For 1990-2009 cars

The air temperature controls are divided into 4 primary areas:

• | The A/C cycle

• | HVAC Control Components

• | Instrument Panel - Driver Information Center (DIC)

• | The Heating and Air Conditioning System

### Driver Information Center

#### Instrument Panel - Display

This vehicle may be equipped with a driver information center and it displays outside temperature through the I/P Cluster display. If an outside temperature display concern occurs please refer to Outside Air Temperature Display Inaccurate or Inoperative.

### HVAC Control Components

#### HVAC Control Module

The HVAC control module is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 1 voltage circuits provide power to the control assembly. The control module supports the following features:

Feature | Availability
--- | ---
Afterblow | No
Purge | No
Personalization | No
Actuator Calibration | Yes

#### Air Temperature Actuator

The air temperature actuator is a XXX-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. Both control circuits have a value of 0 volts when the actuator is at rest. In order to move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The polarity of the control circuits to move the actuator in the opposite direction gets reversed by the HVAC control module. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between XXX-XXX volts.

#### A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a XXX-wire piezoelectric pressure transducer. A XXX-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near XXX volts.

The A/C refrigerant pressure sensor prevents the A/C system from operating when an excessively high or low pressure condition exists.

### Heating and A/C Operation

The purpose of the heating and A/C system is to provide the following:

• | Cooled air

• | Heated air

• | Remove humidity from the interior of the vehicle

• | Reduce windshield fogging

The following can effect the rate that the HVAC system can achieve a desired temperature, regardless of the temperature setting:

• | Blower motor speed setting

• | Difference between ambient and desired temperature

• | Difference between inside and desired temperature

• | Mode setting

• | Recirculation actuator setting

The HVAC control module commands or monitors the following actions when an air temperature setting is selected:

• | COLDEST POSITION--The air temperature actuator door position directs maximum air flow around the heater core.

• | WARMEST POSITION--The air temperature actuator door position directs maximum air flow through the heater core.

The A/C system is engaged by selecting the A/C switch for the blower motor and the mode switch on the HVAC control module. The HVAC control module sends a A/C request message to the body control module (BCM) which sends a high speed serial bus message to the powertrain control module (PCM) or engine control module (ECM) for A/C compressor clutch operation. To turn on the compressor clutch the following conditions must be met in order for the PCM or ECM:

• | HVAC control module

- | Control module operating range 9 and 16 volts.

- | Evaporator temperature more than XXX°C (XXX°F).

• | PCM or ECM

- | Engine coolant temperature (ECT) is less than XXX4°C (XXX°F).

- | Engine RPM is between 0 RPM and 5,000 RPM.

- | Correct pressure in A/C system.

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

• | Throttle position is 100 percent.

• | ECT is more than XXX°C (XXX°F).

• | Engine speed is more than 5,000 RPM.

• | Transmission shift.

• | PCM or ECM detects excessive torque load.

• | PCM or ECM detects insufficient idle quality.

• | Incorrect pressure in A/C system.

• | PCM or ECM detects a hard launch condition.

When the compressor clutch disengages, the electrical system is protected from a voltage spike by the compressor clutch diode.

### Remote Vehicle Start HVAC Operation

When vehicle is started in remote start status, the HVAC control head settings will power up at the same settings it was at prior to shutdown. If the fan knob is left in the OFF position, the climate control system will not operate during remote start.

### Engine Coolant

The engine coolant is the key element of the heating system. The engine thermostat controls the normal engine operating coolant temperature. 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 ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. The HVAC module distributes heated air to the passenger compartment for consistent 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 is recirculated back through the engine cooling system.

### A/C Cycle

Refrigerant is the key element in an air conditioning system. R-134a is an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air and it is presently the only EPA approved refrigerant for automotive use.

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 with 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 (TXV) is located in the liquid line between the condenser and the evaporator. The TXV is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the pressure of the refrigerant is lowered. Due to the pressure differential of the liquid refrigerant, the refrigerant will begin to vaporize at the TXV. 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 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.