For 1990-2009 cars

The air temperature controls are divided into 2 areas:

• | HVAC Control Components

• | Heating and A/C Operation

### HVAC Control Components

#### HVAC Control Assembly

The HVAC control assembly is a device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. An ignition 3 voltage circuit provides power to the control assembly. The mode, air temperature and recirculation actuators are electrically controlled. The control assembly does not support the following features:

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

#### Air Temperature actuator

The temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 8-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. When the actuator is at rest, both control circuits have a value of 0 volts. In order to move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-8 volts. The HVAC control module uses this value to index the actuator position. When the module sets a commanded, or targeted value, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

### 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. Regardless of the temperature setting, the following can affect 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 manual HVAC system is a single temperature zone system. Moving the air temperature control to the heat position diverts most of the airflow through the heater core, which increases the outlet air temperature. Moving the air temperature control to the cold position diverts most of the airflow around the heater core, which decreases the outlet air temperature.

Pressing the A/C button enables the HVAC control module to request A/C compressor engagement. The HVAC control module sends a signal to the Engine Control Module (ECM) for A/C compressor engagement. The ECM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn ON:

• | Ambient air temperature above 2°C (35°F)

• | A/C refrigerant pressure sensor parameter is less than 3234 kPa.

• | ECM receives an A/C request from the HVAC control module.

• | Engine coolant temperature (ECT) is less than 124°C.

• | The engine RPM is less than 5,800 RPM.

The sensor information is used by the ECM to determine the following:

• | The A/C high side pressure

• | An A/C system load on the engine

• | An excessive A/C high side pressure

• | The heat load at the A/C condenser

• | Outside air temperature

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

• | Ambient air temperature is less than 4°C (39°F).

• | Throttle position is 100 percent.

• | A/C high side pressure is more than 1931 kPa.

• | A/C low side pressure is less than 151 kPa (22 psi).

• | Engine coolant temperature (ECT) is more than 124°C.

• | Engine speed is more than 5,800 RPM.

• | ECM detects excessive torque load.

• | ECM detects insufficient idle quality.