Air Temperature (Part 2)

Heating and A/C Operation
The purpose of the heating and A/C System is to provide the following:
- Heated air
- Cooled air
- Remove humidity from the interior of the vehicle
- Reduce windshield fogging

Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:
- Recirculation actuator setting
- Difference between inside and desired temperature
- Difference between ambient and desired temperature
- Blower motor speed setting
- Mode setting

The HVAC control module commands or monitors the following actions when an air temperature setting is selected.
- WARMEST POSITION-The air temperature actuator door position directs maximum air flow through the heater core.
- COLDEST POSITION-The air temperature actuator door position directs maximum air flow around the heater core.
- BETWEEN THE WARMEST AND COLDEST POSITION-The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:
- Sunload
- Ambient temperature
- Inside temperature

The A/C system is engaged by selecting any switch on the HVAC control module except the A/C OFF switch. The A/C switch will illuminate A/C OFF when the A/C switch is selected. The control module sends a GMLAN A/C request message to the engine control module (ECM) for A/C compressor clutch operation. The following conditions must be met in order for the ECM to turn on the compressor clutch:
- HVAC control module
- Evaporator Temperature more than 4°C (39°F)
- Control module operating range 9-16 volts
- ECM
- Engine coolant temperature (ECT) is less than 125°C (257°F) or 120°C (248°F) for PV8
- Engine RPM is between 0-6,000 RPM.
- A/C pressure is between 3137 kPa (455 psi) or 2850 kPa (413 psi) for PV8 and 193 kPa (28 psi) or 200 kPa (29 psi) for PV8.

Once engaged, the compressor clutch will be disengaged for the following conditions:
- Throttle position is 100 percent.
- A/C pressure is more than 3137 kPa (455 psi) or 2850 kPa (413 psi) for PV8.
- A/C pressure is less than 193 kPa (28 psi) or 200 kPa (29 psi) for PV8.
- Engine coolant temperature (ECT) is more than 125°C (257°F) or 120°C (248°F) for PV8.
- Engine speed is more than 5,500 RPM.
- Transmission shift
- ECM detects excessive torque load.
- ECM detects insufficient idle quality.
- ECM detects a hard launch condition.

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Dual Zone Operation
The HVAC control module has temperature settings for the driver and the passenger. If the passenger setting is turned OFF then the driver setting controls both driver and passenger temperature actuators. The passenger setting cannot be used without the driver setting also being ON. The passenger setting can be turned ON or OFF by pressing the power button in the center of the passenger temperature rocker switch. When the passenger setting is ON, the passenger temperature can be adjusted independently from the driver setting and the passenger temperature is displayed on the passenger side of the control module. A different sunload on one side of the vehicle may cause different discharge air temperatures, even when the passenger setting is not turned ON.

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:
- The blower motor switch must be in the AUTO position.
- The air temperature switch must be in any other position other than 60 or 90 degrees.
- The mode switch must be in the AUTO position.

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:
- Regulate blower motor speed
- Position the air temperature actuator
- Position the mode actuator
- Position the recirculation actuator
- Request A/C operation

When the warmest position is selected in automatic operation, the blower speed will increase gradually, until the vehicle reaches normal operating temperature. When normal operating temperature is reached, the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation, the blower will stay on high and the air temperature actuators will stay in the full cold position.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting, but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extremely cool temperature will not cool the vehicle any faster.

Auxiliary HVAC Control Module
The auxiliary HVAC control module is a non-GM LAN device that interfaces between the operator and the auxiliary HVAC system to maintain air temperature and air distribution settings. Five volts and ground are supplied to the rear HVAC control module. The front HVAC control module monitors the position of the rear air temperature actuator and the rear mode actuator. The front HVAC module will move the actuators to the proper position when it receives a request from the rear HVAC control module on which position to place the actuator.

