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 Module

The HVAC control module is a GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The HVAC module assembly is operated manually by a combination of electrical and mechanical components. The case is of a 3-piece, plastic construction. The left and right cases are assembled with a centre divider that houses the doors and are secured together with screws. The case sump is secured to the upper case assembly with screws. The blower resistor is integrated into the HVAC control module which is screwed to the HVAC blower motor fan assembly, which in turn is secured to the HVAC unit with a combination of pivot pins and screws.

#### A/C 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 engine control module (ECM) converts the voltage signal to a pressure value.

It provides a 0-5 volt output and requires a 5-volt regulated power supply. In operation, the transducer senses applied pressure via the deflection of a 2 piece ceramic diaphragm with one half being a parallel plate capacitor. Changes in capacitance influenced by the refrigerant pressure under the ceramic diaphragm are converted to an analogue output by the transducers integral signal electronics. The pressure transducer's electronics are on a flexible circuit board contained in the upper section of the transducer. They provide linear calibration of the capacitance signal from the ceramic sensing diaphragm. Benefits of using the pressure transducer over a normal type pressure switch is that the transducer is constantly monitoring pressures and sending signals to the ECM. The normal type pressure switch only has an upper and lower cut out point. The ECM will disengage the A/C compressor at low or high refrigerant pressures and electronic diagnostic equipment can be used to extract system pressure information making it easier when diagnosing concerns. As well as acting as an input to the ECM for A/C operation, the ECM also uses the information provided by the pressure transducer to determine when to turn ON and OFF the 2nd Stage cooling fan operation.

A/C Pressure Sensor Operation
Low Pressure Cut Out 180 kPa (26 psi)
Low Pressure Cut In 240 kPa (35 psi)
High Pressure Cut Out 2900 kPa (420 psi)
High Pressure Cut In 2000 kPa (290 psi)
1st Stage Fan Operation On 1517 kPa (220 psi)
1st Stage Fan Operation Off 1214 kPa (176 psi)
2nd Stage Fan Operation On 1758 kPa (255 psi)
2nd Stage Fan Operation Off 1517 kPa (220 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 compartment temperature by adjusting the air temperature switch. 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 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. Pressing the A/C switch, the IRC (Radio) sends switch information via GMLAN to the HVAC. The HVAC unit then requests permission to engage the compressor from the ECM. Upon receiving permission from the ECM, the HVAC then engages the compressor.

• | A/C Pressure is between 196 kPa (28 psi) and 3250 kPa (470 psi).

• | Engine coolant temperature (ECT) is less than 119°C (250°F)..

• | Engine RPM is more than 400 RPM.

• | The A/C request signal is set to engage.

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

• | A/C Pressure is more than 3250kPa (470 psi)..

• | A/C Pressure is less than 196 kPa (28 psi).

• | ECT is more than 119°C (246°F).

• | Throttle position is over 95 percent.

• | Transmission shift

• | ECM detects higher than predicted torque load.

• | ECM detects insufficient idle quality.

• | ECM detects a hard launch condition.

#### A/C Request Signal and A/C Control

The compressor is an externally controlled variable displacement (ECVD) compressor. The HVAC controls the compressor displacement with a pulse width modulated (PWM) signal. When the A/C switch is pressed, the HVAC requests permission to engage the compressor from the ECM. The ECM must grant this permission before the HVAC can engage the compressor. The BCM acts as a gateway to pass these messages on GMLAN between the HVAC (LS bus) and ECM (HS bus). If the conditions listed above are not met, the engine will not grant this permission and A/C will remain at approximately 4 cc (OFF). The A/C indication (LED/Icon) will remain ON. If A/C pressure is below 196 kPa the A/C will be reduced back to approximately 4 cc (turned OFF) and the A/C indication (LED/Icon) will turn OFF. This is a normal condition in very cold ambient temperatures. While the compressor is running the HVAC transmits a Normalised Load value (representing compressor torque) to the ECM. The ECM uses this value to manage engine torque requirements and maintain idle stability. The A/C will be reduced back to approximately 4 cc (turned OFF) if the HVAC fan is turned to zero (OFF).

#### Heater Core

The heater core is located within the HVAC case. The engine coolant flows through the heater core providing heat to warm the vehicle interior and to provide windscreen defogging. The heater core is of a tube and fin design and is constructed of aluminium. It is fitted with a detachable inlet and outlet pipe assembly. Each pipe is attached and sealed to the heater core. Sealing foam is bonded to the sides and around the top of the heater core to prevent air leakage from the HVAC case and to make sure that all air passes through the heater core in the full hot mode.

