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Part 2 of 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 and 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 RPM and 6000 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 passengers setting is turned off then the drivers setting controls both driver and passenger temperature actuators. The passengers setting can not be used without the drivers setting also being ON. The passengers setting can be turned ON or OFF by pressing the power button in the center of the passengers temperature rocker switch. When the passengers setting is ON, the passenger temperature can be adjusted independently from the drivers setting and the passenger temperature is displayed on the passengers side of the control module. A different sunload on one side of the vehicle may cause different discharge air temperatures even when the passengers setting is not turned ON.

Air Distribution

The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. The mode switch provides the vehicle operator with the ability to override the automatic setting. When the mode door is moved to the defrost position, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. In the outside air position, the incoming air is filtered by the passenger compartment air filter.

The HVAC system allows air to the outboard panel outlets in all air distribution modes. This allows air to be circulated higher in the cabin under any operating condition. The panel outlets have three operating positions, open, side window defogger and closed. To eliminate air from these outlets, the individual outlet thumb wheel must be turned to the OFF or side window defogger position.

Recirculation

The HVAC control module controls the air intake through the recirculation actuator. Recirculation is not available when the mode is in defrost. When the mode is in defog, Recirculation will only be available for 10 minutes. In the outside air position, the incoming air is filtered by the passenger compartment air filter.


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. The mode actuator will remain in the panel position and the recirculation actuator will remain in the recirculation 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 extreme cool temperature will not cool the vehicle any faster.


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 an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.


The Denso 10S20 fixed 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 continued 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.




Automatic HVAC Description and Operation



The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum and aluminum cooling fins, which allows 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.
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