary HVAC control module.

Air Delivery Improper - Auxiliary (W/O Sunroof)

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

1. 3: This step tests for a auxiliary blower motor that does not operate regardless of the desired blower motor speed.
2. 4: This step tests for a auxiliary blower motor that operates, yet does not accurately respond to the desired blower motor speed.
3. 5: This step tests for an inoperative auxiliary mode actuator in one mode position(s) or in the wrong mode position(s).
4. 10: The front auxiliary HVAC control assembly mode switch control circuit will normally vary between 0-12 volts.
5. 16: An incorrectly installed auxiliary mode actuator, damaged attaching linkage(s). Any mechanical repairs will not require the mode actuator-auxiliary to be replaced.

Air Delivery Improper - Auxiliary (W/Sunroof)

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

1. 2: This step tests for a inoperative mode actuator in one mode position(s) or in the wrong mode position(s).
2. 3: This step tests for a inoperative recirculation actuator in one mode position(s) or in the wrong mode position(s).
3. 4: This step tests for A/C system performance that is correct for the for the conditions at hand; that would mandate a state of automatic recirculation mode being active. Automatic recirculation mode will become active when the A/C high side pressure reaches 2206-2620 kPa (320-380 psi) and remain ON until the high side pressure drops below 1447-1861 kPa (210-270 psi). In automatic recirculation mode their will be no indication on the HVAC controller that the mode is active, the out side air (OSA) and recirculation (REC) switches and indicators will be inoperative on the HVAC controller.
4. 8: This step tests for a internally shorted A/C high pressure recirculation switch causing the automatic recirculation mode to be active.

Air Recirculation Malfunction

The numbers below refer to the step numbers 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.

Outside Air Temperature Display Inaccurate or Inoperative

Re-Calibrating Actuators

The HVAC control module will relearn the actuator limits whenever power is disconnected and reconnected to the module.

Use the following steps to perform the calibration update

1. Turn OFF the ignition.
2. Remove the battery positive voltage circuit fuse of the HVAC Control Module.
3. Wait 60 seconds.
4. Install the fuse.
5. Turn ON the ignition.
6. Wait 60 seconds before installing a scan tool.

Air Delivery Description and Operation

The air delivery controls are divided into eight areas

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

Recirculation Operation

When air recirculation is selected, a signal is sent from the HVAC control module to the recirculation actuator through the recirculation door control circuit. The recirculation actuator 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 vent and bi-level mode switch positions. Including the OFF position. The mode switch must be placed in either the vent 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, floor, and mix-blend 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.

When the recirculation door moves to a desired position, a variable resistor within the actuator is used to create the recirculation door position signal. The HVAC module uses the recirculation door position signal to determine the actual recirculation door position. The left instrument panel fuse block provides power to the recirculation actuator through the ignition 3 voltage circuit and the IP wiring harness junction block. Power and ground are provided to the HVAC control module by the fuse block through the ignition 3 voltage circuit and the ground circuits through the right instrument panel junction block. A 5 volt reference signal is sent to the actuator through the 5 volt reference circuit, through the right instrument panel junction block, to the recirculation actuator.

When the automatic setting has been selected, the HVAC control module will recirculate air whenever system temperature performance is insufficient to provide the desired temperature. The recirculation switch LED will not be illuminated. If either the outside air or recirculation switch is pressed during automatic operation, then automatic operation will be over ridden. The appropriate recirculation or outside air switch LED will be illuminated. The recirculation door will be opened or closed depending upon the selected override. When the ambient air temperature is below 4°C (40°F) the recirculation override of the automatic setting will only be available for 10 minutes.

When the automatic setting has not been selected, only the outside air or recirculation switch will be used to determine the recirculation door position. When the ambient air temperature is below 4°C (40°F) any recirculation selection will only be available for 10 minutes.

The A/C high pressure recirculation switch can cause the HVAC system to recirculate air. When the high side pressure reaches 2206-2620 Kpa (320-380 psi), the PCM will place the HVAC system in recirculation mode. The high side pressure is lowered when the inside air cools the refrigerant within the A/C evaporator. When the high side pressure reaches 1447-1861 Kpa (210-270 psi), the PCM will place the HVAC system out of recirculation mode.

Automatic Operation

The automatic HVAC system will maintain the interior temperature of the vehicle by controlling the blower motor, air temperature, mode and recirculation actuators to achieve the desired temperature. For full automatic operation, both the blower and mode switches must be in the AUTO position and the air temperature set between 19°C (66°F) and 28°C (82°F). Blower speeds will be gradually increased or decreased, and can be reduced based on vehicle speed in order to maintain airflow into the vehicle. At start up, the automatic HVAC system will use to the last settings used prior to vehicle shut down.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. At initial vehicle start up, the blower motor is not turned on until the coolant reaches a predetermined temperature. If the coolant does not warm up within a preset time, the blower will be activated. The HVAC system can quickly warm up the vehicle when the temperature and blower motor settings are set to maximum and outside air and floor modes are selected. Once the desired temperature is reached, the blower motor will slow down, discharge temperatures will be lowered, and the mode actuator will remain in the floor setting. The system can be placed in bi-level if the ambient temperature is near 7-10°C (45-50°F). Regardless of mode selection, some air will be diverted to the defrost setting to keep window fogging at a minimum.

