Part of your job as a certified mechanic is to maintain and repair air-conditioning systems. You will need to understand how air-conditioning systems function before you can become a competent troubleshooter. This manual gives you the foundation to recognize problems and repair them. Your knowledge in this area is essential to the successful completion of a job or unit mission.

This manual introduces you to the fundamental operating principles, parts, and maintenance of air-conditioning systems, and operating problems that you may experience.

When you have completed this manual, you will be able to:

  1. Understand the principles of refrigeration.
  2. Identify components of air-conditioning systems.
  3. Identify malfunction of components in the air-condition systems.
  4. Understand how to inspect air-conditioning systems for leaks.
  5. Understand how to evacuate and recharge air-conditioning systems.
  6. Understand how to add refrigerant to air-condition systems.
  7. Understand how to perform functional testing on air-conditioning systems.


1.0.0 Principles of Refrigeration

2.0.0 Components of the Air-Conditioning System

3.0.0 Malfunctions of Components in the Air-Conditioning System

4.0.0 Inspecting the Air-Conditioning System for Leaks

5.0.0 Purging the Air-Conditioning System

6.0.0 Adding Refrigerant to the Air-Conditioning System

7.0.0 Functional Testing of the Air-Conditioning System

Review Questions


Refrigeration is the process of producing low temperatures. It is usually associated with refrigerators or freezers rather than with vehicles. An understanding of heat transfer, basic refrigeration, pressure temperature relationship, and the qualities of refrigerants is essential for a working knowledge of the air-conditioning system.

1.1.0 Heat Transfer

It may seem odd to cover heat transfer in connection with air conditioning. Keep in mind, however, that heat, like light, is a form of energy. As you remove light, a room grows darker. Likewise, when you remove heat, an area becomes colder. The process of transferring heat is the basis for air conditioning. Generally, when two objects of different temperatures are close to each other, heat energy will leave the warmer object and travel to the cooler. This is quite clearly illustrated in North America each fall and winter. As the rays of the sun become less direct and consequently give off less heat, we experience a drop in temperature. Cooler weather (refrigeration) results from this removal of heat.

Refrigeration applies a physical principle that is known to most of us through our everyday experiences. We have experienced the application of rubbing alcohol and its cooling effect. This example illustrates that an evaporating liquid absorbs heat. The evaporating moisture in the air on a hot day soaks up heat like a sponge. This removal of heat is exactly the same process used in automotive air conditioning. Heat is removed from the vehicle by an evaporating refrigerant and transferred into the atmosphere.

1.2.0 Pressure Temperature Relationship

Different liquids have different boiling evaporating points; however, the boiling point of any liquid increases when pressure is increased. When pressure is decreased, the boiling point is then decreased. This process of removing the pressure and allowing the coolant to boil is a vital part of any refrigeration system.

1.3.0 Refrigerant (134A)

With the exception of changes in state, gases used in refrigeration are recycled much like engine coolant. Different pressures and temperatures cause the gas to change state from liquid to gas and back to a liquid again. The boiling point of the refrigerant changes with system pressure. High pressure raises the boiling point and low pressure reduces it. These gases also provide good heat transfer qualities and do not deteriorate system components. A few years ago, vehicles were cooled with refrigerant-12 (R-12). It had a boiling point of 220F and vaporized at room temperature. Because R-12 has been found to be hazardous to the environment, it has been fased out. The replacement refrigerant is R-134a.

1.4.0 Handling Refrigerant

R- 4a is a relatively safe refrigerant; however, you must observe certain precautions when using and handling it:

Figure 1 - Warming the refrigerant with warm water.

