Converting Air Conditioning to R-134

By Rob -

(Editor's note: Please read this article on Refrigerant Recovery.)

I have a 1990 Miata which I wanted to convert to R-134a from R-12. I bought a rebuilt compressor (w/ clutch) from Advance Auto Parts which cost me $350 with a $10 core charge (I kept my factory compressor because I might as well rebuild it as get back a lousy 10 bucks), a conversion kit from AutoZone that contained 3 cans of freon/oil, high and low-side connection adapters, and a filler hose with a low-side gauge on it (each can had 12 oz freon, 3oz oil). I also bought a low-side R-12 to R-134a elbow (I needed the elbow because it's impossible to get the gauges hooked up once an adapter has been added because everything's so close to the intake manifold). I only purchased one elbow because I didn't have any luck finding an adapter for the gauges available to me, which had R-12 connections. The conversion kit I bought had a gauge on the filler tube (low side), and so I left the high side with an R-12 connection so I could use my gauges for the high side. I also purchased a dryer (be careful with this: the dryer I bought had connections that were at angles to each other, while the factory dryer has inline connections. I didn't notice until I was out in the country doing the installation and it wasn't worth going back to complain. If this is the case, it's just a bit of a pain to remake the connections to the dryer, but it's doable). I did not purchase any o-rings because I had the system evacuated at a Midas (they'll take whatever freon you have for free, since they can sell it) and it held vacuum, so I figured there were no serious leaks. I also had no idea what size any of the o-rings were.

The Procedure:

I have a factory manual, which was useful for information. I had a friend help me, which made things easier. We removed the splash shield, but didn't take the front of the car apart as the manual says to do. To remove the belt, my friend had a 22mm wrench for the idler pulley. If you don't have the correct wrench, I found it virtually impossible to use a socket because of lack of space, so you may have to remove the radiator and fans. We took the horn loose from its mounting and unbolted the dryer. Air rushed into the system, so I figured it was still leak free. I then got under to car and removed the bolts on the bottom of the compressor (a long socket extension helps here, because there is some tubing in the way). My friend removed the top bolts and I got the
compressor out through the bottom.

We were going to measure how much oil was in the compressor, but in removing the compressor I managed to spill most of it onto the ground (to avoid this, keep it upright as you remove it). The factory manual specifies adding 10cc of compressor oil if the dryer is replaced. However, it doesn't say anything about how much oil to add if the compressor is changed, so we didn't have any information. The new compressor had no oil in it, and it also did not come with any o-rings. My friend happened to have a couple that fit (I think one was green and one was black), and I have heard you can use any color other than black, but since I wasn't changing any other o-rings, I thought I might as well use another black o-ring. The dryer did come with o-rings and they were blue.

We coated all the new o-rings with compressor oil, and put the inlet and outlet pipes into the new compressor. We bolted in the compressor, and then bolted in the dryer, although its angled connections meant bending the tubing a little to get it to bolt up. Because the elbow connector I bought didn't have anything to depress the Schraeder valve in the low side, we had to remove it before installing the elbow. We installed the belt, again using the handy 22mm end wrench for the idler. NOTE: If you don't tighten the belt sufficiently, it will chirp when the compressor is turning. We then connected a vacuum pump (which also had an R-12 connection) to the high side and let it suck out the system for about an hour.

After the system was sufficiently evacuated, I connected the filler hose to the low side and the A/C gauge to the high side. I then closed off the valve on the filler hose and punctured the R-134a/oil can. Nothing happened. It
turns out the outlet side of the filler hose was blocked, so I had to take it apart and blow it out with compressed air. However, after this minor setback, we reconnected to the system and drained the first can into the low side. The factory manual says not to invert cans because you risk liquid freon hitting the compressor, but you have to get the oil out somehow. Also, the manual was written for an R-12 system, so I decided to take a risk and everything turned out OK. You can decide what you're willing to chance. You're probably safe with the first can because the freon will expand, and you haven't started the car yet.

After the first can was empty, we started the car and proceeded to put in the second can. I didn't have it completely inverted, just a little past horizontal because I was still a little worried. The second can went in without a problem, but at this time there was about 24oz of freon and 6oz of oil in the system. The manual calls for 28oz of R-12. I'm not an A/C expert, but my friend thought we were probably doing OK on freon because he'd heard that you need about 85% of the R-12 you would normally need when using R-134a. Cold air was
coming out as well. However, there were still bubbles in the sightglass, so I decided to add another little bit of freon and hooked up the third can. This was, of course, a mistake and after a test drive, the compressor was
engaging and disengaging about once per second. After hooking the gauges to the high side, we found that the high side pressure was around 350 psi (normal high-side pressure is max 240 psi). You're probably saying I should have thought of that before, and you're right. Anyway, we bled freon from the system slowly (trying to avoid losing a bunch of oil) until the pressure was normal. The system still produced cold air, and the compressor operated normally. However, the sight glass still has bubbles in it, so I guess the old R-12 method of adding freon until all the bubbles are gone doesn't work in this conversion. Anyway, we ended up, after bleeding the system, to about the equivalent of 24oz of freon and 6oz of oil added to the system. I have been driving it for 2 months in the hot summer weather of Florida with no problems.

Again, I'm no A/C expert, so I probably made several mistakes. However, I'm cool now whereas I was dying before in the heat. I don't claim that this will work for everybody, but it did work for me. As far as the connections go, if you have gauges with R-134a connections on them, buy two elbows. It not, buy the low-side elbow and get a filler hose that has a gauge on it. Again, I couldn't find a description of the upgrade in the garage or the discussion group. Maybe it's there and I wasted my time, but anyway, I hope this helps somebody.

By Tom Mak -

Here are what you need

  1. 2 Cans of Freon
  2. 1 Can of oil (you can buy the freon with the oil)
  3. 2 new nipples. You have to put these nipples on the low and high side. Some have said you don't have to change the high side nipples.
  4. A connecting hose to connect the cans to the car.
  5. Gauge to measure how much freon in the system


  1. Cool down the car and open your hood
  2. Locate the high port side of the A/C and the low port side of the A/C. The high port side is the one with the small hose. It is located right above alternator on the 92-93 car. On the 90 car that I have seen, it is right under the low side nipple. I am not sure about 91. Anyway, you just twist the new nipple on and cap it off. You wont use the high side.
  3. The low port side of the A/C is right near the firewall on the passenger side. Just look for that big nipple on the big A/C hose. Simply remove the cap and screw the new nipple on. Easy job
  4. What you do next is grab a can of freon. Shake it hard and put the connecting hose to it. Slip the hose to the low side of the A/C line. Turn the car on and run the A/C at full blast. Once you do that, start to open the nipple and allow the freon to go in. After 1 can of freon, put in some oil. I used about 8 oz of oil to lubricate the whole system. After the oil, I put in another half a can of freon.
  5. Check how much freon is in your system with the gauge. The Miata really should not take more than 1 1/3 can of freon. Maybe 1.5 can the most. Check the gauge.

If my memory serves me right, Miata has a 1lb 8oz. capacity. A can of $4.50 freon from Wal-Mart is about 12 oz. I figure I put in almost 1.5 can of freon and almost 6-7 oz of oil would be good enough. Use the gauge to check, don't overcharge it.

How well does it work? Well definitely not as good as before, but for $30. It sure beats paying $300 to recharge the R-12. I also have to say that my old R-12 never really worked that great anyway. My 90 RX-7's AC was cold enough to give me frost bites!


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12 June, 2005