Section 3 A/C System Components In the previous sections, we referred to the functions of the A/C components. Here’s how each one works in closer detail. A/C System Components Evaporator Expansion Valve Receiver-Drier* Condenser Expansion Valve Compressor Fig. 3-1 752f301 The expansion valve receives liquid refrigerant from the high-pressure components (compressor and receiver/drier). In order for the system to develop pressure, the flow of refrigerant must be met with a restriction.
Section 3 Most Toyota models now use a block-type expansion valve where both the evaporator inlet and outlet pass through the valve assembly. The capillary tube is located inside the stream of refrigerant leaving the evaporator. Due to the low temperature at this point, the valve is subject to blockage by microscopic debris or internal ice if any water is present in the refrigerant. Because of this, every system has some method to filter out these elements.
A/C System Components Evaporator The action between the expansion valve and the evaporator is the key to heat transfer in the system. The evaporator is the heat exchanger for the low-pressure side of the system. It is the key heat exchanger in the A/C system. All incoming or recirculated air passes through the evaporator. In doing so, the evaporator absorbs heat from the cabin air (car interior) or incoming fresh air so this heat can be carried to the condenser.
Section 3 Expansion Valve/ Evaporator Interaction Since the evaporator surface temperature can be close to 32° F (0° C), there could be a problem in high humidity conditions when moisture vapor condenses on the evaporator and freezes. This frozen water forms an insulating layer that prevents air from reaching the evaporator to exchange heat. In order to prevent icing, the expansion valve can change the size of the spray orifice (opening).
A/C System Components An important difference between a compressor and a pump is that a compressor cannot pump liquids. Since liquids cannot be compressed, a compressor filled with liquid will either lock up or break depending on the amount of torque applied to its pulley by the engine crankshaft. More commonly, the reed valve assembly that controls gas flow inside will be destroyed when a compressor hydraulically locks.
Section 3 crankshaft supported by ball bearings. Lubrication is provided by a splash from a sump in the compressor base. Construction of reciprocating compressors are very similar to a four-stroke cycle lawn mower engine. Discharge Suction Piston-type A/C Compressor Reed Valve Construction. Down-Stroke Up-Stroke Fig. 3-6 752f306 Through-Vane (TV) compressors replaced reciprocating, piston-type compressors.
A/C System Components This valve prevents damaging the through-vanes or the reed valve (which should “see” only refrigerant gas) by allowing liquid oil to escape from the compression area into an oil chamber. Some Toyota vehicles use a swash-plate (also called “wobble plate”) type compressor. Some models use multiple, opposed pistons arranged around a single swash-plate with two compression chambers for each cylinder. The cylinders connect to reed valves and common inlet and outlet passages at each end.
Section 3 The Variable Capacity swash-plate compressor uses a solenoid control valve that opens and closes to adjust the low-pressure inlet to the compressor. Controlling the suction side of the compressor changes the volume capacity according to the cooling load of the A/C system. This change in pressure affects the swash-plate angle. It also changes the piston stroke and thus the amount of refrigerant discharged to the condenser.
A/C System Components A Scroll Compressor is a spirally wound, fixed scroll and variable scroll that form a pair. The fixed scroll is integrated with the housing. The rotation of the shaft causes the variable scroll to rotate while maintaining in the same space. Thus, the volume of the space that is created by both scrolls varies. This changing volume creates the suction, compression and discharge forces needed for refrigerant gas flow through the compressor.
Section 3 Scroll Compressor Cycle Suction Discharge Port Intake Port Fixed Scroll Variable Scroll Compression Discharge Fig. 3-11 752f311 Some scroll compressors in Toyota vehicles contain a built-in oil separator. This chamber helps separate the compressor oil from the refrigerant that circulates in the refrigeration system. Excess oil in the scroll section of the compressor can lower compressor efficiency and in some cases, damage it.
A/C System Components Compressor Clutch A drive belt from the crankshaft pulley drives the compressor. Some systems use V-belts and some use a flat, multirib belt to help reduce frictional loss and noise. On some models, a single serpentine V-belt drives all the engine accessories including the A/C compressor. In this kind of system, an automatic tensioner maintains the correct serpentine belt tension. Idler Pulley Serpentine Belt System Some models have a single belt that drives multiple components.
Section 3 The electromagnet allows A/C operation to be controlled by an electric circuit. The compressor clutch relay is also controlled by a temperature signal from the evaporator and a pressure switch in the refrigerant line. In most systems, the compressor clutch cycles ON and OFF periodically to allow the evaporator to warm up (defrost) during periods of high cooling demand. An evaporator cold enough to freeze moisture around it does not transfer heat as well.
