HVAC Control Components through Heating and A/C Operation
AIR TEMPERATURE DESCRIPTION AND OPERATIONThe air temperature controls are divided into 4 areas:
- HVAC Control Components
- Heating and A/C Operation
- Engine Coolant
- A/C Cycle
HVAC CONTROL COMPONENTS
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 HVAC unit is operated manually by a combination of electrical, mechanical and vacuum components. The case is of a 4-piece, plastic construction. The front and rear housing are assembled without the use of any fasteners. The front housing clips over the rear housing at 6 locations. Two equally sized recirculation doors are used to control airflow into the HVAC unit. Two air mix doors are used to control the airflow through the heater core. A single door is used to control airflow to the front defrost, floor and panel outlets. All doors excluding the air mix doors are vacuum operated. The air mix doors are operated mechanically. The blower motor/fan assembly, blower motor resistor, heater core and evaporator are all contained within the HVAC case. A vacuum tank is mounted to the left side of the case. Four externally mounted vacuum actuators are used to provide the selected ventilation modes. Contained within this assembly is a printed circuit board retained within the rear housing.
Temperature Switch
When the temperature switch is turned clockwise from the coldest position to the hottest position, the pinion and crescent gear mounted to the rear of the controller are rotated. This action simultaneously opens airflow through the heater core and cuts off vacuum to the water valve allowing heated coolant to flow through the heater core.
Cold Operation
Vacuum generated within the inlet manifold of the engine is stored within the vacuum tank mounted on the side the HVAC unit. Vacuum is retained within the HVAC system by the one way check valve and is directed to the water valve vacuum switch mounted to the rear of the HVAC controller. When the temperature switch is turned to the Full Cold position, the pinion gear rotates the crescent gear so that the ramp on the crescent gear pushes the plunger inside the water valve vacuum switch inward against spring pressure. In this position, the water valve vacuum switch allows vacuum to be directed to the water valve. When vacuum is applied to the water valve vacuum actuator, no coolant can flow through the heater core. However, coolant is still able to flow from the engine through the water valve via its engine side ports and engine side heater hoses. The crescent gear is also mechanically connected to the HVAC air mix doors via the actuating rod and levers. As well as operating the plunger of the water valve vacuum switch, the crescent gear simultaneously locates the air mix door in a position that does not allow any air to flow through the heater core in the Full Cold mode. The result is that all air entering the vehicle cabin will be cold air.
Warm Operation
When the temperature switch is turned from the Full Cold position, the crescent gear rotates backwards moving the ramp away from the plunger of the water valve vacuum switch. Spring pressure moves the plunger outward and at the third detent position, the vacuum line to the water valve actuator is vented through the exhaust port of the water valve vacuum switch. When the actuator is relieved of vacuum, the disc in the water valve will rotate and allow hot water to flow through the cabin side water valve ports and the cabin side heater hoses into the heater core. As the crescent gear rotates backward, it pulls the air mix doors open. When the temperature switch is turned to a Warm position, the air mix doors will be partially open. This will cause some incoming air to pass through the heater core and some to air to bypass the heater core. The mixture of heated and cool air will result in warm air entering the vehicle cabin.
Hot Operation
When the temperature switch is turned to in the Full Hot position the water valve vacuum switch plunger remains in the same position. Therefore, the water valve remains in the fully open position because the water valve actuator is devoid of vacuum. In the Full Hot position the crescent gear will be rotated fully rearward. This action will move the air mix doors to a position that directs all incoming air through the heater core. Therefore, all air entering the vehicle cabin will be heated air.
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 PCM converts the voltage signal to a pressure value.
The pressure transducer is a sealed gage reference capacitive pressure sensor with on board signal conditioning. 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 powertrain control module (PCM). The normal type pressure switch only has an upper and lower cut out point. The PCM 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 problems. The A/C pressure transducer is located to the left side of the radiator assembly. Removal of the upper radiator shroud is required to gain access. As well as acting as an input to the PCM for A/C clutch operation, the PCM also uses the information provided by the pressure transducer to determine when to turn ON and OFF the 2nd Stage cooling fan operation. If there is a failure in the A/C Pressure Sensor circuit DTC P0530 A/C Refrigerant Pressure Sensor Circuit, will set.
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 module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:
- When the air temperature switch is placed in the warmest position, vacuum commands the air temperature door to divert maximum air past the heater core.
- When the air temperature switch is placed in the coldest position, vacuum 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. Pressing the A/C switch the control module grounds A/C request signal circuit from the powertrain control module (PCM). The following conditions must be obtained before A/C compressor engagement is allowed:
- A/C Pressure is between 180 kPa (26 psi) and 2 900 kPa (420 psi).
