Introduction: Adventures in Auto Air-Conditioning. or Retrofit a NA Miata and Recharge It.

Picture of Adventures in Auto Air-Conditioning.   or Retrofit a NA Miata and Recharge It.

I picked up a 92 Mazda Miata for a fun/backup to a Chevy Aveo, If you saw my 2x4 tower game instructable you will see that i sort of pushed it beyond the cost benefit analysis to repair by breaking my windshield. It just needed too much work and would require about $1500 front end work to remain safe and roadworthy. Unfortunately the car was only worth about $2000 in good condition so repairing it along with the upcoming timing belt it was time to send it off to the junk man.

There is just 2 problems with my little plan the 92 was outfitted by the factory with R12 and 2 It had not worked for several years as the previous owner only drove it 6000 miles over 8 years. Great for me as I got a 23 yr old car with under 90,000 miles, but they were just not interested in retrofitting it since it was only driven on perfect days.

If you have the ability to turn a wrench you can perform this process by yourself. Also if you have a leak these directions will assist you in troubleshooting your system.

Scope of project, I wanted to restore air conditioning to my almost antique roadster by inexpensively converting the system to R134a.

SAFTY NOTE* R12 is some nasty stuff for the environment. If you think your old A/C system has any Freon at all take it to a shop that can safely recover it for you. It is Illegal in the US to vent A/C systems to the atmosphere there are some pretty big fines so don't do it.

If you are lucky they will probably not even charge you due to R12 being long out of production and quite expensive but if your system is empty they may charge you a diagnosis fee.

As we are talking pressurized gasses and automobiles make sure you don’t put anything in the path of a fan or a belt. Watch out for engine bay temperatures and wear safety glasses in case a fitting pops off.

Tools needed:

  • Combination wrenches
  • R134A Manifold Gauges (Borrowed from Auto Zone via $100 deposit)
  • Air Conditioning Vacuum Pump (Borrowed from Auto Zone via $200 deposit)
  • Valve Core removal tool ($4 in the tire section)
  • R134a can tap
  • Thermometer
  • Syringe and length of silicone tubing. (Not necessary but helpful for changing compressor oil)

Parts Required:

  • R134a Low side conversion fitting
  • R134a High side conversion fitting
  • Accumulator dryer for the vehicle being serviced
  • Assortment of R134a safe seals
  • 8 ounce bottle of PAG 100 compressor oil
  • 3 16oz cans of R134a Refrigerant containing a UV dye but not stopleak

Before beginning work; obtain the correct fittings for your vehicle, the tools required, and if possible a schematic of the A/C system to assist in locating seals. The cost of this after the loaned tool deposits were returned was about $65

If retrofitting an old R12 system (cars made before 1994) everywhere there is a connection, for example the lines running to the evaporator and condenser there will be a rubber seal. Those seals will need to be changed to a 134a safe version. If you are just charging an 134a system skip ahead to step 7 on page 3 After chocking the wheels and setting the parking brake.

System Capacities for various systems can be found here

If you are just topping off a weak system then you don't need a full set of manifold gauges the little low side only cans will give you a fill but will not help you trouble shoot a faulty system.

Pressures for a given temps.

And more in depth info for using manifold gauges.

Step 1: Retrofit.

Picture of Retrofit.

Step 1 Ensure your vehicle is in park (or neutral if a manual) with the parking brake set and the wheels chocked.

Step 2 With the engine off and cold perform a visual inspection that all of the A/C components are still there. Locate the High and Low A/C service ports. We know that the system has been fully discharged, in a EPA compliant manner before this point, it is now safe to use the valve core tool to remove the existing valve cores from the service ports. Discard the used valve cores.

Step 3 with a drop of PAG100 oil lightly lubricate the threads of the existing service ports and screw the conversion fittings on finger tight. You will notice from my fittings that the Miata requires 90 degree fittings to allow 134a compression fittings to clear the manifold. Most cars do not require these special fittings.

Step 4 I then began taking apart A/C line junctions and replacing the existing rubber seal with the R134a compliant equivalent.

Step 5 When I reached the A/C compressor I was presented with two options. I could either remove the existing compressor and dump the mineral oil into a container to measure and refill with the same amount of PAG100 or I could do a trick that I heard of that involved a syringe and a length of tubing. I went for the tubing method and after using a suction bulb syringe with a piece of silicone tubing I was able to recover about 4 fluid ounces (half a cup) of mineral oil.

I thought that was pretty good as the linked chart indicated that it should have about 6 1/2 ounces of oil in the system, and I am sure some settled into the dryer accumulator. Although not exact I split the difference and injected about 5 ounces back into the compressor using my makeshift suction bulb. My reasoning is that most shade tree mechanics would not put any R134a safe oil in the system as they figured that R134a would just use the oil existing. Unfortunately 134a does not mix with mineral oil and the oil would settle in the low parts of the system. With the system is rated for R12 capacities I did not want to overfill the system with oil to decrease the cooling capacity with less R134a. If I am wrong please correct me in the comments but so far my compressor is quiet and I get a 30 degree drop below ambient.

