1 - We start by doing an air conditioning system diagnosis. While this is outside the scope of this article, you want to find out if the system is currently working, and if not, why. Hook up gauges to the system and check system pressure. If there is no pressure, you can immediately begin disassembling the system. Check the pressure of both the high and low pressure sides of the system. A properly-working system will have 75psi or more pressure on both sides when shut off, and the pressure should be equal. Inequal pressure indicates system blockage.
2 - We next want to acquire all of our parts. At minimum this will involve a new dryer, service fittings, lubricant, and refrigerant. In addition you will need replacement parts for anything broken that you discovered in step 1, and a R-134a conversion sticker which details what has been done to the system by who. On cycling-clutch systems, it may be a good idea to install a brand new pressure switch, retaining the original as a spare. Check all your A/C-related electrical connections for damage as well.
Your best bet for lubricant is a synthetic ester oil, which works equally well with both R-12 and R-134a. The best way to get refrigerant is in a large bottle, but without a license you can only purchase 1-lb cans. These are typically actually only 12oz now; be aware of this when purchasing refrigerant. You will need 90% (0.9) of the R-12 capacity in R-134a. (If your system takes 40oz. of R-12, you will need 36oz. of R-134a.)
3 - Now we want to array all of our tools. In systems with snap-ring connectors (primarily Fords and their ilk) you will need snap-ring disconnect tools which you should be able to purchase at any auto parts store. In other systems there are typically large nuts holding lines together and you will need either large combination wrenches or some good-sized (10" or longer) crescent wrenches to disconnect them. These are usually aluminum connections and lines, so be cautious and always use the proper size tool or you will damage them. You will need a schrader valve removal/installation tool when you install your service fittings. If your system uses an orifice tube, you will need a removal tool to extract it.
In an ideal world you will need a refrigerant recovery machine for the R-12 you will be removing, if there is any in the system, and a charging system for the R-134a you will put in. It is possible to use an empty R-12 bottle and a vacuum pump to remove refrigerant from the system but it is not recommended and it is illegal to store or transport refrigerant in such a container. You can safely and reasonably charge the system with a vacuum pump (you will always need a vacuum pump capable of pulling 29 inches of mercury of vacuum for this process) and either a big can or several small cans of refrigerant. If using small cans you will need some tool to get the refrigerant out of them, either one which punctures the can (tap-a-can), or one which attaches to the top.
4 - We begin by recovering refrigerant, if any, from the system. This is outside the scope of this article for two reasons - one, it is best covered elsewhere; and two, if you cannot achieve this step, you should not be converting the system. If the system is empty, move on.
If you are going to use an R-12 system with flush capability to flush the A/C system, recover the system, then do this now. Otherwise we will flush in step 7.
5 - Disconnect the lines from anything you intend to replace, as well as the evaporator and condenser. Disconnect all of the electrical connectors in the system, especially the clutch coil field connector when present.
6 - Remove the drive belt from the compressor, then remove the compressor and drain it of oil by tilting it such that the discharge side of the compressor (often marked with a "D") is pointing down into a container. Slowly rotate the compressor until the oil has drained from the unit.
7 - Flush the evaporator and condenser. Depending on how you are flushing the system this step will vary significantly. A typical flush system is connected to the inlet and outlet of the evaporator or condenser such that the fluid (usually an ester-based cleaner, perhaps just ester oil) is pumped through it in the direction opposite to the usual travel of refrigerant. This process called "backflushing" is more likely to remove anything from the lines. You can flush the system by putting a chemical flush product into the line with a funnel and blowing it out into a container with compressed air if necessary, repeating the process until the fluid comes out clean. If you used an R-12 flush system back in step 4, you can ignore this step, but it is still a good idea.
8 - Remove anything else you will be replacing from the vehicle now, including lines, the dryer, or any other components. Install, but do not connect any parts. Leave the covers on the fittings of the dryer. It is filled with desiccant which will be rendered useless if you leave its fittings open to the air. These fittings are the very last thing that you should connect.
9 - Add oil to the compressor and install it. The lubricant capacity of your system should be listed on the air conditioning sticker, or you can find it from a shop manual or some other source. Put about half of the lubricant in the compressor. Connect all the lines in the system, lubricating all seals with the ester oil before sealing them. This will help form a good seal, keeping contaminants out of the system, and refrigerant in, as well as making the fittings come apart easier when the system must be serviced at a later time. Every seal which you have disturbed should be replaced with a new one at this time. Connect the dryer last. In systems with a pressure switch, which is typically located on the dryer, move this from the old dryer or line to the new location now.
10 - Install R-134a service fittings. Typically one removes the schrader valve (like the valve in your tire fill neck) from the original fitting, then screws the service fitting over the top. If that is not feasible, you can also install a saddle valve next to the existing fitting. Be sure to install it in the proper place. For example, when the fitting is on the dryer, be sure to install the fitting on the proper side of the dryer. Saddle valves fit over existing lines, puncturing them to allow refrigerant to pass through their valve stem, and sealing around the puncture with an O-ring. Don't forget to lubricate that O-ring with ester before installing the saddle valve. It is better, however, to use a screw-on service fitting.
11 - Put the system under vacuum using a vacuum pump or a recovery system. Make sure that it can hold 29 inches of vacuum for at least fifteen minutes before proceeding. If the system does not hold vacuum, locate and repair the leak before continuing.
11 - Add lubricant to the system. Lubricant can be added at this stage by pulling the gauge set (you are using gauges, right?) down to vacuum - the system is already evacuated - and by putting the service fitting of the gauges (the center, yellow one) in the container of refrigerant. When it's almost empty, close off the system so you don't pull in air, and add your refrigerant. Again, you get the proper amount of R-134a for an originally R-12 system by multiplying the original quantity of refrigerant by 0.9 - you want to charge the system with 10% less R-134a than you would R-12, because of R-134a's greater expansion when heated. New refrigerant is introduced into the system on the high side. If refrigerant is reluctant to leave its container, the container can be heated - with a heat gun for bottles, or with a vessel of hot water, plenty hot but cool enough to put your hand in.
12 - Reconnect all electrical connections. Your average system has wires going to the A/C compressor for energizing the clutch coil, and wires going to a pressure switch on the dryer which determine when to cycle the compressor clutch. Other systems will sometimes have other pressure gauges, for example modern Cadillacs typically have transducers on both the high and low sides of the system to aid in troubleshooting via their proprietary scan tool.
13 - Test the system. You should get between 20 and 30 degrees of cooling out of the unit depending on ambient temperature, humidity, and the expected system performance. Consult the factory service manual for precise performance specifications, which should tell you what to expect coming out of the duct (typically the right center duct) given a specific input air temperature and humidity. If the system does not cool as expected, carry out A/C diagnosis as usual.
14 - Fill out the conversion sticker and place it over the original A/C sticker, or in cases in which the original sticker was removed from the system with some equipment, place the new sticker in some easily locatable place, like on the compressor, on the dryer, or on the radiator support. This is not an optional step if you ever want a shop to work on this system.
This article has covered the proper way to safely convert an R-12 refrigeration system to R-134a. Failure to follow these instructions may result in severe injury. Even if you do follow these instructions to the letter, there is still the potential for death or dismemberment. Be careful working around high-pressure systems filled with toxic gases. When in doubt of your ability to complete the job, refer the job to a qualified mechanic. When others are in doubt of your ability to complete the job, heed their warnings. The air conditioning system is simple in principle but devilishly complicated if something is wrong. I will accept no responsibility for any injury or damage to property resulting from your inability to carry out an air conditioning conversion.