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A few questions regarding 134a Conversion

waggy73 on Mon July 07, 2008 2:09 PM User is offline

Year: 1991
Make: Mazda
Model: Miata
Engine Size: 1.6L
Refrigerant Type: R-12
Country of Origin: United States

Hello, I have read the forum archives and have most of my questions answered thanks to previous posts. I do have a few questions that I hope you can help me out with.

I am installing a stock A/C system into my fiance's Miata. Her car did not originally come with A/C, but I purchased the entire system from a friend who had a couple Miata systems laying around. Since the system is in parts, I was able to flush using Hecat Safe Flush and blow dry compressed air through each part individually, and I am confident that the used parts are clean. The original parts all used R-12, I am converting the car to 134a so I can do the work myself and because the rest of the cars I work on are usually 134a.

The compressor had mineral oil in it I think, since it used to push R-12. I tried to put some PAG in it and flush it, but I can't get the compressor to push out any PAG/mineral oil. Is it because I haven't put enough PAG in to be pushed out? The compressor was drained pretty much completely, and I only put in 2-3 ozs of PAG so far. I am using a low speed drill to turn the compressor over in a vice in my attempt to flush the compressor. Also, do you put the replacement PAG oil directly into the compressor? I know you are not supposed to put liquid coolant in the compressor as it doesn't compress liquids, so it seems contrary to me to put the oil in there.

Should I replace the expansion valve? I did buy a new receiver/dryer.

Is there a micron scale vacuum gauge you recommend to get a better reading on the vacuum? ACKits does not sell one that I can see.

I have a 30lb tank of 134a, how do I know how much coolant to bleed into the system? I don't think the tank has a gauge on it. I have a set of the mastercool high/low pressure gauges from ACKits that I will use to monitor the pressures.

Since there are no printed pressures that I can find for the high and low, is the general consensus to have the low pressure between 20-30 psi and the high pressure right around 200?

Thanks for all your help.


CCWKen on Mon July 07, 2008 4:33 PM User is offlineView users profile

Wow, that's a lot to answer but while I'm here I give it a whirl. The Miata called for 29ozs. of R12 and 6.5ozs of mineral oil. For 134a, the figure is ABOUT 24ozs and use 6.5ozs of PAG-100 (DEC) or BVA-100 located here: BVA

24ozs is two 12-ounce cans but since you have a tank, you need a tank scale to measure what is installed plus allow for line volume. Trying to guess based on pressure is not very accurate and changes with temperature. What part of the equation do you assume is correct? Your gauges or the temperature of the refrigerant? You will measure the refrigerant temperature, right?

The micron gauges are pretty pricey for the home shop or for just doing a job or two. A good vacuum pump pulling and holding a 29-30 on the dial will be good enough. (Check your ZERO before each use.) At high elevations, a 27 may be all you'll see.

Put about 2-3 ozs. of oil in the compressor. You can distribute the rest of the 6.5ozs. in the system. (Usually placed in the accumulator/drier.)

Add a 1/4oz. of dye to the system too. It may help you or someone in the future to diagnose a leak.

Ken Kopsky

Custom Car Works
"Rules are for the guidance of wise men and the obedience of fools."

Edited: Mon July 07, 2008 at 4:37 PM by CCWKen

bohica2xo on Mon July 07, 2008 9:31 PM User is offline

Your Miata uses a TV14 Vane type compressor. It should flush just fine with the procedure in the FAQ's. Are you turning it the correct direction? Of course if the vanes are stuck, it will not pump very well. Try covering the suction port while you turn it the proper direction - it should pull a vacuum.

If it was bone dry, you may need to push a few ounces through it to see good ejection.

Micron gauges are available from places like Johnstone Supply & Grainger. An inexpensive unit like the Supco VG60 works fine for MVAC work, but it is still about 135 bucks.

An expansion valve that has been sitting around may be ok, or it may not. Depending on how hard it is to get at after the system is assempled, and the cost to buy a new one - the decision is hours/dollars.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Mon July 07, 2008 at 9:33 PM by bohica2xo

waggy73 on Tue July 08, 2008 9:34 AM User is offline

Thanks for all the help. I'll see if I can source a tank scale for the 30 lb tank before this weekend. I have the correct PAG fluid for the conversion, I'll try covering the suction port and spinning the compressor again. I did verify that I was turning the compressor the correct direction.

