Written by: Jon Delucia
Nostalgic Air Parts, Inc.
www.jeepair.com
Automotive air conditioning systems have not changed much over the past 40 years. The system still has all the major components that were used when a/c first became common on automobiles. There are five major components to an automotive a/c system: the compressor, evaporator, condenser, accumulator or drier, and expansion valve or orifice tube. The components all have a task to perform while working together as a team to produce cold air.
The first major component of the a/c system is the compressor. A compressor is made of two parts, the compressor, and the clutch assembly. The compressor has one job in an a/c system. It is used to pump refrigerant and oil throughout the system. The compressor is the starting point for the high side of the a/c system, and the ending point for the low side. The compressor sucks refrigerant and oil into the body on the low side, which is where the name suction side comes from. On the discharge side or high side the compressor is pushing the oil and refrigerant out at a higher pressure. The compressor has a clutch assembly attached to the front. The clutch assembly is made up of three parts (on most cars today); the coil, the pulley, and the hub. The coil is simply a magnet that gets power from the a/c system’s control head. The pulley is the part that has the belt routed around it. The pulley has a bearing pressed into it that rides on the compressor’s nose. The hub is located on the front of the clutch assembly; it is bolted to the shaft coming out of the compressor. The hub is the part of the clutch that actually spins the internals of the compressor to achieve the pumping action needed to move the refrigerant and oil. The hub is normally mounted with a nut or bolt on the front of the compressor. The shaft of the compressor will have splines or keyway to keep the hub and the shaft locked together. The way the complete assembly works is as follows: When the driver turns on the a/c the coil will magnetize. The pulley is always turning because the engine is turning the belt that is attached to it. When the coil is magnetized the hub is then pulled against the pulley to spin the compressor shaft. At this time the compressor will pump. On most vehicles only two things cause a clutch assembly to fail. Lack of oil in the compressor (from a leak) or wear over time. On Jeeps with a Sanden compressor (Wranglers, Cherokees, Liberties) the compressor clutch’s metal surface will wear down increasing the clutch gap. The clutch gap is the gap between the hub and the pulley. As the metal wears down, the coil is not able to pull the hub to the pulley. This normally happens on Jeeps at 90,000 to 110,000 miles. The most common sign of this problem is the vehicle’s a/c cools when it is first driven with the a/c on then it quits after a while. You can then turn off the a/c for 30 or so minutes and reuse the a/c. The other cause of clutch failure is a leak in the system. The compressors on most Jeeps are similar to an engine; they have pistons, and valves and they always have to have lubrication. If a system develops a leak in any component the refrigerant will escape as well as some oil. As oil and refrigerant leave the system the compressor will starve for lubrication causing the metal on metal friction to produce severe heat. The heat in turn burns up the clutch assembly. The first part of a clutch to burn up is the bearing inside the pulley. The bearing is sealed with grease; heat will melt the grease causing the pulley to get hot which in turn heats up the coil melting the epoxy that holds the metal coils inside. Then the hub’s rubber dampeners melt. The misdiagnosis of this problem by the majority of repair shops is to replace the clutch assembly rather than fix the problem. When a clutch burns up it is because something caused it to. The only time a clutch should be replaced is when they wear out due to high mileage.
The evaporator is a component that operates on the low pressure side of the a/c system. The evaporator is normally located on the inside firewall of the Jeep behind the glove box on. The evaporator is encased in the plenum or evaporator housing. The housing also contains the heater core, the blower motor, and the blend doors for mixing the outside and inside air, and temperature. The evaporators function is to get the heat out of the vehicle, hence the term heat exchanger. The way an evaporator works is by pulling warm air across the fins and tubes of the core thus removing hot air from the vehicle, which allows cold air to exit the evaporator core into the ventilation system. The air coming out of the vents is the same air that is in the vehicle just without any hot air. The best analogy is when you are in a house in the winter, you exit the house feeling warm, and after a few minutes you start to feel cold. The outside air is removing the body heat from your body to make you feel cold. The evaporator’s functions are to convert the refrigerant (R-12 or R-134a) into a vapor from a liquid form, and to remove the heat from the air inside the vehicle. The refrigerant (Freon) in the evaporator has to exit in a vapor form in order to pass through the compressor. If the refrigerant enters the compressor as a liquid it can cause damage to the internals of a compressor. A compressor is not designed to pump liquid refrigerant and oil. Evaporators were originally made from copper, as Jeep air conditioning systems became more efficient the evaporators were made out of aluminum as they are today. Copper evaporators do not leak unless they are physically punctured, or they have a hole rubbed in them from movement inside the case. Copper evaporators are not as efficient for getting rid of heat, so aluminum was introduced. Aluminum evaporators work much better with R-12 and R-134a refrigerants, but they develop leaks a little easier. There are two major causes of leaks in aluminum evaporators. The first is caused by the blower motor. The blower motor has copper wires on the armature of the motor. As the motor runs the copper flakes off in micron sized pieces, these pieces are blown into the evaporator core where they can cause a reaction with the aluminum. The reaction leads to the copper eating into the aluminum causing pin-hole leaks. The second cause for leaks in aluminum a/c components is from moisture in the a/c system. If moisture mixes with R-12 or R-134a in the system it can create an acid that will eat away the aluminum from the inside out, this will be covered more when we speak about driers and accumulators.
The last component on the low side of the a/c system is the expansion valve, or orifice tube. The expansion valve and orifice tube both perform the same function in the system. They are used on two different types of systems. Orifice tubes are used on CCOT (Cycling Clutch Orifice Tube) systems which use an accumulator, not a drier. The orifice tube is used to meter the refrigerant and filter debris in the system. The expansion valve meters the refrigerant, while the drier filters debris. The purpose of the orifice tube and expansion valve is very similar. The two parts are the changing point from high pressure to low pressure in the system. If there is a problem with the high side of an a/c system is it not an expansion valve or orifice tube problem. If the low side has problem it most likely is an expansion valve or orifice tube problem. The most common sign of a clogged or damaged valve or tube is a low pressure on the low side (less than five pounds) or the low side is in a vacuum. If the high side is 300 psi it does not have anything to do with the expansion valve. The function of both the tube and the valve is to take the liquid refrigerant in the system and reduce it down to a mist before entering the evaporator. If this process does not happen the refrigerant will flood the evaporator causing the a/c system to not operate properly. The misting of the refrigerant allows the evaporator to transform the refrigerant at a faster and more effective rate into a vapor.