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Need help with weak A/C diagnosis - 91 Miata

super77 on Thu August 12, 2010 1:02 AM User is offline

Year: 1991
Make: Mazda
Model: Miata
Engine Size: 1.6
Refrigerant Type: R12
Ambient Temp: 82F
Pressure Low: unknown
Pressure High: unknown
Country of Origin: Japan

First time post, please pardon any noobness.

I have a 91 Mazda Miata, with an all-original R12 system. AFAIK, it has never been serviced. I have had the car for 3-1/2 years, and the A/C has always worked fine. Since I live in the Pacific Northwest, I use it mostly in winter as a dehumidifier (we get a lot of rain). I use it occasionally in the summer as well, but being a convertible, it's hardly necessary. I noticed this summer that it was not blowing quite as cold as before. The clutch seemed to cycle normally, and the sight glass showed foamy bubbles at idle and went to clear a few seconds after turning off the engine.

I am a die-hard DIY'er, so I assumed a top-up was necessary and started on the track of getting a 609 cert and a couple of cans of R12 from a private seller on CL near me. The system only takes 2 lbs., and I figured it might cost a bit less than going to a tech, plus I'd learn something. I obtained a new set of R12 gauges for an initial pressure check, and that's where the problem began. The fittings were impossible to thread on (yes, I checked to make sure they were R12 fittings), and in the process I let about 10 seconds worth of hiss out of the high side and maybe 1-2 seconds out of the low. Never did get them on. The engine bay space in a Miata is really tight, and my hose ends just didn't have a bend radius tight enough to fit lined up with the A/C nipples. I figure I need right angle adapters.

I'd still like to try DIY, but I think I may be over my head, especially if I am accidentally venting. I'm not sure how to determine the need for a top-up vs. purge and refill, either. I don't mind paying a tech if that makes more sense. Could I have made things much worse by releasing some pressure from the system like I did? The A/C still blows, but the compressor seems to cycle off for a couple seconds once a minute now. Not sure if it did this before. Sight glass looks the same. Thoughts? Thanks.

Chick on Thu August 12, 2010 8:14 AM User is offlineView users profile

What exactly was the problem hooking your hoses up? Was the high side "thinner" than the low side? If so, you need an adaptor.. You can get it at the sponsors store, just e-mail and they can set you up.. If they are R12 service ports, the hoses have to screw on if you're using R12 hoses.. And yes, foamy sight glass, you need a top off....If you decide to bring it to someone, make sure they use R12, a lot of shady people out there might try to talk you into a product with the number 12 in it...Not the same...

Email: Chick


Freedoms just another word for nothing left to lose

super77 on Thu August 12, 2010 1:56 PM User is offline

The threads on the female hose fitting seem to match the threads on the system nipples. The problem is that there very little room between the nipple and some engine hoses, and the hose fitting is hard to thread on since it does not easily line up. In the process of lining up, I end up pressing the pin in the center of the nipple, which releases gas. I suppose with enough patience, I could get the hose screwed on, but I would probably be releasing gas from the system the entire time I am fiddling with it. Is that normal? I have never hooked up a manifold gauge, but I'd imagine you're supposed to screw it on without depressurizing the system in the process. Could I have made things worse just in my attempts so far?

super77 on Fri August 13, 2010 12:23 PM User is offline

Thanks for the replies. Yes, I did determine that that high side has a smaller diameter nipple, which explains why I could not screw the hose on. So am I simply looking for a "Ford-style" adapter, or is it called something more specific?

1stbscout on Sat August 14, 2010 1:55 AM User is offlineView users profile

Go up to the auto parts or look at the sponsor’s site for an adapter for the smaller high side fitting. They can supply you with a 90 degree adapter as well for the low side which should make it easier to install the hoses there as well. Ford and GM use the same size port on the high side. I believe it is a 3/16 port but could be wrong so don't quote me.
Don't worry too much about the little bit you lost just don't operate the system other than for testing until it is replaced.
The site glass should be clear then an additional few ounces added to the system. Watch the high side pressure. The pressure will "hang" (seem like it is not taking refrigerant) as you slowly add. It is filling the receiver/drier at this point. Depending on the location of the sight glass it can take additional refrigerant to proper fill the system after the glass is clear or ............

Most on this site may get a bit upset with what I am going to say next........ But as you are a DYI type....

Each system whether or not there is a label and/or a spec for the proper amount of refrigerant, will be slightly different. Manufacturing being what it is there are too many variables to say all like kind vehicles with the same system will need exactly the same charge. Overall, weighing the amount and following the "rules" is the simple way to make sure you are “in the ballpark". BUT (once again that word) I work on all types of refrigeration equipment with several different refrigerants and from experience each system is ever so slightly different. Close enough for government work fits here. If you are a perfectionist and want to truly understand what is happening and how to go about determining the "proper charge" for your system I can recommend a very good book for you to read.
It will require advanced math skills and will teach you more than you probably want to know but you will understand after you have read it. It may take a day or two for me to find it as I have not read it in a few years.

Edited: Sat August 14, 2010 at 2:01 AM by 1stbscout

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