HVAC System - Mechanism & Function: Other Subaru Outback BP

Heater System

The heater control unit is located in the middle portion of the instrument panel.

The heater unit has mode doors and an air mix door. The intake unit has intake doors and a blower motor. The heater unit and the intake unit are regulated by their control units.

Fresh outside air is introduced into the cabin through the center and side ventilators when the blower fan is operated.

All models are equipped with front side window defrosters.

An optional filter is provided in front of the evaporator inlet.

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AIR DISTRIBUTION RATIO

The following diagram shows air distribution for each position of the mode selector switch.

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Blower System

The blower relay is ready to be activated when the ignition switch is in the ON position. With the ignition switch ON, placing the fan switch in any position other than OFF activates the relay, allowing electric current to flow from the battery to the ground through the blower motor, the resistor, and the selected fan switch contacts. The connected resistor(s) vary depending on the selected position of the fan switch and cause the blower motor speed to change.

Scheme 10

Filter (Option)

The optional filter is located in front of the cooling unit's evaporator inlet.

The air conditioning system may fail to exhibit its full performance if the filter is excessively clogged with dust and dirt. It is essential to replace the filter with a new one at the specified interval.

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GENERAL

The refrigerant that circulate in the air conditioning system flow out of the compressor, pass through the condenser, receiver drier and evaporator, and return to the compressor.

The flow of refrigerant to the evaporator is controlled by an expansion valve located inside the evaporator.

The compressor operates and stops repeatedly to maintain the evaporator temperature within a specified range. When the evaporator temperature falls below the specified temperature, the ther-mo-control amplifier stops the compressor operation. When the evaporator temperature rises above the specified temperature, the thermo-control amplifier puts back the compressor into operation.

The refrigerant system is protected against excessively high or low pressures by a pressure switch. If the system pressure rises or drops excessively, the pressure switch is activated to prevent the compressor from operating.

Scheme 12

The compressor is a rotary type that has a rotor fitted with five radially movable vanes. The rotor rotates together with the vanes in an elliptical cylinder. As the rotor rotates, the volume of each closed space formed between two adjacent vanes (referred to as "cylinder chamber" in the following description) decreases, so that the pressure of the refrigerant gas confined in the cylinder chamber increases. In this way, the rotary compressor performs its function as a pump. The pumping cycle consisting of suction, compression and discharge takes place 10 times during every rotation of the rotor.

On the discharge side of the cylinder, a roll valve is provided that opens at a predetermined high pressure. Air tightness between the rotor shaft and front head is ensured by the shaft seal. The trigger valve incorporated in the front side block provides the function of applying back pressure to the vanes. The compressor contains necessary quantity of compressor oil. The oil is distributed to all the parts requiring lubrication and sealing by utilizing the discharge pressure of the refrigerant.

Scheme 13

As the rotor rotates, the volume of each cylinder chamber changes. This creates the compressor's suction, compression and discharge functions as explained in the following

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1. Suction: Low-pressure gaseous refrigerant is forced out from the evaporator by rotation of the compressor. It enters the low-pressure chamber in the rear head through the check valve. The refrigerant is then drawn into the cylinder by rotation of the vane-fitted rotor through the two suction ports provided in the rear side block. Air tightness of the cylinder chambers is maintained by the compressor oil.
2. Compression: Further rotation of the rotor after suction makes the volume of each cylinder chamber smaller, thus compression occurs.
3. Discharge: When the pressure of refrigerant in the cylinder chamber exceeds a predetermined pressure, the roll valve opens to discharge the refrigerant through a pipe-shaped passage built in the front side block into the high-pressure chamber in the front head. The gaseous refrigerant in the high-pressure chamber is led to a baffle, which separates the compressor oil contained in the refrigerant before it flows into the high-pressure piping.

TRIGGER VALVE

This valve has a function of maintaining a proper level of pressure behind the vanes (vane back pressure) such that they can move easily upon start of the compressor. The trigger valve is incorporated in the front side block and its end opens to a cavity called K-ditch that is provided in the rotor side end of the side block. The valve consists of a check ball and a spring.

The vanes are prone to chatter if there is only small difference between the high- and low-pressures. This condition typically occurs when the compressor is started. In such a condition, the spring raises the ball to open the valve and allows the back pressure to act on the vanes, thereby ensuring smooth operation.

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Scheme 19

1. When compressor starts or when load is low: When the compressor starts or when the load is low (the high-pressure level is low), the spring can raise the check ball clear of its seat, so the trigger valve is opened. The pressure of the high-pressure chamber then acts on the back end surface of each vane to prevent it from chattering.
2. When compressor is in regular operation: When the pressure in the high-pressure chamber of the compressor increases, the pressure overcomes the spring tension and pushes the check ball against its seat, so the trigger valve closes. The oil port pressure coming through the side block is applied to the end surface of vane to maintain proper back pressure.

