Air Temperature Description and Operation

Air Temperature Description and Operation

The air temperature controls are divided into three primary areas. The first, Heating, is related to how the heater system responds when a heater mode is selected, and how the HVAC system provides the desired temperature for each setting. The second, Air Conditioning, is related to how the A/C system responds when an A/C mode is selected by the vehicle operator, and how the HVAC system provides the desired temperature for each setting. The third, A/C Cycle, describes the complete A/C cycle.

Heating

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 adjusting the temperature control, located on the HVAC control module, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater, A/C or OFF.

When the air temperature switches are pressed, the HVAC control module monitors the position of the air temperature door through the air temperature door position signal circuit. The air temperature actuator receives its power and ground through the air temperature door control A and air temperature control B circuits. The polarity is switched to acquire the desired temperature.

A 5 volt reference signal is sent out over the 5 volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position. That variable resistor is grounded through the low reference circuit.

If the air temperature actuator or wiring fails, the air temperature LED graph on the HVAC control module will flash. Power and ground are provided to the HVAC control module by the ignition 3 voltage, battery positive voltage and ground circuits.

Engine Coolant

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

Air Conditioning

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 in any mode, on the left mode switch, by pressing the A/C or MAX A/C switches.

The A/C compressor clutch relay coil receives its power from the ignition 3 voltage circuit. When the A/C high and low pressure switches are closed, ground is provided to the A/C compressor clutch relay coil from the HVAC control module, through the A/C compressor clutch relay control circuit. When the A/C compressor clutch relay is engaged, voltage is supplied to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. The A/C compressor clutch is grounded through the ground circuit. Whenever the compressor is turned off, the A/C compressor clutch diode prevents a voltage spike from entering the vehicles electrical system. Power and ground are provided to the HVAC control module by the ignition 3 voltage, battery positive voltage and ground circuits.

Max A/C Mode

When the driver selects the MAX A/C mode, the A/C compressor clutch is activated. The recirculation door is activated, bringing air from inside the vehicle for maximum cooling. You can only use MAX A/C, when the mode is selected with the MAX button.

A/C Mode

When the driver selects the A/C mode, the A/C compressor clutch is activated. You can only use A/C, when the mode is selected with the MAX button or the A/C button.

A/C High Pressure Switch

The A/C system is protected by the A/C high pressure switch. The A/C high pressure switch turns off the A/C compressor when the line pressure exceeds a predetermined value. The switch does this by opening the A/C compressor clutch relay control circuit.

A/C Low Pressure Switch

The A/C system is protected by the A/C low pressure switch. The A/C low pressure switch turns off the A/C compressor when the line pressure exceeds a predetermined value. The switch does this by opening the A/C compressor clutch relay control circuit.

Recirculation Mode

Recirculation is only available in MAX A/C. When MAX A/C mode is selected, power and ground are applied to the recirculation actuator through the recirculation door control A circuit and the recirculation door control B circuit. The polarity is reversed to move the recirculation door back to the outside air position. Recirculation brings air from inside the vehicle instead of fresh air from the outside. You can only use MAX A/C, when the mode is selected with the MAX button.

Air Temperature Actuator

When the air temperature switches are pressed, the HVAC control module monitors the position of the air temperature door through the air temperature door position signal circuit. The air temperature actuator receives its power and ground through the air temperature door control A and air temperature control B circuits. The polarity is switched to acquire the desired temperature.

A 5 volt reference signal is sent out over the 5 volt reference circuit to the air temperature actuator. When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door position. That variable resistor is grounded through the low reference circuit.

If the air temperature actuator or wiring fails, the air temperature LED graph on the HVAC control module will flash. Power and ground are provided to the HVAC control module by the ignition 3 voltage, battery positive voltage and ground circuits.

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 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 the moisture and heat from the ambient air. 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.