How to Convert R12 to 134A on a Miata

by Lee Sallings

The conversion process from R-12 (Freon) refrigerant to R-134A refrigerant will drastically lower the cost of repairing the air conditioning system in your Mazda Miata. The use of R-12 has been discontinued; and as a result, the cost of freon has skyrocketed over the years. A special certification is required to purchase R-12, but no such certification is required to purchase R-134A, and servicing an R-134A system is still within the capabilities of the average weekend mechanic.

Evacuate the remaining R-12 refrigerant from the air conditioning system. This requires a very expensive recovery machine and must be done by a certified technician. Most auto repair shops will perform this service for a nominal charge. Never vent R-12 into the atmosphere.

Remove and replace the drier located on the driver's side of the engine compartment next to the radiator. Remove the mounting and A/C line attaching bolts using a 10 mm socket and ratchet with a long extension. Each of the A/C lines attached to the drier has a rubber O-ring that should be replaced with new ones supplied with the new drier. Pour three ounces of PAG oil into the new drier before bolting it to the car.

Place shop rags under the A/C lines where the lines connect to the compressor. Remove the 10 mm bolt that attaches the A/C lines to the back of the compressor and allow the oil to drain from the compressor. Replace the O-rings on the lines before bolting them onto the compressor.

Install the retrofit service valves onto the service ports located on the passenger's side of the engine compartment. The high-pressure port is located next to the radiator on the high-pressure line, and the low-pressure port is on the passenger's side fender well on the low-pressure line. Screw the retrofit adapters onto the original ports.

Attach the A/C gauge set to the service ports. The blue hose attaches to the low-pressure side, and the red hose attaches to the high-pressure side. Attach the yellow hose to the vacuum pump. Plug the pump into an electrical outlet and turn it on. Open both valves on the gauge set and allow the pump to run for one hour to remove any moisture from the system.

Close the valves on the gauge set. Turn off the vacuum pump and disconnect the yellow hose from it. Attach the can tap to the yellow hose and screw a can of refrigerant onto it. Open the can by screwing the knob on the tap all the way in and all the way out. Open the blue valve on the gauge set to allow refrigerant to enter the system. Start the engine and turn the A/C on maximum.

Add 1.75 cans of refrigerant. The Mazda Miata holds 24 ounces of R-12. The replacement equivalent of R-134A is about 21 ounces so two 12-ounce cans should be sufficient. Close the valve on the gauge set and allow the pressures to settle. A fully charged system should have a low-side pressure around 40 PSI and a high-side pressure between 350 and 400 PSI.


  • close Wear safety glasses and work gloves, when working around a running engine, to prevent serious injuries.
  • close It is illegal to vent any kind of refrigerant into the atmosphere. If the air conditioning system in your car still contains refrigerant, have a qualified repair technician remove it prior to doing the conversion.

Items you will need

About the Author

Lee Sallings is a freelance writer from Fort Worth, Texas. Specializing in website content and design for the automobile enthusiast, he also has many years of experience in the auto repair industry. He has written Web content for eHow, and designed the website. He began his writing career developing and teaching automotive technical training programs.

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