CHECK VALVE

A check valve consisting of a spherical plate and spring is provided at the suction port of the rear head. Immediately after the compressor has stopped, there is large difference between the high and low-pressures. This would cause reverse rotation of the compressor and consequent reverse flow of refrigerant to the evaporator if no check valve is provided. Immediately after the compressor has stopped, the high-pressure refrigerant forces the check valve plate upward and closes the suction port to prevent flow of refrigerant from the high-pressure side to the low-pressure side.

Scheme 20

MAGNET CLUTCH

The magnet clutch serve to transmit engine power to the compressor module. It is built into the compressor shaft. When current flow through the magnet clutch coil, the drive plate is attracted so that the pulley and compressor shaft rotate as a module. When the compressor is not in use, the pulley alone rotates freely.

The compressor used with the six-cylinder engine has a lock sensor. If the sensor detects locking of the compressor resulting from a fault, it causes disengagement of the magnet clutch to protect the engine and the power steering drive.

Scheme 21

The high-temperature and high-pressure gaseous refrigerant discharged from the compressor is cooled down and converted into liquid by the condenser.

The condenser consists of tubes and radiating fins.

The heat of the refrigerant flowing through the condenser tubes is released into to the ambient air which is caused to flow across the fins by the cooling fan.

Scheme 22

The amount of refrigerant necessary to circulate in the system varies with change in the heat load.

The receiver drier stores part of the liquid refrigerant until an increased heat load requires its use again. The receiver drier also has the following functions

Scheme 23

1. It removes bubbles from the liquid refrigerant. (If bubbles are present, the refrigerant passing through the expansion valve varies in quantity, temperature, and pressure, resulting in insufficient cooling.)
2. It removes moisture from the refrigerant.
3. It removes foreign substance from the refrigerant. The receiver drier contains a strainer to remove foreign substance and desiccant to absorb moisture from refrigerant.

SOHC MODELS

The pressure switch is located on the high-pressure line to the receiver drier. When an abnormally high or low pressure occurs in the high-pressure line, the pressure switch turns OFF to stop operation of the compressor.

Scheme 24

1. When the pressure is abnormally low [177 kPa (1.8 kgf/cm 2 , 26 psi) or less] The pressure switch turns OFF assuming that the refrigerant is lost due to leakage.
2. When the pressure is abnormally high [2,940 kPa (30 kgf/cm 2 , 427 psi) or more] The pressure switch turns OFF to prevent the system from being damaged.

DOHC MODELS

Installed to the high pressure side of the refrigerant cycle and contains a diaphragm that receives pressure, a snap plate, a rod, contacts which operate when the low pressure side pressure is abnormally high, and contacts which operate at mid pressure.

Scheme 25

1. Prevents operation of the system when refrigerant is lost due to leakage (when pressure is too low)
2. Protects the system from abnormally high refrigerant pressures (when pressure is too high)
3. Detects compressor load (mid pressure contact: electric cooling fan output control)

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Scheme 27

1. High/low pressure contact (compressor ON/OFF control)
2. Mid pressure contact (electric cooling fan output control)

Air pushed by the blower passes through the cooling fins and tubes of the evaporator. Since the air is warmer than the refrigerant, the heat of air moves to the refrigerant through the fins and tubes. As the low-pressure refrigerant moves through the evaporator, heat from the air causes the refrigerant to boil. By the time the refrigerant has passed through the evaporator, it becomes vapor. Moisture in the air condenses to water drops as it moves around the tubes and fins of the evaporator. Water and dirt are then discharged outside the vehicle through a drain hose.

The evaporator is a laminated type and consists of thin, rectangular aluminum plates arranged in multiple layers and fins that are attached between them. During flow through the evaporator, the state of the refrigerant changes as follows

Misty refrigerant (very close to liquid form) from the expansion valve at a low-pressure, enters the lower tube of the evaporator, where it soaks up heat from the compartment. The refrigerant boils and vaporizes quickly due to the rapid heat exchange. Then the refrigerant is pushed upward by the force of the bubble generated during the heat exchange and enter the upper tube. When it reaches the upper tank, the refrigerant is in a thoroughly vaporized state.

The evaporator has a single tank, and its surface has been given the following treatments.

Scheme 28

1. Rustproof treatment
2. Waterproof treatment
3. Moldproof treatment

The expansion valve is connected to both the evaporator inlet and outlet pipes. It converts high-pressure liquid refrigerant which comes from the receiver drier to misty, low-pressure refrigerant which is delivered to the evaporator. Being at low pressure and low temperature, this refrigerant can easily evaporate in the evaporator and remove heat from the cabin air. The valve performs this conversion by automatically controlling the flow rate of refrigerant according to the cooling ability required by the heat load.

