For 1990-2009 cars

The air temperature controls are divided in five primary areas:

• | HVAC Control Components

• | Heating and A/C Operation

• | Automatic Operation

• | Engine Coolant

• | A/C Cycle

### HVAC Control Components

Important: Anytime a repair or the HVAC control module connection is opened, the control head will not display correct temperature information for approximately 5-10 minutes after reconnecting.

#### HVAC Control Module - Automatic Climate Control

The HVAC control module is a class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. To change the HVAC display from degrees Fahrenheit to degrees Celsius press the Auto, Econ and Mode buttons together for 3-5 seconds. The battery positive voltage and ignition 3 voltage circuits provide power to the control module. The body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features:

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

#### Air Temperature Actuator

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts. The HVAC control module uses a range of 0-255 counts to index the actuator position. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

#### Inside Air Temperature Sensor

Replacement of the inside air temperature sensor means replacing the HVAC control module, auto climate control head. The inside air temperature sensor is built into the HVAC control module. The HVAC control module monitors the passenger compartment air temperature through an inside air temperature sensor. The sensor is a thermistor used to control signal voltage at the HVAC and heater control. The module supplies a reference voltage of 5 volts internally to sensor and measure voltage drop in the reference circuit to obtain a temperature signal. The HVAC control module requests A/C compressor clutch engagement or heating, controlling the air temperature actuator door positions in order to maintain the selected air temperature.

#### A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-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 5 volts.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The PCM/ECM disables the compressor clutch under the following conditions:

• | A/C high side pressure is more than 2900-2997 kPa (420-435 psi). The clutch will be enabled after the A/C high side pressure decreases to less than 1998 kPa (290 psi).

• | A/C low side pressure is less than 180 kPa (26 psi). The clutch will be enabled after the A/C low side pressure increases to more than 248 kPa (36 psi).

#### Evaporator Low Ambient Protection (ELAP)

ELAP is a BCM controlled cycling of the compressor clutch to prevent freezing of the evaporator core. ELAP is only enabled at ambient temperatures between 0-20°C (32-68°F)

#### Outside Air Temperature Sensor

The outside air temperature sensor is a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:

• | Outside

• | Inside

A 5-volt reference and signal circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases.

The sensor operates within a temperature range of -40 to +60°C (-40 to +140°F). The outside air temperature sensor is mounted in the forward engine area of the vehicle. In this position, it is exposed to the airflow through the grill before it reaches the radiators. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a working temperature range.

If the HVAC control module detects a faulty class 2 signal, then the control module software will use a defaulted air temperature value.

The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the system is fixed.

#### Ambient Light Sensor

The ambient light sensor is a 2-wire photo diode. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor conductance increases. The sensor signal decreases as the conductance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts.

The ambient light sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted solar load value. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. A resistance check of the ambient light sensor should not be performed as it will damage the sensor.

### Heating and A/C Operation

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:

• | Recirculation actuator setting

• | Difference between inside and desired temperature

• | Difference between ambient and desired temperature

• | Blower motor speed setting

• | Mode setting

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

• | When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.

• | When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

• | When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air through the heater core in order to achieve the desired temperature:

- | Ambient light

- | Outside air temperature

- | Inside air temperature

The following shows what blower motor, recirculation actuator, and air temperature actuator adjustments are made for each air temperature switch selection:

The A/C system can be engaged by either pressing the A/C switch, setting the mode switch in Mix Blend, Defrost or pressing the automatic operation switch. The A/C ON status will illuminate on the HVAC control module display when the A/C switch is pressed. The control module sends a class 2 A/C request message to the body control module (BCM). The HVAC system uses a variable displacement rotary vane type compressor that incorporates a thermal link fuse in the A/C compressor supply voltage circuit that opens once the compressor temperature is more than 211- 217°C (380 - 454°F). The thermal link fuse is a serviceable item. The following conditions must be met in order for the PCM/ECM to turn on the compressor clutch:

• | HVAC control module

- | Ambient temperature more than 0°C (32°F)

- | Evaporator Temperature more than 0°C (32°F)

• | BCM

- | Battery voltage between 9-18 volts

- | A/C request from the HVAC control module

• | PCM/ECM

- | Engine coolant temperature (ECT) is less than 114-118°C (237-244°F)

- | Engine rpm is below 5000 rpm

- | A/C Pressure is between 248-2997 kPa (36-435 psi)

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

• | Compressor thermal switch is opened

• | Throttle position is 100%

• | A/C Pressure is more than 2997 kPa (435 psi)

• | A/C Pressure is less than 248 kPa (36 psi)

• | Engine coolant temperature (ECT) is more than 120°C (248°F)

• | Engine speed is more than 5000 rpm

• | Transmission shift

• | PCM/ECM detects excessive torque load

• | PCM/ECM detects insufficient idle quality

• | PCM/ECM detects a hard launch

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

To protect the system from high pressure, a pressure relief valve is located in the front head of the compressor. This valve will open at 3965 - 3375 kPa (575 - 490 psi) and reseat at 2930 kPa (425 psi). If this valve ever opens the A/C system must be serviced and the valve must be replaced.

### Automatic Operation

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation. To place the HVAC system in Automatic mode you must press the Auto mode switch located on the left side of the HVAC module. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

• | Monitor the following sensors:

- | Outside air temperature sensor

- | Ambient light sensor

- | Inside air temperature sensor

• | Regulate blower motor speed

• | Position the air temperature actuator

• | Position the mode actuator

• | Position the recirculation actuator

• | Request A/C operation

### Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal operating temperature of the engine. 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 presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

A Zexcel rotary vane compressor with variable displacement is 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 TXV.

The TXV is located at the evaporator inlet. 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 on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil 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 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.