A/C System Components Belt Protection Sensor To reduce the overall length of the engine, accessories may be driven by just one or two belts. The A/C compressor and power steering pump are usually driven by the same belt. However, if the compressor were to seize, the belt could break which would cause a loss of power steering assist. A belt protection system is used on all Toyota models to reduce the potential safety hazard of a loss of power steering.
Section 3 Other conditions can cause the sensor to detect a difference in speed between the engine and compressor: • A system that is overcharged with refrigerant or refrigerant oil. Compressor lockup is possible due to the extremely high pressures that could result. • A slipping A/C drive belt due to loose tension and/or oil or water on the belt. • A loss of signal to the amplifier from either the ignitor or the rpm sensor.
A/C System Components Many Toyota vehicles now use a sub-cool condenser that helps separate the liquid from the gaseous refrigerant. In this design, the condenser redirects gaseous refrigerant to the top for further cooling (gas-to-liquid) while the liquid refrigerant exits from the bottom. This ensures all refrigerant sent to the evaporator is in a liquid state. Sub-Cool Condenser • Current models use sub-cool type.
Section 3 The outlet of the receiver-drier connects to a siphon tube that goes to the bottom of the container. This acts as a liquid/vapor separator and ensures only liquid refrigerant is supplied to the expansion valve. In addition, the end of the siphon tube has a very fine mesh screen to filter debris from the refrigerant and oil. This protects the expansion valve and the compressor from mechanical damage.
A/C System Components Pressure Relief Devices For safety, every pressurized system must have some sort of pressure-relief system to reduce excess system pressure before it can become a hazard. In an A/C system, a fusible plug was one type of safety device. A fusible plug is a hollow bolt filled with a soft, low-temperature solder. The plug then threads into the top of the receiver-drier.
Section 3 Multipressure Switch There may be one or more pressure switches in the refrigerant lines. Current models use a Multipressure Switch that contains two or even three pressure-sensing circuits. It is located in the high-pressure line of the system (between the compressor and expansion valve). • Low pressure – The low-pressure sensing circuit switches the compressor OFF. This prevents system damage due to the reduced amount of lubricant as a result of low system pressure.
A/C System Components refrigerant changes. As a rule: • The high-pressure lines are smallest in diameter. • The low-pressure lines have the largest diameter. Block-Type Fitting Block-type fittings help position and secure piping. Fig. 3-21 752f321 Unlike stationary refrigeration systems (appliance or building), mobile A/C systems must operate under high levels of vibration and motion. For this reason, all joint fittings use a gasket or O-ring to help seal the system.
Section 3 A quick-disconnect type fitting uses a plastic clamp lock to connect the tube endings. This type of fitting has no threads. One tube end has an O-ring that fits into a mating tube end. The plastic clamp keeps the tube ends together to create a leak-free seal. To service the system (e.g. evacuating the system — the system must be empty), a special remover tool is used to release the clamp. Quick-Disconnect Fitting Clamp • Clamp holds tubes in place. A/C Tube • See removal SST page 4-11.
A/C System Components Cooling Fans The effectiveness of the A/C system depends on removing heat as the hot refrigerant flows through the condenser. Because of this, cooling fans become more critical when the A/C system is ON. There are two fans that contribute toward heat transfer in the engine compartment: • A/C Condenser Fan • Engine Cooling Fan A/C Condenser Fans are driven either manually from the engine or electrically. Some Toyota vehicles use a belt-driven fan.
Section 3 Electric Cooling Fan Circuit 1 2F Fan relays activate cooling fans at low or high speed depending on coolant temperature or system pressure. 1 5 Engine Main Relay 30A RDI FAN 3 2 30A CDS FAN 4 10 2A 1 W 1 2D 3 2 Radiator Fan Relay L M 4 1 R 2 A1 A/C Condenser Fan Motor 8 2A 2 2D 4 2A 6 2D 5 B L B-R L-B 4 Radiator Fan Relay No.
A/C System Components • Refrigerant Pressure Switch. This switch normally monitors refrigerant pressure in the high-pressure side of the system (between the compressor and the expansion valve). If the pressure is too high or too low, the pressure switch opens to stop the compressor (via the compressor clutch). A mid-pressure setting on many cars also controls the high speed operation of the electric condenser fans.
Section 3 Review of Refrigeration Circuit Based on the physics of heat transfer, an automotive A/C system works on these principles: • Heat is absorbed from the passenger compartment by the evaporator. This happens because the expansion valve restricts the flow of liquid refrigerant and increases pressure. However, when the expansion valve opens, there is a drop in pressure which causes the refrigerant to evaporate and absorb heat.
A/C System Components TOYOTA Air Conditioning and Climate Control – Course 752 Home Bookmarks Next Print Exit 3-25