The refrigerant temperature is sensed by the temperature sensing element located in the low-pressure refrigerant passage of the expansion valve, and the flow rate of the refrigerant is controlled by changing the lift of the valve ball located in the high-pressure passage.

Scheme 29

When the heat load to the air conditioning system increases, the refrigerant temperature at the evaporator outlet rises and therefore the pressure P 1 around the temperature sensing area increases. As this pressure P 1 becomes higher than the sum of the evaporator outlet (low-pressure side) pressure P 2 and the spring force F (P 1 P 2 + F), the diaphragm is pressed down, moving the valve ball connected to the diaphragm clear of its seat. This increases the flow of the refrigerant.

When the heat load is small, the action of the valve's inner elements is contrary to the above; the valve ball closes and the flow of the refrigerant decreases.

Scheme 30

Compressor Clutch ON Delay System

When the A/C switch and fan switch are turned ON, a signal is sent to the engine control module. The engine control module then judges whether the engine is in operation. If the engine is operating, the engine control module activates the A/C relay. The maximum clutch "ON" delay times is 0.8 seconds after the A/C relay is activated.

Scheme 31

1. When the A/C switch and fan switch are turned ON, the A/C relay is activated. The compressor starts operating, and then the main and sub fans also operate.
2. The thermo control amplifier, when activated, disengages the compressor clutch and the main and sub fans.
3. When the pressure switch turns on, the compressor clutch is disengaged and the main and sub fans also stop.

THERMO CONTROL AMPLIFIER

The thermo control amplifier disconnects the magnet clutch circuit to prevent the evaporator from becoming frosted when the temperature of the evaporator fin drops close to 3°C (37°F). When the limit temperature is reached, the thermistor (located on the evaporator fin) interrupts the base current of the amplifier. This deactivates the A/C relay, which in turn disconnects the magnet clutch circuit.

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ACCELERATION CUT SYSTEM

The A/C switch turns the air conditioning system ON and OFF. The on-off signals from the switch are transmitted to the engine control module (ECM).

When the ECM receives a full-throttle signal from the throttle position sensor during compressor operation, it deactivates the A/C relay to interrupt electric current to the compressor magnet clutch. This prevents the degradation of acceleration performance. The A/C relay is in the main fuse box located on the left side of the engine compartment.

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IDLE SPEED CONTROL

The idle air control solenoid valve increases the engine idling speed when the compressor is in operation.

The engine control module activates the idle air control solenoid valve when it receives an A/C switch ON signal so that necessary by-pass air is introduced into the throttle body to ensure proper idling speed for an increased engine load.

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FAN CONTROL

The main fan and sub fan are switched ON and OFF according to the operating modes as shown in the following table.

ENGINE COOLANT TEMPERATURE SPECIFICATION

Scheme 35

Note. The beeper for button operation confirmation has been discontinued.

CABIN TEMPERATURE SENSOR

The cabin temperature sensor sends signals to the ECM.

This sensor consists of an aspirator and a thermistor, the resistance of which changes in inverse proportion to the temperature. The aspirator uses the vacuum created by the heater unit to direct cabin air to the thermistor. (The cabin temperature sensor, therefore, functions only while the blower fan is in operation.)

Scheme 36

AMBIENT TEMPERATURE SENSOR

This sensor uses a thermistor to detect the ambient temperature and sends signals the ECM.

The thermistor can detect only an average temperature of the outside air but cannot respond to sharp changes in the temperature because its exterior is made of a plastic to increase the thermal capacity.

The ambient temperature sensor is located on the radiator stay behind the front grille for efficient exposure to the outside air.

Scheme 37

SUN-LOAD SENSOR

A photodiode is used in the sun-load sensor. The photodiode detects changes in the sunbeam intensity and converts the results into current signals to send to the ECM.

The sun-load sensor is built into the front defroster grille.

Scheme 38

AIR MIX SERVO MOTOR

According to signals from the ECM, the servo motor forming integral part of the air mix damper rotates in one or the other direction to change the opening of the damper via a link.

The motor has a built-in potentiometer which detects the opening of the air mix damper and sends the result to the ECM.

Scheme 39

AIR OUTLET SWITCHING SERVO MOTORS

According to signals from the ECM, the servo motor incorporated into each air outlet switching damper rotates in one or the other direction to open or close the damper via a link to control the air from the corresponding outlet(s).

Each motor has a built-in potentiometer which detects the position of its damper and send the result to the ECM.