- Engine coolant temperature (ECT) is less than 121°C (250°F).
- Engine RPM is more than 550 RPM.
- The A/C request signal circuit is grounded.
Once engaged, the compressor clutch will be disengaged for the following conditions:
- A/C Pressure is more than 2 900 kPa (420 psi).
- A/C Pressure is less than 180 kPa (26 psi).
- Engine coolant temperature (ECT) is more than 121°C (250°F).
- Engine speed is more than 5,500 RPM.
- Throttle position is 100 percent.
- 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.
A/C Request Signal and A/C Clutch Control
The BCM requests the PCM to turn the A/C compressor ON or OFF via the powertrain interface module (PIM) and the serial data bus normal mode message. The BCM monitors the voltage at the BCM A/C request circuit in order to determine the status of the momentary A/C switch. When the A/C switch is pressed, 12 volts is applied to the BCM A/C request circuit. The BCM sees this high voltage as an A/C switch input signal. On receiving the A/C switch input signal, the BCM requests the PCM to energize the A/C clutch via the PIM and the serial data bus normal mode message. If the ignition is ON and the blower motor is operating. The BCM also provides a ground to illuminate the A/C ON LED within the A/C switch. If the A/C switch is pressed again the BCM will request the PCM to turn OFF the A/C compressor. The operating status of the system will be remembered by the BCM, when the ignition is switched from ON to OFF or when the blower is switched OFF. If the blower is OFF and the A/C switch is pressed, then the next time the blower is switched ON the air conditioning will be turned ON. Turning the ignition OFF will cancel this button press function.
The system will reset to OFF when the battery is disconnected. The PCM uses this signal to:
- Adjust the idle air control (IAC) valve position to compensate for the additional load placed on the engine by the air conditioning compressor.
- Energize the A/C compressor relay to operate the A/C compressor if the pressure in the A/C system is within the correct operating range.
The BCM monitors the blower motor switch setting to determine whether the blower motor has been selected or not. When the blower motor is running, BCM blower input circuit is pulled to ground. This causes the voltage at the BCM to change from battery voltage, +12 volts in the OFF position, to less than 0.3 volts, which is seen by the BCM as the blower ON signal. The BCM does not have any direct control over the operation of the air conditioner. When the A/C switch is turned ON at the HVAC controller, it passes this request received at the BCM to the PCM. The PCM then determines from other engine and A/C parameters whether the A/C clutch will be energized or not. Communication between the PCM and the BCM takes place via the Primary Serial Data bus.
The PCM monitors the A/C pressure sensor to determine A/C system pressure. There are 2 DTCs associated with the A/C system:
- P1539-Sets when the PCM detects voltage on the A/C clutch status terminal when the system has not requested A/C. A short to voltage at any point in the A/C status circuit, or the A/C relay contacts are stuck.
- P1546-Sets when the PCM activates the relay and does not detect the presence of a voltage at both the A/C compressor clutch and the A/C clutch status circuit at the PCM.
Heater Core
The heater core is located within the HVAC case. When the water valve is in the open position, 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 by a single screw, retaining clip and O-ring. 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 ensure that all air passes through the heater core in the full hot mode.
Water Valve
The heater water valve is located in the engine bay. The vacuum line attached to the water valve vacuum actuator is connected to the water valve vacuum switch mounted on the HVAC controller. When full vacuum is applied to the water valve actuator, full closure of the valve occurs and no coolant will flow through the heater core.
Radiator
The condenser is mounted to the front of the radiator and is located and supported by 4 clips moulded into the front of the plastic radiator tanks. The lower clips lock the condenser in place and can be released by hand to facilitate condenser removal. The cooling fans motors are each attached by 3 screws to the one piece plastic fan shroud. The fan shroud is mounted to the rear of the radiator and is located and supported by 4 clips moulded into the rear of the plastic radiator tanks. The upper clips lock the fan shroud in place and can be released by hand to facilitate fan shroud removal. The shroud must be removed to facilitate fan motor and blade assembly removal. Two harness connectors are mounted to the upper section of the fan shroud allowing the fan motor and blade assemblies to be removed individually from the shroud. The fan motor and blade is balanced as an assembly. These 2 components are serviced only as a unit and must not be separated. The condenser, filter drier receiver, radiator and the fan motors/blades/shroud assembly can be removed and installed individually from the vehicle.
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. The condenser is of a parallel flow design. It has 38 horizontal, flat section tubes with side mounted header tubes. All tubes and fins are constructed of aluminium. It is mounted to the front of the radiator and is attached to the radiator tanks by 4 identical mounting clips that are assembled with screws to the condenser sides. The filter drier receiver is attached to the front of the condenser on the left side.
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 moving the ball off its seat, 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.