Some systems may still have R-12 in them. Here are some differences in the systems to help you identify them and the precautions involved:

1.5.0 Refrigeration Cycle

The refrigeration cycle is a continuous closed-loop system. The refrigerant is pumped constantly through the components in the system. By changing the refrigerant pressure and by removing and adding heat, the refrigeration cycle is completed. The refrigeration cycle operates as follows:

  1. The receiver/drier collects high-pressure refrigerant in a liquid form. Also, moisture and impurities are removed at this point.
  2. The refrigerant is routed to the expansion valve through high-pressure lines and hoses.
  3. The expansion valve reduces refrigerant pressure to the evaporator by allowing a controlled amount of liquid refrigerant to enter it.
  4. A stream of air is passed over the coils in the evaporator as refrigerant enters.
  5. As the low-pressure refrigerant moves through the coils in the evaporator, it absorbs heat from the airstream, which produces a cooling effect.
  6. As the refrigerant nears the end of the coils in the evaporator, greater amounts of heat are absorbed. This causes the low-pressure liquid refrigerant to boil and change to a gas as it exits the evaporator.
  7. As the refrigerant enters the compressor, the pumping action increases refrigerant pressure, which also causes a rise in temperature.
  8. The high-pressure, high-temperature gas enters the condenser, where heat is removed by an outside ambient airstream moving over the coils. This causes the gas to condense and return to a liquid form again.
  9. The high-pressure liquid refrigerant now enters the receiver again to begin another cycle. This continuous cycle, along with the dehumidifying and filtering effect, produces a comfortable atmosphere on hot days.


Test Your Knowledge

1. The boiling point of any liquid is increased in what way?

A. By raising the evaporation point
B. By decreasing the pressure on the liquid
C. By increasing the pressure on the liquid
D. By lowering the evaporation point


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Each air-conditioning system (Figure 2) must have a receiver/drier, an expansion valve or metering device, an evaporator, a compressor, and a condenser. Without these components, an air-conditioning system will not function. Additionally, the system must have some means of control. The following information briefly covers each air- conditioning component and the controls involved.

Figure 2 - Air-conditioning system.

2.1.0 The Receiver/Drier

The receiver (Figure 3), otherwise known as a filter-drier or accumulator-drier, is a cylindrical-shaped metal tank that has two purposes; separate liquid from gas and to remove moisture and filter out dirt. The tank is hollow with an inlet to the top of the hollow cylinder. The outlet port has a tube attached to it that extends to the bottom of the receiver. This tube assures that only liquid refrigerant will exit the receiver because any gas entering will tend to float above the liquid.

Figure 3 - Receiver and components.

Some other components of the receiver/drier are:

Figure 4 - Possible sight glass conditions.

2.2.0 The Expansion

The refrigerant expansion system is designed to regulate the amount of refrigerant entering the evaporator and to reduce its pressure.

2.2.1 Expansion Valve

One type of expansion system used on modem vehicles is the expansion valve (Figure 5, View A). The valve action is controlled by the valve spring, suction manifold, and pressure exerted on the diaphragm from the thermal bulb. Operation of the valve is as follows:

  1. High-pressure liquid refrigerant flows into the valve and is stopped at the needle seat.
  2. If the evaporator is warm, pressure is developed in the thermal bulb and transferred to the diaphragm through the capillary tube.
  3. The diaphragm overcomes the pressure developed in the equalizer tube and valve spring pressure, causing it to move downward.
  4. This movement forces the valve-actuating pin downward to open the valve.

As the refrigerant flows, it cools the evaporator and therefore reduces pressure in the thermal bulb. This allows the valve to close and stop refrigerant from flowing into the evaporator. By carefully metering the amount of refrigerant with the expansion valve, the evaporator cooling efficiency is increased greatly.

2.2.2 Expansion Tube

The expansion tube (Figure 5, View B) provides the same functions as the expansion valve. A calibrated orifice is built into the expansion tube. The tube retards the refrigerant flow through the orifice to provide the metered amount of refrigerant to the evaporator. The tube also has a fine screen built in for additional filtration.

Figure 5 - Expansion valve and tube.

2.3.0 The Evaporator

The evaporator is designed to absorb heat from the airstream directed into the driver's compartment. It is a continuous tube looped back and forth through many cooling fins firmly attached to the tube. The evaporator dehumidifies the air by passing an airstream over the cooling fins. As this happens, the moisture condenses on the fins and drips down to collect and exit under the vehicle. Also, dust and dirt are collected on the moist fins and are drained with the moisture. The temperature of the evaporator must be kept above 32F. Should the temperature fall below 32F, moisture condensing on the evaporator will freeze and prevent air from passing through the fins. A typical evaporator is shown in Figure 6. There are basically three methods of regulating evaporator temperature; each is examined below.

