GM Service Manual Online
For 1990-2009 cars only

The air temperature controls are divided into four primary 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. The battery positive and ignition 3 voltage circuits provide power to the control assembly. Two integrated potentiometers control air temperature door position and blower motor speed. The integrated vacuum system controls the mode door position. The control assembly supports the following features:

Feature

Availability

Afterblow

No

Purge

No

Personalization

No

Actuator Calibration

No

Air Temperature Actuator

The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit uses a 0-12-volt linear-ramped signal to command the actuator movement. The 0-12-volt control values represent the opposite limits of the actuator range of motion. The values in between 0-12 volts correspond to the positions between the limits. The control signal sets to a value of between 0-12 volts when the HVAC control assembly sets a commanded, or targeted, value. The actuator shaft rotates until the commanded position is reached. Once a new commanded value is needed the HVAC control assembly control value changes.

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 disables the compressor clutch under the following conditions:

   • 

A/C pressure is more than 2979 kPa (432 psi). The clutch will be enabled after the pressure decreases to less than 1510 kPa (219 psi).

   • 

A/C pressure is less than 186 kPa (27 psi). The clutch will be enabled after the pressure increases to more than 207 kPa (30 psi).

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 assembly makes the following actions when an air temperature setting is selected:

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

The A/C system can be engaged by pressing the A/C switch. The A/C switch will illuminate when the A/C switch is pressed to the on position. The A/C compressor will also run automatically when the mode settings are in the Defog or Defrost setting to help eliminate humidity from inside the passenger compartment. The HVAC control assembly sends a A/C request message to the body control module (BCM). The PCM receives the ambient air temperature reading from the Intake Air Temperature Sensor during startup. The following conditions must be met in order for the PCM to turn ON the compressor clutch:

    • BCM
       - Battery voltage between 9-16 volts
       - A/C request from the HVAC control assembly
    • PCM
       - Ambient temperature more than 3°C (38°F)
       - Engine coolant temperature (ECT) is less than 121°C (250°F)
       - Engine RPM is below 5,000 RPM
       - A/C Pressure is between 1510 - 2979 kPa (219 - 432 psi)

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

    • Compressor thermal switch is opened.
    • Throttle position is 95 percent or more
    • A/C Pressure is more than 2896 kPa (420 psi)
    • A/C Pressure is less than 186 kPa (27 psi)
    • Engine coolant temperature (ECT) is more than 121°C (250°F)
    • Engine speed is more than 5,500 RPM
    • Battery voltage less than 10.5 volts
    • Transmission shift
    • PCM detects excessive torque load
    • PCM detects insufficient idle quality
    • PCM detects a hard launch condition

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

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal engine operating coolant temperature. The thermostat also creates a restriction for the cooling system that promotes a smooth coolant path 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.

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 Delphi model V-5 compressor is used on this model year vehicle.

The A/C system used on this vehicle is a non-cycling system. Non-cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal to the compressor clutch, if the refrigerant pressure becomes excessive. After the high and the low sides of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor clutch. The A/C system is also 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 continues to rise, the high pressure relief will pop open and release refrigerant from the system.

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. The refrigerant is discharged from the compressor through the discharge hose, and forced through the condenser and then through the balance of the A/C system.

Compressed refrigerant enters the condenser at a high-temperature, high-pressure vapor state. As the refrigerant flows through the condenser, the heat is transferred to the ambient air passing through the condenser. Cooling 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, causing the refrigerant to vaporize at the orifice tube. The orifice tube also measures 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 to boil inside 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 flows 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 condenses, and discharges from the HVAC module as water.