Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."
Repairing a Car Air Conditioning System
In this article I discuss how to remove refrigerant from your car's AC system, how to replace various components, and how to refill the system with refrigerant when done.
Part 1. Recovering the Refrigerant from the Car AC System
What the Law Requires When Removing Refrigerant
Working on a car's air conditioning system at home, whether to change the refrigerant or replace components, requires you to decide what to do with the refrigerant. The law (Section 609 of the EPA Clean Air Act) requires that no refrigerant be released into the atmosphere under Section 609 of the EPA Clean Air Act. It also specifies that if the technician doing the work is paid, he or she needs to be 609-certified. This means if you are doing the work at home you will either have someone remove the refrigerant with a recovery machine or violate the law. High fines can be imposed for the release of refrigerant into the atmosphere.
Many shops will remove the refrigerant and only charge you for the recovery and recharge. You can save the labor for replacing the components.
How to Use a Recovery Machine to Remove Refrigerant
In order to remove the refrigerant that is under pressure, a recovery machine will need to be hooked up. With R134a refrigerant, the connections are quick-connect snapped-on fittings similar to an air hose connection. The low-side (blue) and high-side (red) fittings are different sizes so they are not confused and connected to the wrong place. The older R12 fittings were screwed on, and were the same with most vehicles (except GM).
When using a recovery machine, once the hoses are hooked up, screw in the knobs on the quick connectors to depress the Schrader valves. If there is no pressure on the gauges, that means there is no refrigerant in the system, and so no refrigerant needs to be recovered. If there is pressure, then press the recovery button. The pressures should go down. Once the pressure is at zero, the machine will pull a slight vacuum of about 10" Hg (inches of mercury). It should then shut off, or you can turn it off. Watch the pressure. If the pressure goes up, recover again.
Keep track of how much refrigerant was removed and how much oil was lost in the oil recovery bottle. Any oil lost should be added back in before or during charging.
Part 2: Replacing AC Components
Replacing Hose and Line Connections
Hose and line connections have several ways to fasten together: screw together using line fittings, bolt together using a flange and bolt, and spring-lock for a quick-disconnect type of fitting. All of the lines use at least one O-ring or O-ring/gasket to seal the connections. Always replace the O-rings when the connection is apart. Some lines coming off the compressor are made into a manifold where both the suction and discharge lines are connected together and have to be replaced together.
- Screw-together line fittings have been used for many years. With most of the lines being made out of aluminum, it is easy to kink the lines if two wrenches are not used. Corrosion between the steel nut and the aluminum line could also affect how this comes apart. Lube the O-ring with the proper lubricant oil.
- Bolt-together connections are becoming more widely used. One or two bolts hold the connection together. One or two O-rings seal the connection.
- Spring-lock connectors have been used by Ford Motor Company for many years on fuel and AC lines. A special tool is required to disconnect the lines. The tool must be the right size for the line being disconnected. As the tool slides into the line connection, it spreads the garter spring over the other line. The lines can then be pulled apart. Two O-rings seal each connection. To reassemble, lube the O-rings and then push the lines together until the garter spring clicks and locks the connection. Double-check the lock by trying to pull the lines apart.
Service Tip: Never reuse O-rings or gaskets on an AC system. The seals and gaskets are a high-leak occurrence anyway. Lubricate the O-rings with mineral oil. Another item to replace while you have the system apart is the Schrader valves. It’s very disappointing to finish an AC job, only to have the Schrader valves leak after the service hoses are removed.
Replacing a Compressor
To replace a compressor, remove the drive or serpentine belts. Two lines connect to the compressor either individually or in a manifold. Disconnect the AC clutch coil wiring connector. Unbolt the compressor from the engine.
A variety of compressors are used and each has its own procedure for adding oil to the replacement compressor. Some compressors have a sump that stores a volume of oil where a dipstick is fashioned to check the level, while most do not. Some compressors require a given amount of oil added to the suction line opening and the compressor is rotated in the proper direction a given amount of times.
To install the compressor, bolt the compressor up to the engine. Place new O-rings or gaskets on the line connections and bolt up the lines. Connect the AC clutch coil connection. Install the drive belts. Note: with V-belts the tension must be adjusted. With serpentine belts, there is an automatic tensioner.
Service Tip: When a compressor is replaced because it is locked up or noisy, it will have pushed metal out into the system. Since the condenser is the next component down line, much of this material may have lodged in the condenser. You may be able to backflush a single-pass condenser using flushing solvent, but parallel condensers should be replaced in this situation. It is also recommended that a new receiver-drier or accumulator be installed.