If the ambient air temperature is less than 2°C (35°F) and the coolant temperature is less than 10°C (50°F), then a 75 second purge will occur. The purge is performed in order to reduce window fogging until the coolant warms up. The purge will begin as soon as the coolant temperature reaches 4°C (40°F). The mode door will move to the defrost position and the blower motor will be near half speed.

In warm temperatures, the automatic HVAC system will provide A/C in the most efficient manner. The HVAC system can quickly cool down the vehicle when the temperature is set to minimum, blower motor is set to maximum, and recirculation and vent modes are selected. Once the desired temperature is reached, the blower motor will slow down, discharge temperatures will be raised, and the mode actuator will be switched to bi-level for better passenger comfort. The recirculation actuator can be partially opened to assist in temperature control. Regardless of mode selection, some air will be diverted to the floor setting to keep occupant comfort at a maximum.

Air Temperature Description and Operation

The air temperature controls are divided into eleven areas

1. HVAC Control Components
2. Heating
3. Air Conditioning
4. Automatic Operation
5. Engine Coolant
6. A/C Cycle
7. Auxiliary HVAC Combinations
8. Automatic Auxiliary HVAC System
9. Manual Auxiliary HVAC w/o CF5
10. Manual Auxiliary HVAC w/CF5
11. Manual Auxiliary w/C36 or C69 Only

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 warmest 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 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 the warmest 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 lower air temperature sensor, the inside air temperature assembly and the sunload 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, the A/C low pressure switch signal circuit needs to be grounded and the A/C request signal circuit needs to have 12 volts applied to it from the HVAC control module. A 12-volt reference signal is sent out over the A/C request signal circuits from the HVAC control module, through the A/C high pressure switch and then to the PCM when the vehicle operator makes an A/C request. A separate 12-volt reference signal is sent out over the A/C low pressure switch signal circuit, through the A/C low pressure switch and the ground circuit.

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).

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 the coldest position or the warmest position.
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 coldest position, 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 warmest position, 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

Auxiliary Heating Operation

The operation of the front HVAC control assembly when put into the OFF position disables the auxiliary controls. When the front HVAC control assembly is put into the REAR position the rear auxiliary HVAC control module controls the auxiliary HVAC operation. Only the rear seat occupants can control the HVAC settings. When AUTO is selected on the front auxiliary HVAC assembly, the rear auxiliary HVAC control module uses calculations based on these following components to regulate the air temperature

1. Auxiliary inside air temperature sensor
2. Upper auxiliary air temperature sensor
3. Lower auxiliary air temperature sensor
4. Auxiliary air temperature actuator
5. Mode actuator

The signal from the front auxiliary control assembly for auxiliary air temperature signal is a varied 12 volt signal. When a warm air request is made the signal voltage is high. When a cool air request is made the signal voltage is low.

Auxiliary A/C Operation

The operation of the front HVAC control assembly when put into the OFF position disables the auxiliary controls. When the front HVAC control assembly is put into the REAR position the rear auxiliary HVAC control module controls the auxiliary HVAC operation. Only the rear seat occupants can control the HVAC settings. When AUTO is selected on the front auxiliary HVAC assembly, the rear auxiliary HVAC control module uses calculations based on these following components to regulate the air temperature

1. Auxiliary inside air temperature sensor
2. Upper auxiliary air temperature sensor
3. Lower auxiliary air temperature sensor
4. Auxiliary air temperature actuator
5. Mode actuator

When an auxiliary warm air request is made the auxiliary HVAC control processor will send a high voltage signal to the auxiliary air temperature actuator on the auxiliary air temperature door position signal circuit. This voltage signal is a varied 12 volt signal. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested. The signal from the auxiliary HVAC control assemblies to the auxiliary HVAC control processor is a varied 12 volt signal. This signal, when warm air is requested, is a high voltage signal.

When an auxiliary cool air request is made the auxiliary HVAC control processor will send a low voltage signal to the auxiliary air temperature actuator on the auxiliary air temperature door position signal circuit. This voltage signal is a varied 12 volt signal. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested. The signal from the auxiliary HVAC control assemblies to the auxiliary HVAC control processor is a varied 12 volt signal. This signal, when cool air is requested, is a low voltage signal.

When an auxiliary warm air request is made the front auxiliary HVAC control assembly will send a high voltage signal to the auxiliary air temperature actuator on the auxiliary air temperature door position signal circuit. This voltage signal is a varied 12 volt signal. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested.

When an auxiliary cool air request is made the front auxiliary HVAC control assembly will send a low voltage signal to the auxiliary air temperature actuator on the auxiliary air temperature door position signal circuit. This voltage signal is a varied 12 volt signal. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested.

With the C36 option there is no auxiliary A/C system. This system has only a blower motor switch to regulate air speed. It does not use an air temperature actuator or a mode actuator. The lowest blower speed will have high auxiliary duct temperature. The highest blower speed will have low auxiliary duct temperature. The slower the air travels past the auxiliary heater core the more time it has to absorb heat.

With the C69 option there is no auxiliary heater system. This system has only a blower motor switch to regulate air speed. It does not use an air temperature actuator or a mode actuator. The lowest blower speed will have low auxiliary duct temperature. The highest blower speed will have high auxiliary duct temperature. The slower the air travels past the auxiliary evaporator core the more time the evaporator has to absorb the heat.