The air temperature controls are divided into 5 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, two separate request signals need to be grounded. A 12-volt reference signal is sent out over the A/C request signal circuits, through the A/C high pressure switch and grounded through the HVAC control module when the vehicle operator makes an A/C request. The A/C request signal circuit output from the PCM to the HVAC control module can not survive a short to power. A separate 12-volt reference signal is sent out over the A/C refrigerant low pressure cut-out switch signal circuit, through the A/C low pressure switch and the ground circuit. When both these reference signals are grounded, the PCM may activate the A/C compressor clutch.

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.