The air temperature controls are divided into six primary areas:

HVAC Control Assembly

The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 3 voltage circuit provides power to the control assembly. An integrated potentiometer controls the air temperature door position. The blower motor speed is controlled by a rotary switch. The integrated vacuum system controls the mode door position. The control assembly supports the following features:

Air Temperature Actuator

The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and position signal circuits enable the actuator to operate. The position signal circuit uses a 0-12 volt linear-ramped signal to command the actuator movement. The 0 and 12 volt control values represent the opposite limits of the actuator range of motion. The values in between 0 and 12 volts correspond to the positions between the limits.

When the HVAC control assembly sets a commanded, or targeted, value, the position signal is set to a value between 0-12 volts. The actuator shaft rotates until the commanded position is reached. The assembly will maintain the control value until a new commanded value is needed.

A/C High Pressure Switch

The A/C high pressure switch protects the A/C system. The A/C high pressure switch is a two wire normally closed switch. The A/C high pressure switch opens the A/C request signal and disables the A/C compressor when line pressure exceeds 2965 kPa (430 psi). The switch will then close once the pressure reaches 1586 kPa (230 psi).

A/C Low Pressure Switch

The A/C low pressure switch protects the A/C system. The A/C low pressure switch is a two wire normally closed switch. The A/C low pressure switch opens the A/C low pressure switch signal circuit and disables the A/C compressor when line pressure falls below 152 kPa (22 psi). The switch will then close once the pressure reaches 303 kPa (44 psi).

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The control assembly takes the following actions with a mode operation selected, and an air temperature setting selected:

Ignition 3 voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The Mode switch must be in one of the following positions for an A/C request:

Once the mode switch is in one of these positions, the mode switch applies ignition 3 voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the A/C compressor relay and the engine control module (ECM). The voltage applied to the ECM is an input, this ensures idle quality. For the relay to close its internal contacts to supply the A/C compressor clutch with battery positive voltage, the A/C low pressure must be closed to provide a path to ground for the relay coil through the A/C low pressure switch signal circuit.

Once engaged, the compressor clutch will be disengaged for the following conditions:

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

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The control assembly takes the following actions with a mode operation selected, and an air temperature setting selected:

Ignition 3 voltage is supplied to the mode switch from the blower motor switch. The blower motor switch must be in any position except for OFF for A/C operation. The Mode switch must be in one of the following positions for an A/C request:

Once the mode switch is in one of these positions, the mode switch applies ignition 3 voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the powertrain control module (PCM), for the 8.1L (L18), or engine control module (ECM), for the 6.6L (LB7). The voltage applied to the PCM/ECM is an A/C request signal. The PCM/ECM then monitors the voltage on the A/C low pressure switch signal. If the voltage is low, near ground, then the PCM/ECM will enable the A/C compressor clutch relay. The PCM/ECM provides a ground for the relay so it can close it internal contacts. The A/C low pressure must be closed to provide a path to ground for input to the PCM/ECM through the A/C low pressure switch signal circuit. The following conditions must be meet in order for the PCM/ECM to turn ON the A/C compressor clutch relay:

Once engaged, the compressor clutch will be disengaged for the following conditions:

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

The purpose of the auxiliary heating system is to supply heat to the rear interior of the vehicle. The heater auxiliary blower motor switch is located on the headliner and controls the auxiliary blower motor speeds for rear heater operation. Placing the heater auxiliary blower motor switch in any position other than Off, turns on the solenoid allowing vacuum to open the water valve. This action allow coolant to flow through the auxiliary heater core. The location of the hot water shut off valve is under the right rear area of the cab. The only way to regulate the temperature of the air is to regulate the blower speed. The slower the blower speed, the higher the output air temperature. The slower moving air has more time to absorb heat from the heater core.

Engine coolant is the essential element to the heating system. The thermostat controls the normal engine operating coolant. 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 hose, in a pressurized state. Engine coolant flows through the main inlet hose and through the hot water shutoff valve to the auxiliary inlet heater hose at the rear of the vehicle, in a pressurized state. The heater core is located inside of the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculates 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 Sanden model 4754 (C60), the Sanden model 4796 (C60), and the Delphi model HT-6 (C60, C69) A/C compressors are used on this model year vehicle.

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.