Figure 6 - Typical evaporator.

2.3.1 Thermostatic Switch

This system uses an electrically operated thermostatic switch (Figure 7) to engage and disengage the compressor. The switch is operated by a sensing bulb placed in the airstream after the evaporator. As the evaporator temperature falls, the thermostatic switch opens to disengage the magnetic clutch in the compressor. When the coil temperature reaches the proper level, the switch again closes to engage the clutch and drive the compressor.

Figure 7 - Thermostatic switch.

2.3.2 Hot Gas Bypass Valve

The hot gas bypass valve (Figure 8) was used on some older models to control evaporator icing. The valve is mounted on the outlet side of the evaporator. The high-pressure gas from the compressor joins with the low- pressure gas exiting the evaporator. These two gases mix, causing a pressure increase. Also, the boiling point increases, which results in a loss of cooling efficiency. This, in turn, causes the evaporator temperature to increase, thus eliminating freeze-up. The compressor is designed to run constantly (when it is activated) in the hot gas bypass valve system.

Figure 8 - Hot gas bypass valve operation.

2.3.3 Suction Throttling Valve

Figure 9 - Suction throttling valve.

The suction throttling valve (Figure 9) is now used in place of the hot gas bypass valve system. It is placed in line with the outlet of the evaporator. This system is designed to limit the amount of low-pressure vapor entering the compressor. The suction throttling valve operates as follows:

  1. The outlet pressure enters the valve on the bottom.
  2. The gas pressure passes through a fine screen and small bleeder holes to act on a diaphragm.
  3. The valve spring and atmospheric pressure oppose the gas pressure on the opposite side of the diaphragm.
  4. As the outlet pressure of the evaporator overcomes the opposing forces, the diaphragm and piston move upward, allowing low-pressure gas to flow through the valve and flow to the inlet of the compressor.

As pressure again drops on the inlet side of the valve, atmospheric pressure and valve spring pressure close the valve again. A vacuum power unit is mounted to the top of the valve to help reduce valve spring pressure and prevent icing at high elevations.

2.3.4 Pilot-Operated Absolute Suction Throttling Valve

The pilot-operated absolute (POA) suction throttling valve (Figure 10) maintains the proper minimum evaporator pressure regardless of compressor speed, evaporator temperature, and changes in altitude. The POA suction throttling valve is operated by a bellows containing an almost perfect vacuum. The expanding and contracting action of the bellows operates a needle valve, regulating its surrounding pressure. As inlet and outlet pressure are equalized, spring pressure closes the valve. The pressure differential across the valve then forces the piston toward the lower pressure, therefore opening the valve to allow refrigerant to flow.

Figure 10 - Pilot-operated absolute suction throttling valve.

2.4.0 The Compressor

The compressor increases the pressure of vaporized refrigerant exiting the evaporator. When the system is activated, a coil produces a magnetic field that engages the drive pulley to operate the compressor (Figure 11). Some compressors are protected from overheating by a superheat switch located inside the compressor. Should the compressor develop an excess amount of heat due to a loss of refrigerant or oil, the superheat switch disengages the compressor by completing a circuit and opening a thermal fuse. Sometimes a compressor discharge pressure switch is used to protect against a low refrigerant condition. This switch disengages the compressor drive to protect the system when discharge pressure drops below approximately 35 psi (241 kPa). Often a muffler is used on the outlet side of the compressor. The muffler helps reduce compressor pumping noise and line vibrations.

Figure 11 - The compressor and its components.

2.5.0 The Condenser

The condenser (Figure 12) is designed to remove heat from the compressed refrigerant, returning it to a liquid state. Generally, condensers are made from a continuous tube looped back and forth through rigidly mounted cooling fins. They are made of aluminum and can encounter pressures of approximately 150 to 300 psig and temperatures ranging from 120F to 200F (48C to 93C). Usually, the condenser is mounted in front of the radiator and is subjected to a steady stream of cooling air.

