Air Temperature Part 2

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

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 O-rings.

Filter Drier
The filter drier receiver 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 ensures 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 Delphi V7 compressor can match the air conditioning demand under all conditions without cycling. The basic compressor mechanism is a variable angle wobble-plate with 7 axially oriented cylinders. The compressor has a pumping capacity of 179 cc.

The control valve is installed in the compressor rear head. The wobble-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 wobble plate to change its angle to towards vertical which reduces the stroke of the pistons and reduces the output of the compressor to approximately 14.5 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 bellows in the control valve assembly has expanded to allow 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 wobble 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 wobble plate to change its angle away from vertical which increase the stroke of the pistons and increases the output of the compressor to approximately 164 cc. When suction pressure is above the control point, it will compress the control valve bellows. This will close off the discharge valve as the ball valve is now on its seat. The shuttle valve moves towards the suction port and opens the suction valve. Crankcase pressure will then bleed from the compressor crankcase past the suction valve to the suction port. As the crankcase pressure behind the pistons is reduced, the wobble plate will tilt from vertical causing the pistons to move towards maximum stroke. The compressor will then have a corresponding increase in its displacement.

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 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. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

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
The ECM determines when the engine cooling fans should operate at Stage 1 (engine cooling fan relay 1 is energized and both fans, being connected in series, run at low speed) based on inputs from the A/C request signal, VSS and the ECT sensor. 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) or;
- A/C refrigerant pressure is greater than 1500 kPa (218 psi) or;
- ECT is greater than 98°C (208°F) or;
- ECT is greater than 1130°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) or;
- An ECT sensor fault is detected and a DTC such as P0117, P0118, P1114 or P1115 is set.

Stage 1 operation will cease when:
- There is no A/C request and the engine coolant temperature is less than 95°C (203°F) or;
- 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 1170 kPa (170 psi) and the ECT is less than 95 °C (203 °F) or;
- The vehicle speed is greater than 104 km/h (65 mph).

Stage Two - Both Fans Operate at High Speed
The ECM also determines when the engine cooling fans should operate at Stage 2 (that is, engine cooling fan relays 1, 2 and 3 are energized and both fans, each being connected to battery voltage, run at high speed) based on inputs from the A/C request signal, VSS and the ECT sensor. 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 2400 kPa (348 psi) or;
- The ECT is greater than 108 °C (226 °F) or;
- An ECT sensor fault is detected and a DTC such as P0117, P0118, P1114 or P1115 is set.
- There is a body control module (BCM) message response fault, which will cause a powertrain interface module (PIM) DTC B2002 to 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 engine coolant temperature is less than 102 °C (216 °F) and;
- There is no A/C request or;
- There is an A/C request and the A/C refrigerant pressure is less than 1900 kPa (276 psi) or;
- 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 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.

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 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 vaporize 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 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.