#### Radiator

The radiator is a heat exchanger. It consists of a core and 2 tanks. The aluminium core is a down flow tube and fin design. This is a series of tubes that extend vertically down from the inlet tank to the outlet tank. Fins are placed around the outside of the tubes to increase heat transfer from the coolant to the atmosphere. The inlet and outlet tanks are moulded with a high temperature, nylon reinforced plastic. A high temperature rubber gasket seals the tank flange edge. The tanks are clamped to the core with clinch tabs. The tabs are part of the aluminium header at each end of the core.

The radiator removes heat from the coolant passing through it. The fins on the core absorb heat from the coolant passing through the tubes. As air passes between the fins, it absorbs heat and cools the coolant.

During vehicle use, the coolant heats and expands. The coolant that is displaced by this expansion flows into the recovery reservoir bottle. As the coolant circulates, air is allowed to exit. This is an advantage to the cooling system. Coolant without bubbles absorbs heat much better than coolant with bubbles.

#### Evaporator

The evaporator is located inside the vehicle housed behind the instrument panel fascia in the HVAC case. It is constructed of aluminium and is of a plate and fin design. The evaporator core is the actual cooling unit of the A/C system. As the low pressure, low temperature refrigerant enters the evaporator, it begins to boil and evaporate. This evaporation process absorbs heat from the air being circulated through the evaporator core by the blower fan. Due to the evaporator being so cold, condensation forms on the surface. This condensation is moisture taken from the air (humidity). Also any dust particles in the air passing through the evaporator become lodged in the condensate water droplets, thus filtering the air from contaminants. The evaporator is constructed of aluminium and is fitted with a detachable inlet and outlet pipe assembly. It is attached and sealed to the evaporator by a single bolt and sealing washers.

#### Condenser

The purpose of the condenser is the opposite of the evaporator. The condenser receives high pressure, high temperature refrigerant vapour from the compressor. It is exposed to a flow of ram air from the movement of the vehicle and as the high pressure high temperature vapour flows inside the condenser tubes, heat is given off to the cooler ambient air flowing past the condenser core. The vapour then condenses into a high pressure, high temperature liquid. Two cooling fans fitted to the rear of the radiator, are activated when required to assist in drawing cool air through the condenser.

#### Receiver Dehydrator

The receiver dehydrator acts as a particle filter, refrigerant storage container and most importantly, a moisture absorber. Moisture, temperature and R-134a causes hydrofluoric and hydrochloric acid. The silica gel beads (desiccant) located in the filter drier receiver absorb small quantities of moisture thus preventing acid establishment.

#### Thermal Expansion Valve (TXV)

The thermal expansion valve (TXV) controls refrigerant gas flow to the evaporator and make sure that complete evaporation takes place. It has 2 refrigerant passages. One is in the refrigerant line from the condenser to the evaporator and contains a ball and spring valve. The other passage is in the refrigerant line from the evaporator to the compressor and contains the temperature sensing element.

#### TXV Opening

As the non-cooled refrigerant from the evaporator core flows through the TXV outlet (suction), it makes contact with the underside of the thin metallic diaphragm and reacts on the refrigerant contained above that diaphragm. This refrigerant then expands, forcing the pin downwards and moving the ball off its seat, then compressing the spring and allowing more refrigerant to enter the evaporator.

#### TXV Closing

Operation is similar to opening but now the refrigerant from the evaporator is cold. The refrigerant contained above the diaphragm now contracts. The ball moves towards the seat aided by the compressed spring, reducing refrigerant flow. Low pressure liquid R-134a passing through the evaporator should be completely vaporized by the time it reaches the TXV outlet side. The TXV is installed in the engine bay to the right side of the instrument panel.

#### Compressor

The Denso compressor can match the air conditioning demand under all conditions without cycling. The basic compressor mechanism is a variable angle swash-plate with 6 axially oriented cylinders. The compressor has a pumping capacity of 160 cc.

The electronic control valve is installed in the compressor rear head. The swash-plate angle of the compressor and the resultant compressor displacement, are determined by the compressor crankcase to suction pressure differential which is governed by the control valve.

