The air temperature controls are divided into 8 areas.

HVAC Control Module

The HVAC control module is powered up by the ignition 3 voltage circuit. When the air temperature switch is rotated a variable resister in the HVAC control module changes the voltage on the air temperature door control circuit for an actuator position change. The module does not utilize keep alive memory (KAM). The module does not have Class 2 communication, personalization and programming available.

Air Temperature Actuator

The air temperature actuator opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The air temperature actuator is a 5 wire actuator that incorporates a electric motor with feed back capability. Power is provided by ignition 3 voltage circuit through the left I/P fuse block. Ground is provided by the ground circuit through the HVAC control module. The air temperature actuator has a potentiometer integral to it. A 5-volt reference signal is sent out over the 5-volt reference circuit to the air temperature actuator. A feed back signal is provided by the air temperature door position signal circuit. As the actuator moves the voltage on the door position signal circuit changes. The HVAC control module monitors this signal to calculate the actual door position.

The control of the air temperature actuator is provided by the air temperature door control circuit. When a request of actuator position change from the HVAC control module, the air temperature door control circuit voltage is varied. A 2.5 volt signal from the HVAC control module keeps the actuator stationary. A 0 volt or 5 volt signal from the HVAC control module allows the actuator to rotate to a position determined by the HVAC control module.

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 control switch, located on the HVAC control module, to any setting. The air temperature actuator position is directly effected by the position of the air temperature switch. This position will not change with an A/C request.

When a warm air request is made the HVAC control module will position the air temperature actuator to divert most of the air flow through the heater core. Blower motor and system mode can be changed regardless of the air temperature setting.

Heater Option Only (C42)

On non-A/C equipped vehicles, the air temperature is controlled by moving the temperature knob. Moving the air temperature knob mechanically moves the air temperature door. They are linked together by the air temperature cable. The air temperature door position determines the amount of air directed to flow across the heater core. The recirculation mode will only be available when the HVAC control module is in Bi-Level mode only.

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 placing the mode switch in one of the following positions: Panel, Bi-Level, Defog, and Defrost. The A/C system can operate regardless of the temperature setting, however, recirculation is only available when the HVAC control module is either in A/C or Bi-Level mode only.

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 5000 RPM and the A/C is requested unless any of the following conditions exist:

A/C Pressure Sensors

The A/C system is protected by two pressure switches. The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure exceeds 2896 kPa (420 psi). The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure falls below 145-172 kPa (21-25 psi). When the PCM sees an open in either signal, the A/C clutch relay control circuit is no longer grounded, thus shutting off the compressor. The low pressure switch will close when pressure reaches 262-290 kPa (38-42 psi).

Recirculation Mode

The recirculation door will move automatically with an input from the A/C high pressure recirculation switch at approximately 2413 kPa (350 psi). The PCM will place the A/C system in recirculation mode when a signal is sent over the A/C refrigerant high pressure cut-out switch signal circuit. The recirculation actuator door control circuit is grounded by the PCM to ensure that the actuator is forced to the recirculation position. This allows for the cooler inside air to flow over the A/C evaporator and cool the refrigerant, until the high side pressure returns to normal. This action will allow the high side pressure to return to normal pressure at a faster pace. The recirculation door will move back to the outside air position when the high side pressure reaches 1724 kPa (250 psi).

Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

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 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.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing 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 orifice tube.

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

Refrigerant exiting the orifice tube 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 through the suction line and back to the compressor, in a vapor state, and 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.

The auxiliary A/C system operates from the vehicles primary A/C system. The front or primary A/C system must be ON to allow the rear A/C system to function.

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 A/C system used on this vehicle is a non cycling system. Non cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal, to the compressor clutch, in the event that the refrigerant pressure becomes excessive. After the high and low side of the A/C system pressure equalize, the high pressure switch will CLOSE. Closing the high pressure switch will complete the electrical circuit to the compressor clutch. The A/C system is also 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.

The 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.

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 tubing 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. The liquid line flow is split and the liquid refrigerant flows to both the front or primary A/C system, and to the liquid line for the rear A/C system.

The liquid refrigerant, flowing to the rear A/C system, flows into the rear TXV. The rear TXV is located at the rear evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the rear A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the expansion device. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the rear A/C 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 through the suction line and back to the primary A/C systems suction line. Refrigerant in the primary A/C system suction line flows back to the compressor, in a vapor state, and completes 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 rear A/C module for passenger comfort. The heat and moisture removed from the rear passenger compartment will also change form, or condense, and is discharged from the rear A/C module as water.