Figure 12 - The condenser.

Refrigeration oil provides lubrication for the compressor. Each system has a certain amount of refrigeration oil (usually approximately 6 to 10 ounces (177 to 296 Ml)) added to the system initially. If the system stays sealed, the oil will not break down or need to be changed. Refrigeration oil is highly refined, must be free of moisture, and is designed for use in automotive air-conditioning systems.


Test Your Knowledge

2. In an air-conditioning system, what is the purpose of the receiver/drier?

A. It collects high-pressure refrigerant.
B. It lowers the pressure of the refrigerant.
C. It raises the pressure of the refrigerant.
D. It separates the liquid refrigerant to a gas.


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Problems in automotive air-conditioning systems are not uncommon. An ordinary industrial system does not have to contend with the vibration that a mobile unit does. What follows is a list of common problems and possible causes associated with each air-conditioning component. This is by no means a complete list, so you should have the manufacturer's vehicle repair manual handy.

3.1.0 The Compressor

A thumping noise in the compressor or a cool and sweating compressor suction line accompanied by no cooling is usually caused by too much refrigerant in the system. If there is no moisture in the system, the excess refrigerant should be removed and stored for proper disposal. If moisture is present, you must discharge, evacuate, and recharge the system.

3.2.0 The Condenser

The condenser unit could have clogged fins that limit the cooling ability of the unit. This could be caused by bugs, leaves, or other debris caught in the tins. This can be corrected by using air pressure to blow out the coils, Check for any icy or frosty spots on the condenser. An abnormally cold spot usually indicates partial restriction inside the condenser coils at that point. Restrictions are normally caused by foreign matter. Correct this condition by discharging and purging the system.

3.3.0 The Evaporator

The evaporator is normally maintenance free for the life of a vehicle. If the evaporator does develop a leak, it will be necessary to remove the assembly for repair. An evaporator is repaired in the same manner as a radiator. If the evaporator does not get the right amount of refrigerant, the expansion valve is most likely at fault.

3.4.0 The Expansion Valve

The most common malfunction in the expansion valve is icing caused by moisture in the air-conditioning system. The system must be discharged and evacuated to remove all moisture. On occasion, the expansion valve may stick open or closed; in this case, you must replace the valve.

3.5.0 The Receiver/Drier

The receiver/drier may become saturated with moisture, or the filter may become restricted. If the receiver/drier is saturated or restricted, replace it. For any of these repairs, comply with the appropriate maintenance manual.


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Approximately 80 percent of all air-conditioning service work consists of your inspecting for and repairing leaks. Many leaks will be located at points of connection and are caused by vehicle vibration. They may only require the retightening of a flare connection or a clamp. Occasionally, a hose will rub on a structural part to create a leak, or a hose may deteriorate and require replacement. The compressor shaft seal may also require occasional replacement. Anytime the system requires more than one-half pound of refrigerant after operating during one season, a serious leak is indicated that you must locate and repair. The following information covers a few of the various means of detecting leaks.


When any tests or repairs are being made on a charged air-conditioning system, always wear adequate eye protection.

4.1.0 Internally Charged Detector

This detector is a specially colored leak detector available in a pressurized can and mixed with R-134a. It can be introduced into the air-conditioning system with regular charging equipment. When a leak occurs in the system, a bright red-orange spot appears at the point of leakage and remains until it is wiped off. The internal leak detector remains in the system and will spot future leaks in the same manner. A sticker is usually placed under the vehicle hood to indicate that the system is charged with a leak detector.

4.2.0 Bubble Detector

The bubble detector is a solution applied externally at suspected leak points. Leaking refrigerant will cause the detector to form bubbles and foam.

4.3.0 Electronic Detector

This instrument indicates leaks electronically by flashing a light or sounding an alarm. There are several different types of electronic detectors. Directions for using the instruments are furnished by the manufacturer. This type of electronic leak detector is the one most widely used today (Figure 13).

