Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

Can I DIY R12 to R134 conversion? No R12 currently in the system...

f2racer on Wed May 26, 2010 10:45 AM User is offline

Year: 1993
Make: Mazda
Model: RX-7

I bought the car back in '99 and the AC has never worked. I figured out why several years ago when I changed the radiator. Because the receiver/dryer is in the way, you have to push it out of the way. When pushing the receiver/dryer back, the bolt head on one of the two bolts at the top of the unit popped off! There appeared to be glue (epoxy) residue underneath the bolt head. I'm guessing that someone overtightened the bolt at some point breaking off the head. They attempted to fix it with glue of all things, but all of the refrigerant probably vented into the atmosphere after the bolt head broke...

In any case, I extracted the broken bolt and replaced it with a new one and simply never used the AC (since at that time I was considering selling the car). I've decided to keep the car and I'm looking to get the AC working again. I'd like to convert it to R134A. Since there is no R12 in the system, can I simply buy one of the R134 conversion kits and do it myself? Should I get a new receiver/dryer? Are there special tools required to flush the mineral oil out of the system? I have no AC experience so any help would be greatly appreciated.


TRB on Wed May 26, 2010 12:02 PM User is offlineView users profile

Minimum requirements for converting a system to R134a.

If system has any refrigerant R12 left in the system it must be reclaimed by an approved recovery machine.

Accumulator/drier must be replaced with an R134a compatible replacement. Conversion fittings and label must be added to the system. Label should have the amount of R134a used and quantity and oil type listed. If vehicle is not equipped with a high pressure cut off switch it must be added. That is the required minimum! While complying with the EPA laws may not provide the best performance.

Flushing the system to remove the mineral oil and debris should also be done. R134a and mineral oil do not work well together so leaving it in the system with R134a is not recommended! O-rings should be replaced with either NBR or HNBR replacements. Adjustment of the pressure cycling switch may also be needed to achieve the best performance. In some cases an upgrade of the condenser may be required to achieve the original performance. In many cases the parallel flow condenser will not be available as a direct replacement. Using a universal parallel flow is an option. A new custom hose set would be needed for the different style fittings used on the various parallel flow condensers.

There are many different suggestions when charging a R12 system with R134a. System must be evacuated before starting the charging procedure. I suggest you start with about 70 percent of the original R12 charge and add an ounce at a time until vent and pressure readings reach the best available results. Remember it is easy to over charge a system with R134a so patience is important.


When considering your next auto A/C purchase, please consider the site that supports you:

94RX-7 on Thu May 27, 2010 11:53 PM User is offline

SO...first thing to figure out is whether you have the Nippondenso system or the MANA system. You'll need to know so you can get the correct parts. You can tell by looking at the sightglass. Driers are hard to get for the MANA system. Redi-Aire made some, but apparently they didn't machine the o-ring openings correctly, and they leak. Ray Crowe at Malloy Mazda told me he had a batch of MANA driers made up a year or two back, so you can probably get one from him. The Nippondenso drier can be had from the forum sponsor, as can the expansion valve for either system.

Good news is:

You've already got a parallel flow condenser (unless somebody replaced your condenser with a cheap Chinese tube and fin knockoff). Bad news is, Mazda decided to increase the fin density when they went to R-134a, so what you've got is likely passable for R-134a, but clearly Mazda/Nippondenso thought they could improve system performance with a higher fin density.
There's also already a high/low pressure cutoff switch (on the dryer lines underneath the intercooler duct), so you don't have to worry about that.
No pressure cycling switch adjustments needed since it is thermostatic, not pressure based.

If it were my car (I have a 1994 Touring), and I lived somewhere where it gets hot (I'm in Houston), I would leave it R-12 (I'm going to replace my compressor here soon, and I'm sticking with R-12). The system performs reasonably well with R-12 but doesn't blow frigid hate like I feel a good auto A/C system should. Performance will go downhill some with R-134a.

Again, if it were my car, and I just HAD to convert it to R-134a, I'd do the following:
Take the entire system out of the car.
Flush everything.
Flush a TON of new PAG or POE oil through the compressor while rotating it by hand.
Replace the expansion valve and drier.
Reassemble with new o-rings.
Charge according to Mazda's spec for R-134a. As I recall, it is 20 ounces. Dump in ~18 and see how it performs.

Parts for these A/C systems are a pain in the ass to hunt down because of the low production volume, and the fact that they put 3-4 different systems on the car, which do not have interchangeable parts. Oh yeah...Mazda quit making A/C parts, too. On the bright side, the ricers like to get rid of their A/C systems so used parts can be had. A full system teardown (assuming you don't screw something up in the process) and flush is the best way to ensure a long service life from the parts you've got.

f2racer on Mon May 31, 2010 11:02 AM User is offline

94RX-7, thanks for the detailed write up. Here's a picture of my drier.

Can you tell which system I have?


94RX-7 on Mon May 31, 2010 1:00 PM User is offline

That is a MANA system. The Nippondenso system has the sightglass in an aluminum block that sits directly on top of the drier.

f2racer on Mon June 07, 2010 2:12 PM User is offline

94RX-7, what are your thoughts regarding Freeze-12? In order to fully flush the AC system will require a full teardown, which I won't have the time to perform anytime in the near future.

Can I get away with replacing the drier (Ray does have them for $95), adding some ester oil (it should be compatible with the original mineral oil right?), evacuating the system with an AC vacuum pump, and filling the system with Freeze-12 (which I believe is a combo of R134a and R142)? I'm looking for something that will last through this summer...


TRB on Mon June 07, 2010 2:45 PM User is offlineView users profile

It's 80 percent R134a. In my opinion you have to follow all the flushing requirements to use Freeze12 just like you would with R134a.


When considering your next auto A/C purchase, please consider the site that supports you:

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.