This system incorporates a front and rear auxiliary HVAC control assemblies that provide inputs to the auxiliary HVAC control processor.

Front Auxiliary HVAC Control Assembly

The front auxiliary HVAC control assembly provides inputs to the auxiliary HVAC control processor. It is located in the overhead console so that front seat occupants can control auxiliary HVAC operation. This assembly provides blower, air delivery mode, air temperature settings and control of which control unit will operate the auxiliary HVAC system. When the REAR position is selected, inputs from this control assembly will not be processed by the auxiliary HVAC control processor. This system does not have Class 2 communication available.

The front auxiliary HVAC control assembly receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit to a splice pack. The front HVAC control assembly will apply a ground to the rear auxiliary enable control circuit when REAR is selected. When the air temperature knob is rotated a variable resistor internal to the assembly will vary a 12 volt input. The 12 volt varied voltage is supplied to the auxiliary HVAC control processor for an auxiliary air temperature actuator position change request. This is done on the auxiliary air temperature door position signal circuit . When the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Rear Auxiliary HVAC Control Assembly

The rear auxiliary HVAC control assembly provides inputs to the auxiliary HVAC control processor. It is located in the rear headliner so that second row seat occupants can control auxiliary HVAC operation. This assembly provides blower, air delivery mode and air temperature settings. When the REAR position is selected, on the front HVAC control assembly, inputs from this control assembly will be processed by the auxiliary HVAC control processor. This system does not have Class 2 communication available.

The rear auxiliary HVAC control assembly receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through the rear auxiliary HVAC control assembly. The front HVAC control assembly will apply a ground to the rear auxiliary enable control circuit when REAR is selected. When the air temperature knob is rotated a variable resistor internal to the assembly will vary a 12 volt input. The 12 volt varied voltage is supplied to the auxiliary HVAC control processor for an auxiliary air temperature actuator position change request. This is done on the auxiliary air temperature door position signal circuit . When the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Auxiliary HVAC Control Processor

The auxiliary HVAC control processor controls all outputs for the auxiliary HVAC system. It receives inputs from the front and rear auxiliary HVAC control assemblies. The processor positions the auxiliary air temperature actuator and auxiliary mode actuator based on these inputs. This system does not have Class 2 communication available.

The auxiliary HVAC control processor receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through rear auxiliary HVAC control assembly and a splice pack. The system receives 12 volt varied voltage input for auxiliary air temperature change request. Then the processor creates a 12 volt varied output for control of the auxiliary air temperature actuator. When the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator opens or closes the auxiliary air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The auxiliary air temperature actuator is a 3 wire actuator that incorporates a bi-directional permanent magnet electric motor.

The auxiliary air temperature actuator receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through a splice pack. The control of the air temperature actuator is provided by the auxiliary air temperature door control circuit. The auxiliary HVAC control processor provides a varied 12 volt signal to the actuator. 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 the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Auxiliary Heating Operation

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.

Auxiliary A/C Operation

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.

Front Auxiliary HVAC Control Assembly

The front auxiliary HVAC control assembly is located in the front headliner. This controller controls all auxiliary HVAC operation. This assembly provides blower, air delivery mode and air temperature settings. This system does not have Class 2 communication available.

The front auxiliary HVAC control assembly receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit to a splice pack. When the air temperature knob is rotated a variable resistor internal to the assembly will vary a 12 volt input. This 12 volt varied signal is then sent to the auxiliary air temperature actuator for a change in door position. This is done on the auxiliary air temperature door position signal circuit . When the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator opens or closes the auxiliary air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The auxiliary air temperature actuator is a 3 wire actuator that incorporates a bi-directional permanent magnet electric motor.

The auxiliary air temperature actuator receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through a splice pack. The control of the air temperature actuator is provided by the auxiliary air temperature door control circuit. The auxiliary HVAC control processor provides a varied 12 volt signal to the actuator. 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 the voltage signal is low a cool air request is made and the voltage signal is high a warm air request is made.

Auxiliary Heating Operation

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.

Auxiliary A/C Operation

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.

Auxiliary Heating Operation

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 a 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.

Auxiliary A/C Operation

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 a 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.