Step 6, Replacing the Accumulator Dryer. I was working on both ends of the system to finish with the Dryer. The reasoning for this is that the accumulator dryer is filled with a desiccant and the longer it is exposed to air the more moisture it will absorb. The more moisture it absorbs the longer the system needs to be under vacuum to boil it out.

The dryer should be shipped filled with nitrogen under pressure to prevent it from being saturated with water when it is installed. This is one of those times that you would need to ensure your safety goggles are on when you unscrew the shipping caps.

To remove the dryer on a NA Miata you can fit a 17mm wrench through the sight glass hole then using a 10mm socket to remove the horn bracket. You will have access to the 10mm bold that is securing the dryer clamp that is retaining the dryer unit.

Replace the last 2 seals then uncap the dryer and secure it into the bracket and reinstall the horn.

Congratulations the A/C is now ready to be placed under vacuum then filled if there are no leaks.

Step 2: CHARGE IT!!!!!!!!!!!

Picture of CHARGE IT!!!!!!!!!!!


Alright now that you have busted your knuckles going through your engine replacing seals. You can finally relax for a bit while the A/C is purged of moisture.

Ensure that all of the knobs are in the closed position and that the quick release connectors and hoses are attached to the manifold blue is the low side and red is the high side. The yellow hose is the service side both your fill hose and vacuum connection.

Connect the blue quick connect to the low side port, the red connector to the high side port and the yellow hose to the vacuum pump. Ensure that the vacuum pump has sufficient oil and switch it on. Then open the manifold valves and the shut offs at each connector.

The low side gauge should now be reading -1 BAR. The reason the system is placed under a deep vacuum is that water vapor will combine with the refrigerant and oil and form hydrochloric acid that will eat up your lines and cause all sorts of nastiness. As I had no idea how long the system was actually opened to air and humidity. I placed the system in a vacuum state for about three (3) hours. Why 3 hours? Well that was how long it took for me to replace the 23 yr old door speakers and tape deck with a modern Kenwood HD receiver with aux inputs and iPhone/Android controls. Oh and some 6 ½” Kicker 2 way speakers for the doors. I just followed the directions from the Crutchfield master sheet so no write up there.

Step 8. After the system was under vacuum for the 3 hours it took me to get the stereo replaced. I closed the valves on the manifold shut off the pump and then let it sit for another 30 minutes or so to ensure that I didn’t have any annoying leaks from changing all the seals

Step 9. Now that the system is verified to be leak free by holding vacuum I disconnected the pump and replaced it for the can tap on the yellow hose. I then closed the high and low side coupling valves and ensured that that the manifold gauges were fully closed. Then with the can tap in the open position thread a sealed can of R134a into the tap then fully close the tap to pierce the seal open ensure that all your couplings and the manifold is closed again.

Go ahead and triple check the high side, to ensure that it is in fact closed. If the high side is opened while you have a can attached to the service manifold it may rupture the can causing injury or death.

Note the ambient temperature on your thermometer and cross reference this chart for what to shoot for.

Start the engine, and turn on the A/C system set it to maximum cool with re-circulation and insert a thermometer into a vent. Note the temperature of ambient temperature. Open the Low/Blue side quick connect, open the can tap valve, and slowly open the Low/Blue side gauge on the manifold you should notice that the pressure will transition from a vacuum to a positive pressure reading. At the time I was filling my A/C I was shooting for about a 45PSI reading and my car took 3 12oz cans to achieve a full reading. It can go faster if you gently warm the cans in the palms of your hands to improve the conversion from a liquid to a gas.

After you have achieved the desired low side pressure, close the can tap and low side manifold valve. Leave the low side quick connect valve open and disconnect the now sealed partial can while threading the yellow service line to its loop valve. If you can safely do so, open the high side quick connect valve to obtain a high side reading. If everything is working properly you’re A/C is now fully charged and you have a reading on your vent thermometer that is about 40-50 degrees some thermometers have a marked target zone. My results were just on the high side of the target but within the range.

Turn off the engine, close all quick connect valves, disconnect the manifold and start cleaning up your mess. If you borrowed tools from your local parts store return them per their directions to receive your deposit. Enjoy your summer and chill out.


jrcrispy2 (author)2015-11-01

Auto shops use the liquid oil like you did, but I have seen them use a special syringe that connects to the service port while the system is under vacuum, which sucks the oil into the system. I personally like to get the oil that is mixed into a pressurized can with a small amount of Freon (im assuming, as oil isn't a gas at room temperature lol). I cant give you any real reason other than it is just more satisfying and reassuring to add it through the manifold lol. My question for you though is do you know anything about synthetic lubricants? Can they be used in old R-12 systems like ours? (I have an 85 f150, that I swapped a newer style compressor and evaporator into) thanks in advance for your knowledge.

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Bio: Howdy, I am a bit of a tinker gnome. I like playing with hardware/technology along with making stuff I want out of old stuff ... More »
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