Auxiliary Heating and A/C Operation
The auxiliary HVAC control module provides airflow direction and temperature control for the back seat passengers. Passengers can operate the rear HVAC control module in both manual or automatic modes. Auxiliary HVAC temperatures can be set cooler or warmer than the front primary HVAC setting. The front HVAC module provides power and ground to the auxiliary air temperature actuator. The front HVAC module receives power through the ignition 1 and battery positive voltage circuits from the underhood fuse block. The auxiliary air temperature actuator is a reverse polarity motor. Each circuit provides both power and ground to the auxiliary air temperature actuator. When the auxiliary air temperature actuator is being held in position, both of the auxiliary air temperature door control circuits have 0 volts applied to both sides of the actuator motor. This holds the actuator stationary. When a cooler temperature is requested, one of the auxiliary air temperature door control circuits will ground, driving the auxiliary air temperature actuator to the desired temperature. When a warmer temperature is requested, the other auxiliary air temperature door control circuit will ground. This moves the auxiliary air temperature actuator into the desired position.

Auxiliary Air Temperature Actuator
The auxiliary air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 volts. In order to move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. 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, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

Steering Wheel Controls
The voice recognition has the ability to control the HVAC output air temperature, fan speed and defrost selections. The radio will recognize the following verbal commands when the steering wheel switch is pressed for controlling specific HVAC controls:

For heater or A/C On the following commands will be recognized:
- Heater On
- Heat On
- A/C On
- Air-conditioning On

For heater or A/C Off the following commands will be recognized:
- Heater Off
- Heat Off
- A/C Off
- Air-conditioning Off

For heater or A/C fan high the following commands will be recognized:
- Heater Fan High
- Heat Fan High
- A/C Fan High
- Air-conditioning Fan High

For heater or A/C fan low the following commands will be recognized:
- Heater Fan Low
- Heat Fan Low
- A/C Fan Low
- Air-conditioning Fan Low

For heater or A/C fan auto the following commands will be recognized:
- Heater Fan Auto
- Heat Fan Auto
- A/C Fan Auto
- Air-conditioning Fan Auto

For heater or A/C temperature change the following commands will be recognized:
- Heater Temp (or Temperature) 16-30°C or 60-90°F
- Heat Temp (or Temperature) 16-30°C or 60-90°F
- A/C Temp (or Temperature) 16-30°C or 60-90°F
- Air-conditioning Temp (or Temperature) 16-30°C or 60-90°F

For heater or A/C Defrost the following commands will be recognized:
- Heater Defog (or Defrost) On
- Heat Defog (or Defrost) On
- A/C Defog (or Defrost) On
- Air-conditioning Defog (or Defrost) On

For heater or A/C Defrost the following commands will be recognized:
- Heater Defog (or Defrost) Off
- Heat Defog (or Defrost) Off
- A/C Defog (or Defrost) Off
- Air-conditioning Defog (or Defrost) Off

For heater or A/C Front Defrost the following commands will be recognized:
- Heater Front Defog (or Defrost) On
- Heat Front Defog (or Defrost) On
- A/C Front Defog (or Defrost) On
- Air-conditioning Front Defog (or Defrost) On

For heater or A/C Front Defrost the following commands will be recognized:
- Heater Front Defog (or Defrost) Off
- Heat Front Defog (or Defrost) Off
- A/C Front Defog (or Defrost) Off
- Air-conditioning Front Defog (or Defrost) Off

For heater or A/C Rear Defrost the following commands will be recognized:
- Heater Rear Defog (or Defrost) On
- Heat Rear Defog (or Defrost) On
- A/C Rear Defog (or Defrost) On
- Air-conditioning Rear Defog (or Defrost) On

For heater or A/C Rear Defrost the following commands will be recognized:
- Heater Rear Defog (or Defrost) Off
- Heat Rear Defog (or Defrost) Off
- A/C Rear Defog (or Defrost) Off
- Air-conditioning Rear Defog (or Defrost) Off

A/C Cycle
Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The Denso 7SBU16 variable displacement swash plate A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C System. The A/C System is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continues to rise, the high pressure relief will pop open and release refrigerant from the system.

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum and aluminum cooling fins, which allow rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the thermal expansion valve.

The thermal expansion valve is located in the liquid line between the condenser and the evaporator. The thermal expansion valve is the dividing point for the high and the low pressure sides of the A/C System. As the refrigerant passes through the thermal expansion valve, the pressure of the refrigerant is lowered. Due to the pressure differential of the liquid refrigerant, the refrigerant will begin to vaporize at the thermal expansion valve. The thermal expansion valve also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the thermal expansion valve flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator back through the thermal expansion valve and into the suction line and back to the compressor, in a vapor state completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water under the vehicle.