How to Repair Automotive Air-Conditioning & Heating Systems. Jerry Clemons
air flowing past the condenser to create a cooling effect that causes the refrigerant to condense into a liquid.
Condenser: The condenser is a large heat exchanger located in front of the radiator. It is used to cool the hot gaseous refrigerant, which causes it to condense back into a liquid. Airflow across the condenser is very important and is accomplished by the ram air effect when the vehicle is traveling at highway speeds and by the fan assembly when the vehicle is traveling slowly or stopped.
It is common to see a puddle of water under the area below the evaporator drain tube when a vehicle is parked after running the AC on a hot and humid day. This water is a sign that the system is working to remove the heat and humidity from the cabin air, which results in cool and dry air being directed into the passenger compartment through the mode doors.
The compressor has a pulley that spins at all times when the engine is operating. The front plate only turns when the AC is commanded to be turned on. Power is sent to the AC clutch coil when the AC compressor needs to run.
The compressor should be carefully tightened with equal torque at all of the mounting bolts to minimize the chance of the body getting in a bind, which could cause a leak. Some compressors use a pigtail connector that connects to the vehicle harness connector.
Many electric and hybrid vehicles use compressors that are driven by a high-voltage battery instead of the engine drive belt. This design increases fuel economy by reducing the need to have the internal combustion engine run. Caution must be followed when performing service and repair on vehicles with electric compressors because these compressors operate on three-phase AC high voltage.
The condenser is the heat exchanger located in front of the engine radiator and allows for generous airflow to pass through the fins. The condenser is connected to the discharge line on the inlet and the liquid line on the exit side.
Orifice Tube: The orifice tube is used as a metering device that causes the pressure to be reduced in the AC system. The refrigerant pressure needs to be lowered in order for it to be able to absorb heat from the duct box air and begin to boil and take on the latent heat of evaporation. Orifice tubes also have a screen that serves as a filter to prevent contaminants from passing into the evaporator core.
Orifice tubes are made in different colors that align with the various sizes, and care should be taken to use the one that is designed for the vehicle being repaired. It is important to remember to use the correct orifice tube during a service procedure.
Evaporator Core: The evaporator core is a heat exchanger located inside the duct box that is used to provide a place for the low-pressure atomized liquid refrigerant to begin to boil. This change of state of the refrigerant causes it to take on the latent heat of evaporation, which absorbs large amounts of heat from the duct box air. The result is that the duct box air is much cooler and drier after moving past the evaporator core. The cool air is distributed to the passenger compartment, and the moisture drops to the bottom of the duct box and drains outside the vehicle.
Accumulator Dryer: The accumulator dryer container is located between the evaporator core and the compressor and is used to store and dry the refrigerant. The dryer contains a desiccant that is used to absorb any moisture that could be present inside the AC system. The location of the accumulator is important on orifice tube systems because it prevents any liquid refrigerant from being supplied to the compressor. The compressor would be damaged if liquid refrigerant was pulled in. The exit point of the accumulator dryer is near the top, which only allows gaseous refrigerant to be sent to the compressor. There typically is a small feed hole at the bottom of the dryer that feeds refrigerant oil to the compressor.
Many condensers used on vehicles with a thermal expansion valve (TXV) have a condenser that combines with the receiver dryer. This design is very effective and it reduces the potential leak points by welding the dryer to the condenser.
The evaporator core is the heat exchanger on the low side of the AC system and is located in the duct box. It is important to transfer the insulation from the old evaporator core to the new unit when replacing it. A block-type TXV would be mounted on the flat surface of this evaporator core.
Evaporator cores often collect dirt and debris in the fins. This problem is usually reduced on vehicles that use a cabin air filter that gets replaced on a regular basis. This evaporator core has one threaded connecting point as well as a quick-connect coupling.
Accumulator dryers can be connected to the lines and hoses with bolted connections that use O-rings to seal the joint from potential leaks. Low side pressure switches are often mounted to the accumulator dryer as well.
Many accumulator dryers are connected to the lines and hoses with quick-connect couplings. The low side service port is often mounted on the accumulator dryer as well as a fitting to mount a low side pressure switch.
Basic Cycle for TXV Systems
A second style of metering device that is used in many AC systems is the thermal expansion valve (TXV). The TXV metering device has been used for many years and is increasing in use in recent years because it varies the flow rate based on operating conditions. There are two styles of TXVs in use: the standard type and the block type. The standard type TXV is almost always located inside the duct box, while the block style is almost always located on the firewall.
As stated, the TXV metering device varies the flow rate through the valve based on operating conditions. This variable operation occurs by the valve sensing the temperature of the exit line of the evaporator and changing the internal opening as the temperature changes. When the sensing element is exposed to warm temperatures, the valve opens, and when the sensing element is exposed to cold temperatures, the valve closes. The continual opening and closing helps control the temperature in the evaporator core, which helps prevent the temperature from reaching freezing levels, which would cause the core to form ice on the surface.
Thermal expansion valves (TXVs) are a popular choice to use as a metering device to feed low-pressure atomized liquid into the evaporator core. The two types of TXVs include the block type (left) and standard type (right). Both styles operate by varying the opening inside the valve to feed refrigerant into the evaporator core.
Block type TXVs are located near the firewall and are visible from under the hood. O-rings and metal gaskets are used to seal the lines to the body of the valve.
Standard-type TXVs are usually located inside the duct box and sealed