When the A/C capacity demand is low, the crankcase pressure behind the pistons is equal to the pressure in front of the pistons. This forces the swash plate to change its angle to towards vertical which reduces the stroke of the pistons and reduces the output of the compressor to approximately 4 cc. The evaporator cooling load is reduced, ambient temperature or blower fan speed is reduced and therefore, the suction pressure is reduced until it reaches the control point. To reach the control point, the electronic control valve assembly allows discharge pressure to bleed past the control valve ball valve seat and into the compressor crankcase. This crankcase pressure acts as an opposing force behind the compressor pistons to cause the swash plate to change its angle towards vertical and therefore, reduce piston stroke.

When the A/C capacity demand is high, the crankcase pressure behind the pistons is less than the pressure in front of the pistons. This forces the swash plate to change its angle away from vertical which increase the stroke of the pistons. The compressor will then have a corresponding increase in its displacement.

### Engine Coolant

The engine coolant is a solution made up of a 50-50 mixture of DEX-COOL and clean fresh water. The coolant solution carries excess heat away from the engine to the radiator, where the heat is dissipated to the atmosphere.

### Cooling Fan Operation

The cooling fans operate in two stages; in both stages both fans run. In stage 1 the two fan motors are connected in series so both fans run at low speed. In stage 2 each fan motor is connected to battery voltage so both fans run at high speed. Cooling fan operation is controlled by the engine control module (ECM) based on inputs from the following:

• | The A/C request signal

• | The vehicle speed sensor (VSS)

• | The A/C refrigerant pressure sensor

• | The engine coolant temperature (ECT) sensor

### Stage One - Both Fans Operate at Low Speed

When the conditions for Stage 1 operation are met the ECM provides a ground to the coil of engine cooling fan relay 1, causing it to operate (turn ON). The fan current path is then from the battery via the large radiator fan fuse, through the large fan motor, cooling fan relay 2, the small fan motor and cooling fan relay 1 to ground.

The conditions for Stage 1 operation are:

• | There is an A/C request and:

• | Vehicle speed is less than 30 km/h (19 mph)

• | A/C refrigerant pressure is greater than 1517 kPa (220 psi).

• | ECT is greater than 98°C (208°F).

• | ECT is greater than 113°C (235°F) when the engine is switched off (in this case stage 1 will operate for approximately four minutes - this is referred to as low fan run-on).

• | An ECT sensor fault is detected and a DTC is set.

Stage 1 operation will cease when:

• | There is no A/C request and the ECT is less than 95°C (203°F).

• | There is an A/C request and the vehicle speed is greater than 50 km/h (31 mph) and the A/C pressure is less than 1214 kPa (176 psi) and the ECT is less than 95°C (203°F).

• | The vehicle speed is greater than 104 km/h (65 mph).

### Stage Two - Both Fans Operate at High Speed

When the conditions for Stage 2 operation are met the ECM provides - in addition to that already provided for the coil of engine cooling fan relay 1 - a ground to the coils of engine cooling fan relays 2 and 3, causing them to operate (turn ON). For the large fan, the current path is then from the battery via the large radiator fan fuse, through the large fan motor and engine cooling fan relay 2 to ground. For the small fan, the current path is from the battery via the small radiator fan fuse, through engine cooling fan relay 3, through the small fan motor and engine cooling fan relay 1 to ground. The conditions for Stage 2 operation are:

• | The A/C refrigerant pressure is greater than 1758 kPa (255 psi).

• | The ECT is greater than 108°C (226°F).

• | An ECT sensor fault is detected and a DTC is set.

If stage 1 operation is off when the conditions for stage 2 operation are met, stage 2 operation will be initiated five seconds after initiation of Stage 1 operation.

Stage 2 operation will cease and revert to Stage 1 operation when:

• | The ECT is less than 102°C (216°F).

• | There is no A/C request.

• | There is an A/C request and the A/C refrigerant pressure is less than 1517 kPa (220 psi).

• | The vehicle speed is greater than 104 km/h (65 mph).

### A/C Cycle

Refrigerant is the key element in an air conditioning system. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

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 a predetermined pressure. The high pressure switch will OPEN the electrical signal to the compressor if the refrigerant pressure becomes higher than required. After the high and the low sides of the A/C system pressure equalise, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor . 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 . The compressor builds pressure on the vapour 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 vapour 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 vapour to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminium tubing and aluminium 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 (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, causing the refrigerant to vaporise at the TXV. The TXV also measures 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 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 vapour 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.