Scheme 40

FRESH/RECIRC SWITCHING SERVO MOTOR

According to signals from the ECM, the servo motor incorporated into the FRESH/RECIRC switching damper rotates in one or the other direction to perform switching between the fresh air introduction and inside air recirculation modes via a link.

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BLOWER SPEED CONTROL POWER TRANSISTOR

The base voltage of the power transistor changes according to blower drive signals from the ECM.

The blower speed changes stepless in accordance with the change in the power transistor's base voltage.

Should an over-current occur, the thermal fuse connected to the circuit (rated to blow at 144°C or 291°F) cuts off the current to the blower.

EVAPORATOR SENSOR

The evaporator sensor detects the temperature at the evaporator outlet and sends the result to the ECM.

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Scheme 43

1. Required blow-out air temperature (TAO) Upon reception of temperature set switch signals in addition to cabin temperature, ambient temperature and sun-load sensor signals, the ECM calculates the TAO first and then, based on the calculated temperature, it determines the outlets from which the air is to be blown out.
2. Calculation of required TAO When the set temperature is 18.0°C (64.4°F), the TAO is fixed at the MAX COOL. When the set temperature is 32.0°C (89.6°F), the TAO is fixed at the MAX HOT. When the set temperature is 18.5°C to 31.5°C (65.3°F to 88.7°F), an optimum TAO is calculated based on the set temperature, as well as the cabin temperature, ambient temperature and sunload data at that time.

TEMPERATURE CONTROL

The temperature control is made based on the drivers inputs from the temperature set switch and the data from various temperature sensors; the ECM determines the TAO using these data and operates the air mix motor so that the TAO can be attained.

The ECM compares the air mix damper opening it has received from the air mix damper potentiometer with the target opening it has calculated and, if necessary, operates the motor to move the damper to the HOT or COOL side and hold the damper in an appropriate position.

The target damper opening is corrected using the sunlight intensity data.

The air mix damper is moved fully to the HOT side and held there when the temperature set switch is placed at the FULL HOT position (32°C, 89.6°F), while it is moved fully to the COOL side and held there when the switch is placed at the FULL COOL position (18°C, 64.4°F).

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1. Normal air flow control When the air conditioning system is in the automatic control mode, the air flow is determined based on the TAO calculated by the ECM. The blower fan speed is controlled accordingly. In the automatic control mode, the minimum air flow is different between DEF mode and the other modes. For the minimum air flow in BILEVEL, HEAT and DEF/HEAT modes, a voltage of 5.5 V is applied to the blower motor, while for the DEF mode a voltage of 9.0 V is applied. The minimum air flow is corrected by the sunbeam intensity if the VENT or BILEVEL mode is selected.
2. Blower fan starting speed control When the blower motor is turned ON in the automatic control mode, the fan speed is initially low and then gradually increases (applied voltage increases by 1V every second until an appropriate voltage is reached) to prevent air from blowing out in a gust.
3. Blower fan control at low coolant temperatures Even when the blower motor is automatically turned ON, the blower fan is kept stopped or allowed to rotate at the minimum speed for a maximum of 150 seconds depending on the cabin temperature and the ambient temperature, if the engine coolant temperature is below 49°C (120.2°F) with the air outlets for the VENT or DEF mode selected. After the conditions for prohibiting blower fan operation or limiting its speed are removed, the voltage applied to the blower motor is increased gradually (by 0.34 V per minute) such that a large amount of cold air does not blow out toward the leg area. Once the coolant temperature exceeds 49°C (120.2°F), the normal blower fan control is performed including the starting speed control.
4. Blower fan stop control with compressor ON The blower fan is stopped for 3 seconds if the compressor is turned ON with the intake sensor-detected temperature is higher than 35°C (95°F).

AIR INLET CONTROL SYSTEM

The air inlet control system determines whether the air inlet damper is to be opened depending on the TAO calculated by the ECM, thus selecting either inside air recirculation or fresh air introduction.

The damper is generally opened for fresh air introduction when the compressor is turned OFF.

It is also opened generally when the DEF position is selected.

Scheme 46

AIR OUTLET CONTROL SYSTEM

The air outlet control system automatically selects the most appropriate air outlet combination depending on the ECM-calculated TAO by activating servo-motors for the VENT, BILEVEL or HEAT modes.

When the OFF switch is pressed position, the air outlet control system is held in the HEAT mode.

Scheme 47

COMPRESSOR CONTROL SYSTEM

In the automatic air-conditioning mode, the A/C relay is activated or deactivated depending on the TAO (required blow-out air temperature), TAM (ambient temperature) and T INT (suction air temperature) to operate or stop the compressor.

The compressor operation circuit supplies current to the magnet clutch as the ECM activates the A/C relay by connecting its coil to the ground.

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Scheme 50

1. Control by TAO and TAM
2. Control by T INT