Figure 13 - Electronic leak detector.

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Anytime an air-conditioning system is discharged and opened before it is returned to service, it must be evacuated and recharged. To perform this operation, you need certain tools, such as a vacuum pump (Figure 14, View A), a gauge manifold set (Figure 14, View B), and a leak detector. Using the vacuum pump, draw the system down to at least 29 inches of mercury at sea level and hold it there for at least 30 to 45 minutes. This will remove all moisture from the system.

Figure 14 - Vacuum pump and gauge manifold set.

As the system is being pumped down, the vacuum should drop to the required inches of mercury. If it does not drop, this is an indication of a leak; in which case you must recharge the system to detect the leak. After you detect the leak, repair the damage and re-evacuate the system.

Once the system is totally evacuated, again close both valves on the gauge manifold set, disconnect the vacuum pump, and connect the refrigerant source.


  1. Any oil lost during the discharge of refrigerant must be replaced or damage to the compressor will result.
  2. During discharge of an automotive air-conditioning system, the vehicle engine must NOT be running.

In the past, when a system was discharged before disassembly, the standard practice was to vent the refrigerant into the atmosphere. For environmental and legal reasons, this is no longer permissible. The proper procedure is to use a refrigerant recovery/recycling device (Figure 15) and reuse the refrigerant. You are to turn in excess used refrigerant to the Defense Recycling and Management Office (DRMO) for proper disposal.

Figure 15 - Refrigerant recovery/recycling device.


Disposal instructions for refrigerants may not be the same at different naval stations. Before you take any action concerning R-134a or any refrigerant, contact your supply department for proper disposal instructions.


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Figure 16 - Adding refrigerant to the air conditioning system.

Now that the system is pumped down, leave the gauge manifold set attached and attach your refrigerant source, as shown in Figure 16. You are to take the following actions:

  1. Loosen the center hose connection at the gauge manifold set.
  2. Open the can valve for several seconds to purge air from the center hose.
  3. Tighten the hose connection and close the can valve.
  4. Start the vehicle engine and operate the air conditioner.
  5. With the system operating, slowly open the low-side manifold hand valve to allow refrigerant to enter the system.

The low side of the system is the suction side, and the compressor will pull the refrigerant from the can into the system.

  1. With the container in an upright (vapor) position, add the refrigerant until the sight glass clears or the test set gauge readings are normal.
  2. Rock the refrigerant can from side to side to increase the flow of refrigerant into the system.


Never turn a can into a position where liquid refrigerant will flow into the system.

  1. Close the low-side manifold valve and the refrigerant can valve.
  2. Continue to stabilize the system and check for normal refrigerant charge.


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Functional testing is required to establish the condition of all components in the system. The engine must be running and the air-conditioning system operating when performing this test. After the initial charge of refrigerant is installed into the system, watch the manifold gauge set. Correct pressure should be 15 to 30 psi for the low side and 175 to 195 psi for the high side. Evaluate the reading you receive against the standard chart in Table 1. If the vehicle you are working on is equipped with a sight glass (Figure 4), the bubbles should disappear at the correct pressures. Close the low side gauge manifold set hand valve. Check the temperature of the air exiting the cooling duct. It should be close to 40F with the blower running on low speed. Stop the engine and disconnect the gauge manifold set.

Table 1 - Temperature Pressure Relationship.


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In this manual, you learned how to inspect, fill, and troubleshoot air-conditioning systems found in late model vehicles. Keeping the operators comfortable is an important aspect of being a certified mechanic. It is also important to know that you cannot let the refrigerant loose into the environment, and you now know some of the other hazards associated with refrigerant. Types of refrigerant and the oils the systems use can be ever changing with the environmental hazards associated with them, so stay up to date on what is required of you and your air-conditioning system. When you have mastered the knowledge of these systems, you will become a better mechanic.


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Review Questions

1. If two objects have different temperatures and are close to one another, heat energy travels in what direction, if any?

