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DIY Auto Service: AC System Service and Component Replacement

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

AC Recovery, Recycling, and Recharging Station

There are recovery-only machines, but most recover refrigerant, clean and dry the recovered refrigerant, evacuate the system and recharge the system with refrigerant and oil.

There are recovery-only machines, but most recover refrigerant, clean and dry the recovered refrigerant, evacuate the system and recharge the system with refrigerant and oil.

Discharging an AC System and the Law

The law requires that no refrigerant be released to the atmosphere under Section 609 of the EPA Clean Air Act. The law also specifies that the technician performing the work must be 609-certified if receiving compensation. 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.

AC System: Three-Part Series

This series is divided into three parts. The AC system operation, AC system service, and the diagnosis of a malfunctioning system. To be able to service and repair an AC system, read through the other two articles, in addition to this one:

Recovering the 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 old R12 fittings were screwed on and with most vehicles were the same size (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, there is no refrigerant in the system and it does not need to be recovered. If there is pressure, then press the recovery button. The pressures should be going 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.

Charging an AC System

Once the components have been replaced, the AC system needs 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 it uses the same hoses as charging, there is no extra hookups that 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.

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R134a Compatible Refrigerant Oil

Most R134a systems use PAG oil. Make sure the specified oil viscosity (thickness) is used.

Most R134a systems use PAG oil. Make sure the specified oil viscosity (thickness) is used.

H-Block Replacement

The H-Block is mounted (bolted) on the Evaporator Inlet/Outlet. The Low and High Side Lines connect to the H-Block.

The H-Block is mounted (bolted) on the Evaporator Inlet/Outlet. The Low and High Side Lines connect to the H-Block.

Replacing AC components

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 their 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 of a locked up or noisy compressor, metal has been pushed out into the system. Since the condenser is the next component down line, much of this material may have lodged in the condenser. Where a single pass condenser may be able to be backflushed using flushing solvent, parallel condensers should be replaced. 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 removal 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 could also affect the efficiency of the condenser. A fin comb is designed to straighten bent fins. Two lines connect to the condenser; one 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.

Replacing a Receiver Drier

The receiver drier is used with the TXV 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

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. This 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 this is made from mostly plastic, care should be taken when taking this 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 having to remove the entire heater box.

AC Condenser Replacement

The AC condenser sits in front of the radiator. Depending on the model, the grill and radiator may have to be removed.

The AC condenser sits in front of the radiator. Depending on the model, the grill and radiator may have to be removed.

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.

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