All pre-1994 vehicles that are equipped with air-conditioning were initially filled with R12 refrigerant. This refrigerant was eliminated in favor of the "ozone-safe" R134a refrigerant. If your R12 filled vehicle's air-conditioning is operating properly and not leaking, there is no reason to retrofit it for R134a refrigerant. However, if the air-conditioning requires repair, a retrofit is necessary as R12 is not only harmful to the environment, but scarce in quantity.
Lubrication
The oil used to lubricate the compressor can become contaminated from the R12 freon. This is not a concern when R12 is continuously used. However, when retrofitting, the key component is to remove all remnants of the R12 freon. For this reason, you must flush and drain the compressor oil and replace it with new PAG oil.
Seals
The rubber O-ring seals have small pores, and the R12 freon particles can become trapped in those pores. For this reason, all auto manufacturers recommend that the O-rings throughout the air-conditioning system be removed and replaced with R134a-compatible seals. There should be anywhere from 5 to 10 O-rings in any air-conditioning system.
Small Components
Most air-conditioning systems have several small components throughout that you should replace to properly retrofit an air-conditioning system. There are two filters that you should replace -- the receiver/drier and the orifice tube. You should also replace the expansion valve, if easily accessed, as well as the high-pressure cut-off switch. All of these components will have remnants of R12 freon and can cause cross-contamination with the R134a.
Check/Fill Ports
The check/fill ports, also know as the high and low service ports, are the components used to check system pressure and fill the system with refrigerant. This ports are completely different from R12 ports; therefore replacement is required to recharge the system, let alone the small remnants of R12 freon present.
Evacuation, Flush and Recharge
The air conditioning system, while low on freon, may still have a small amount remaining throughout. For this reason, a proper evacuation -- removal of freon via suction -- is required, and must be performed by a technician certified for R12 handling. A good addition to the evacuation is a complete system flush, but it is not required. After the system is free of R12 freon, you must fill it with R134a. Only fill the system to 85 to 90 percent of its capacity listed on the under-hood placard. Keep in mind that a certified reclamation company must properly dispose of the evacuated R12 freon.
Labeling
While the R134a fittings on the service ports should be enough to deter a technician from mistakenly putting R12 freon in the system, some inexperienced mechanics may use an adapter and fill it with R12. Doing this will completely negate all of the work performed, as well as potentially cause damage to the system. For this reason, all retrofit kits are required, by law, to include several warning labels advising the mechanic not to use R12. Place these labels in several conspicuous locations, such as near the under-hood placard.
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