A. From the cooler object to the warmer object
B. From the warmer object to the cooler object
C. None; heat energy travels only when the objects actually touch one another

2. A sizeable amount of refrigerant-134 in the atmosphere may cause what result?

A. Fire
B. Explosion
C. Suffocation
D. Halucination

3. When warming a container of refrigerant-12, you should not exceed what temperature?

A. 90F
B. 100F
C. 110F
D. 125F

4. What is the purpose of the desiccant located inside the receiver?

A. It relieves pressure in the system.
B. It acts as a filter.
C. It acts as a bypass for the refrigerant.
D. It removes moisture from the system.

5. The relief valve opens between approximately what pressure ranges?

A. 200 to 300 psi
B. 300 to 400 psi
C. 400 to 450 psi
D. 450 to 500 psi

6. When you observe bubbles in the site glass of an air-conditioning system, what does it indicate?

A. That no refrigerant is in the system
B. That the system is overcharged
C. That the system is undercharged
D. That too much oil is in the system

7. The expansion tube retards refrigerant flow and performs what other function?

A. It acts as a filter.
B. It raises refrigerant pressure.
C. It regulates refrigerant entering the condenser.
D. It opens the valve to allow the refrigerant to flow.

8. The evaporator should be kept above what temperature in degrees?

A. 30F
B. 32F
C. 40F
D. 45F

9. Where is the thermostatic switch-sensing bulb located in an air-conditioning system?

A. In the airstream after the evaporator
B. In the airstream before the evaporator
C. On the compressor clutch
D. In the airstream after the condenser

10. In an air-conditioning system, what does the suction throttling valve limit?

A. Condenser operation
B. Evaporator operation
C. The amount of high-pressure vapor entering the compressor
D. The amount of low-pressure vapor entering the compressor

11. In an air-conditioning system that uses a pilot-operated absolute suction throttling valve, by what means does the valve close as the inlet and outlet pressures equalize?

A. Spring pressure
B. Outlet pressure
C. Inlet pressure
D. Oil pressure

12. A compressor discharge pressure switch is used to protect against what air- conditioning system problem?

A. Overcharging
B. Overspeeding
C. Low refrigerant
D. High-discharge pressure

13. The air-conditioning system compressor muffler reduces noise along with what other problem?

A. High-discharge pressure
B. Low-discharge pressure
C. Line vibrations
D. To aid in sealing

14. In an air-conditioning system, where is the condenser usually mounted?

A. Within the engine compartment
B. In front of the radiator
C. In back of the radiator
D. In the driver's compartment

15. Approximately how much refrigeration oil is contained within each system?

A. 1 pint
B. 2 to 4 ounces
C. 4 to 6 ounces
D. 6 to 10 ounces

16. In an air-conditioning system, when the compressor produces a thumping noise and no cooling, it is an indication of what condition?

A. A clogged condenser
B. A faulty evaporator
C. Low oil level
D. Too much refrigerant

17. An abnormally cold spot on a condenser could indicate what condition?

A. A faulty compressor
B. A partially clogged condenser
C. A faulty evaporator
D. Too much refrigerant

18. What action must you take if the receiver/drier is saturated?

A. Remove it and replace the desiccant.
B. Evacuate the system and recharge it.
C. Replace the receiver/drier.
D. Over-pressurize the system to blow it out.

19. Which of the following is the most widely used refrigerant leak detector in use today?

A. Flame
B. Bubble
C. Electronic
D. Internal charge

20. The air-conditioning system that is being evacuated must be drawn down to 29 inches and held for how many minutes?

A. 10 to 15
B. 15 to 30
C. 30 to 45
D. 45 to 60

21. What is normally done with excess used refrigerant?

A. It is pumped into containers and turned into DRMO.
B. It is turned into the local public works department.
C. It is held in the shop for reuse.
D. It is released into the building's ventilation filters.

22. What is another name for the low side of the compressor?

A. High-pressure side
B. Low-pressure side
C. Fluid side
D. Suction side

23. During the functional testing of an air-conditioning system, what should be the temperature of the air exiting the cooling duct?

A. 32F
B. 35F
C. 40F
D. 45F


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Copyright David L. Heiserman
All Rights Reserved