Replacing a Condenser
Replacing the condenser, which is located behind the grill, may involve removing of the front bumper, hood, or grill, depending on the model. Since the condenser is in front of the radiator, it is also prone to collect dirt, leaves and plastic bags. Bent-over fins can also affect the efficiency of the condenser. A fin comb is designed to straighten bent fins. Two lines connect to the condenser: one is from the compressor and the other is the liquid line. Before installing the new condenser, check to see how much refrigerant oil needs to be added.
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Replacing a Receiver-Drier
The receiver-drier is used with TXV (thermal expansion valve) or H-Block systems only. Many times it will also have high-side switches located on the drier. Replace the O-ring when transferring the switches. Before installing the new receiver-drier, check to see how much refrigerant oil needs to be added. The receiver-drier should be the last component replaced, since as soon as it is unsealed, it starts absorbing moisture.
Replacing a TXV (Thermal Expansion Valve)
The TXV is located on the entrance to the evaporator. The TXV is usually connected by screw-together line fittings and has the thermal bulb attached to the evaporator outlet. When installing a new TXV, make sure the thermal bulb area is clean and that the bulb is fastened securely to the line. After installation, wrap the insulating tape around the connection to prevent engine compartment heat from affecting the bulb. If the TXV is located in the evaporator housing, no insulation is necessary.
Replacing an H-block
The H-block is a TXV that has no need of the external thermal bulb because it is built into the valve. The H-block bolts into both the inlet and outlet of the evaporator. A gasket or O-rings are used for the connection. Unbolt the lines from the H-block and then unbolt the H-block from the evaporator.
Replacing a Fixed Orifice Tube
The orifice tube is usually located at the entrance to the evaporator. The tube sits inside the line on an O-ring and a restriction or dimples in the line hold it in position. A tool pushes in and turns to lock onto plastic tabs to remove the orifice tube. Special tools may be needed to remove a stuck orifice tube. If a compressor has gone out and pushed a large amount of material into the screen, it may be that the orifice tube will not come out. Sometimes the evaporator may have to be replaced or an orifice tube repair kit may be available to replace that section of line.
Service Tip: The material caught in the orifice tube screen is a good indicator of the condition of the system. Black flakes may mean the hoses are deteriorating. Silver flakes may be coming from the compressor. Dark grey mud is usually grindings from the compressor.
Replacing an Evaporator
Most evaporators are located in the heater box under the dash. Replacing them may be a very time-consuming job depending on the vehicle. The entire heater/AC box may need to be removed from the vehicle to replace the heater core or evaporator. Since the box is mostly made of plastic, care should be taken when taking it apart and putting it back together. Before installing the new evaporator, check to see how much refrigerant oil needs to be added.
Some evaporators and heater cores can be accessed by removing an access panel instead of by removing the entire heater box.
Part 3. How to Recharge the AC System After Repairs Are Done
Once the necessary components have been replaced, the AC system needs to be recharged to be placed back into service.
You cannot just put refrigerant in the system, because air and moisture in the system are contaminants. To remove the air and moisture, the system needs to be placed under a vacuum, called evacuation, for at least 30 minutes. The charging stations will have a vacuum pump built in. Because the vacuum process uses the same hoses as charging, no extra hookups need to be done
To evacuate the system:
- Set the timer.
- Push the evacuation button.
- Watch the vacuum gauge to see if it goes to 29" Hg.
- At the end of the timer, it may ask if you want a leak test. This will see if the system holds vacuum.
To charge the system with oil:
- Dial in the amount of oil recovered.
- Place the correct oil in the fill bottle.
- Press the oil charge button and pump in the correct amount of oil.
To charge the refrigerant:
- Dial in the amount of refrigerant the system calls for from the under-hood tag or service information.
- Press the button to charge the refrigerant. Make sure the engine is not running.
Once the system is charged, it can be run to performance-test the system.
Other Articles on Car Air Conditioner Operation and Diagnosis
Here are two companion articles you will find useful if you repair AC systems.
Air Conditioning (AC) System Operation with TXV or Orifice Tube. Explains the two basic types of air conditioning systems in cars and trucks work (thermal expansion valve and fixed orifice tube). Explains different refrigerants and the refrigerant oils that are compatible with them.
AC System Diagnosis by Symptom. Explains how to use pressure measurement and other techniques to find the problem in your car's air conditioning system.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.