For 1990-2009 cars

The air temperature controls are divided into 4 areas:

• | HVAC Control Components

• | Heating and A/C Operation

• | Engine Coolant

• | A/C Cycle

### HVAC Control Components

#### HVAC Control Assembly

The HVAC control assembly is a non-class 2 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 air temperature and mode doors are controlled by cables. The recirculation actuator is electrically controlled. The control assembly does support the following features:

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

#### Air Temperature Actuator

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, control A , control B, 5-volt reference and position signal circuits enable the actuator to operate. Two control circuits enable the actuator to operate. The control circuits use either ground or a 12-volt value to coordinate the actuator movement. The HVAC control assembly reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometer's 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. The door position signal voltage is converted to a 0-255 count range. 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 opens the control circuits.

#### 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 powertrain control module (PCM) disables the compressor clutch under the following conditions:

A/C pressure is more than 2,951 kPa (428 psi). The clutch will be enabled after the pressure decreases to less than 2,068 kPa (300 psi). A/C high side pressure is less than 310 kPa (44 psi). The clutch will be enabled after the A/C high side pressure increases to more than 310 kPa (44 psi).

#### Evaporator Temperature Sensor

The evaporator temperature sensor will not allow an A/C request to the body control module (BCM) at temperatures less than 0°C (32°F).

#### 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

• | Difference between inside and desired temperature

• | Difference between ambient and desired temperature

• | Blower motor speed setting

• | Mode setting

The vehicle operator can activate the A/C system by pressing the A/C switch. The A/C system can operate regardless of the air temperature or mode setting. When the A/C switch is pressed, the HVAC control assembly applies 10 volts to the A/C request signal circuit. The voltage is then sent through the evaporator temperature sensor, if the evaporator temperature is above 0°C (32°F), then the voltage is applied to the other A/C request signal circuit to the BCM. The BCM receives the voltage input and sends a class 2 message to the PCM for an A/C request.

The PCM will operate the A/C system automatically in FRONT DEFROST mode to help reduce moisture inside the vehicle. The A/C LED will not illuminate unless the driver presses the A/C request switch on the HVAC control assembly. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode or Recirc. The following conditions must be met in order for the PCM to turn on the compressor clutch:

• | BCM

- | Battery voltage is between 9-16 volts.

- | A/C request from the HVAC control assembly

• | PCM

- | Engine coolant temperature (ECT) is less than 123°C (253°F).

- | Engine speed is less than 5300 RPM.

- | Engine speed is more than 600 RPM.

- | A/C high side pressure is between 2951-310 kPa (428-44 psi).

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

• | Throttle position is 100 percent for 10 seconds.

• | A/C high side pressure is more than 2951 kPa (428 psi) and will re-engage once the pressure drops below 2068 kPa (300 psi).

• | A/C high side pressure is less than 310 kPa (44 psi).

• | Evaporator temperature is less than 0°C (32°F)

• | Engine coolant temperature (ECT) is more than 123°C (253°F).

• | Engine speed is less than 475 RPM.

• | Engine speed is more than 6000 RPM.

• | PCM detects excessive torque load.

• | PCM detects insufficient idle quality.

• | If the blower is switched to off.

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

#### Engine Coolant

Engine coolant is the key element of the heating system. The thermostat controls 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 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 air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.