The expansion valve is pretty easy to get to, but I'd rather not have to crack the system open again a week from now to replace it when it goes bad. Looks like I'll be ordering one today. Thanks again for all the help.


waggy73 on Mon July 14, 2008 1:54 PM User is offline

Ok, I have the entire system installed in the car. This evening I will be pulling a vacuum on it and if all looks well I'll be charging it. One last question I have is there is a sensor attached to the evaporator. It has a probe that sits in between the evaporator coils, and the probe exits the evap case and enters a "switch" with two spade terminals. I am assuming that the probe is there to monitor the temperature of the evap, and in the event that it gets too cold it will signal the compressor clutch to disengage, preventing you from turning the evap into a block of ice. Is my logic correct?

Also, a shot in the dark here, the sensor switch has the two spade terminals as I previously mentioned. There are two wires that attach to the spade terminals. It is not a "plug" so the wires are interchangeable on the spade terminals. I have no idea which wire goes where. Do it really matter if the sensor is really just a "switch" as I think it is? Or if by chance someone has worked on an early Miata recently, can you tell me which wire goes where? I have a Dark Green w/Red Trace and a Light Green w/Black Trace. The sensor on the evap does not have any markings on it to ease the installation of the wires. Thanks for all the info on the forum, it has made this job infinitely easier.


mk378 on Mon July 14, 2008 2:33 PM User is offline

Yes that is a temperature switch, just a simple open/closed switch so the position of the wires doesn't matter.

waggy73 on Mon July 14, 2008 3:08 PM User is offline

Thanks! Hopefully all goes well tonight.

waggy73 on Fri July 18, 2008 9:48 PM User is offline

Crap! I finally got the acme fitting for my vacuum pump and vacuumed down the system for 2 30 minute sessions tonight. Both times the system held great vacuum. I attach the 30lb tank and turn on the a/c in the car. I can't get the compressor to switch on. I disconnected the pressure switch on the high side and jumped it, still no compressor. Is there ever a need to jump the compressor directly to the battery? The system basically pulled 3 oz until the system pressure equalized with the tank pressure. The a/c fan does turn on when I press the a/c switch in the car, so I know that part of the wiring system is correct. Any help you can provide would be appreciated. Thanks.


TRB on Sat July 19, 2008 10:36 AM User is offlineView users profile

There is no need to hot wire the clutch. Make your your cylinder is opened fully to allow enough refrigerant to flow into the system. If that does not work you need to start tracing down an electrical issue.


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

m32825 on Sat July 19, 2008 8:42 PM User is offline

Check that spade connection you mentioned earlier and make sure it's plugged in good. It's probably a thermistor, not a switch, an open circuit (high resistance) may be fooling your a/c control into not running the compressor, because it thinks the coil is too cold. I had this happen to me when I converted my Camry. I could jump the compressor coil and see it engage so I knew it worked, but wasn't getting juice for some reason. I finally realized I had hooked the thermistor into the wrong connector... it was connected to the glove box lamp power supply!

Good luck!

-- Carl

waggy73 on Mon July 21, 2008 12:04 PM User is offline

Thanks Carl and Tim. I am getting a new a/c relay today, and I will check the connections on the thermistor/switch tonight.

waggy73 on Sun July 27, 2008 7:55 PM User is offline

Ok, finally got a chance to work on the Miata again today. I have power up to the compressor,and the clutch still isn't engaging. This may be a dumb question, but is it possible to rebuild the clutch only? I am guessing it is an electromagnetic switch that engages the clutch, is this correct? If I need to replace the whole compressor (most likely scenario), should I get a later model compressor designed to run with 134a, or would a new "R12" compressor run fine as long as I have the correct 134a oil in it? And one final question, do the compressors usually come with oil already in them, or will it be dry so I can add the necessary oil myself? Thanks for any help in advance.


mk378 on Mon July 28, 2008 9:59 AM User is offline

Assuming your clutch coil is good (3 to 4 ohms resistance and properly grounded), the problem is most likely just the air gap between the pulley and clutch plate. The test is to very carefully tap on the plate with the engine running and A/C switched on and see if it engages. On most non-GM compressors the gap is adjusted by taking the plate off and removing shim washers.

m32825 on Tue July 29, 2008 4:42 AM User is offline

Check to see if you're getting 12V to the magnetic clutch coil when the system is supposed to be running. If you are, then the problem is with the clutch. If you aren't, then the problem is "upstream" somewhere. You could also briefly jump the magnetic clutch to see if it engages.

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