1991 Miata AC - R12 top up

We have a 1991 Miata with factory AC.  We've had the car for a couple years, but the AC hasn't blown cold air.  The clutch seems to work properly, so I'm guessing we need to add some freon.  No idea if there is a significant leak.

Googling R12 produces a huge range of results.  Everything from you'll blow up the universe to just add some R12a and you're fine.  I have no real experience working with AC systems, so I'm struggling to figure out what to do.  Is there any easy way to tell if it's just low refrigerant level?  

My first thought is to try to add some of the R12a 'R12 compatible' refrigerant.  I have the simple R12 hose setup, but it doesn't have a gauge.  How do I know how much to add or when to stop?  

Or should I punt and take it to a shop?

If It has a sight glass on top of the drier just add it until the system kicks on and then you don't see bubbles in the sight glass anymore.

The R12 compatible stuff works fine (R12 is flammable also, so don't worry about that).  Make sure it has some stop leak in it (that refrigerant got out somehow).

The first thing I would do is find a fill port (engine off) and press on it to see if there is any pressure in the system.  If pressure, go ahead and try some equivalent stuff.  If no pressure, it will need to be vacuumed down (and a new receiver/ dryer), so don't bother.

I bought a partially used old 3lb cylinder of R12 off the side of the road for $50, filled the system up to specifications with manifold guages, then sold what was left for $50.    This was on a car that was 25 years old and had never had a fill-up and was only a little bit down, so it wasn't a gross pollution hazard.  

Not endorsing this approach.  In fact, it's a great example of what not to do.  So there, EPA sympathizers.

If the system is empty, the only right way to do it is replace the receiver/drier as mentioned above.  At that point, I would just do the R134-a conversion -- replace the R/D, the expansion valve if it's reasonably accessible, all of the o-rings, and the condenser (since they take all the abuse at the front of the car and usually are where the leak is.)  Draw a deep vacuum and make sure it holds, then refill to specification.

aircooled said:

The R12 compatible stuff works fine (R12 is flammable also, so don't worry about that).  Make sure it has some stop leak in it (that refrigerant got out somehow).

The first thing I would do is find a fill port (engine off) and press on it to see if there is any pressure in the system.  If pressure, go ahead and try some equivalent stuff.  If no pressure, it will need to be vacuumed down (and a new receiver/ dryer), so don't bother.

Not quite. R12 itself is not flammable, but there can be dissolved oil that will burn in the presence of an ignition source, generating phosgene gas. Phosgene falls into the "It is as dangerous as it sounds" category.

"R12 compatible" stuff is generally blends of propane and iso-butane, that are flammable. Currently, these are not EPA approved for use in mobile air conditioning systems.

Also, if you use a refrigerant or blend not on the EPA SNAP list, there is a pretty fair chance no A/C shop will work on that system. Last couple shops I went to checked the contents to make sure R12 really was before they would crack the system. Their concern is contamination of their recovery/recycle systems.

Auto HVAC has been very tentative in the past with flammable hydrocarbon based refrigerants, both CFC-12 and HFC-134a are non-flammable. Global warming is driving mobile A/C to HFO-1234yf, which is minimally flammable. There is also a bunch of discussion about the sustainability of HFO's, and some investigation into revisiting hydrocarbon based systems, either by using a 2 stage system where the hydrocarbon cools a second media outside the passenger compartment and the secondary media is the cooling media in the passenger compartment like the older proposed CO2 systems; or installing hydrocarbon detection systems in the passenger compartment that alarms when a combustible hydrocarbon concentration is detected.

OK, Sorry, went off the rails a little bit

aircooled said:

The first thing I would do is find a fill port (engine off) and press on it to see if there is any pressure in the system.  If pressure, go ahead and try some equivalent stuff.  If no pressure, it will need to be vacuumed down (and a new receiver/ dryer), so don't bother.

That gives me something to check.  Thanks.  I have no idea how long the AC hasn't worked.

Something that has been left out, if the AC system is empty the A/C clutch will not come on or only will come on briefly.  The AC system should have a pressure switch that would prevent the system from running if the system is low.  

Chasing R12 can be frustrating.  If it were an R134 system, I would suggest topping it up and going from there because it's cheap.  You could top it up and see how long it lasts.  If it lasts a day, you obviously have a major leak and it should be repaired.  If it lasts two years, just keep topping it up.

With the cost, environmental impact, and unavailability of R12, that can get expensive fast and also clubs baby seals.

The right way to do it is to check to see if there is any in there.  If there is, have a shop reclaim it, do a vacuum test, repair any leaks, then refill with an R12 replacement.

The other option would be to retrofit an R134 system in there.  Since the NA Miata went through the late 90s (and the switch to R134 happened in 93-94), my hope would be that you could order stock replacement pieces from (for instance) a 96 Miata and install it.  Some poeple have just stuffed R134 into an R12 system and it will cool a little, but one of the things that makes R134 better at not harpooning whales is it's lower volatility.  It needs a higher pressure differential to effectively evaporate and do it's cooling magic, so the whole system is designed differently.

Fortunately, A/C kits are surprisingly not expensive.  My guess is under $200 for a kit that has compressor, oil, dryer, valve, and o-rings.  Then you can charge it yourself.  The caveat there is that its possible that Mazda changed other things like the evap and condenser which may cause things to not bolt up.  All of the retrofits I've done have been on 80s American stuff and they tend to keep it as simple as possible instead of re-inventing the wheel.

For a small parts cost and a weekend of work, you could upgrade to R134 and then you have an easily and cheaply serviceable system.... likely less cost than a single recharge of R12... if you can find a shop that will do R12 at all.  If you can do it with an A/C kit from a later NA, that would be the ticket

Last time I checked, I'm pretty sure the A/C clutch came on.  If that's still the case, it sounds like that's enough to say that the system isn't empty, just low.  Is that correct?

It's not technically legal, but I just redid the R12 system in my 92 ZR-1 with Enviro-Safe R12/R134a replacement refrigerant and it blows ice cold for less than $50 for the fittings, an oil charge and all the refrigerant needed. I don't plan on ever taking it to a shop for AC work, so I don't worry about future issues there, but it's a consideration for others. 

I will ad my extremely positive experience with Enviro-Safe to EDT's comment above. The only caveat is the flammability. No smoking is permitted in my cars, so that is not a problem for me. If it leaks into the cockpit you can smell it, so you don't need a detector onboard.

Around me there are usually people selling old cans of r12 fairly inexpensively. Going rate seems to be ~$25/can right now. Check Craigslist and fb marketplace, you might score a deal. Not much demand for it anymore so prices are fairly low now. If you can find some cheap grab a couple cans and charge it by the bubble method mentioned above. If it doesn't work you are not out much. 

old_ said:

Around me there are usually people selling old cans of r12 fairly inexpensively. Going rate seems to be ~$25/can right now. Check Craigslist and fb marketplace, you might score a deal. Not much demand for it anymore so prices are fairly low now. If you can find some cheap grab a couple cans and charge it by the bubble method mentioned above. If it doesn't work you are not out much. 

So, if I find the clutch is working, I'm thinking this is the easy button.  Can someone walk me through the process of charging it by the bubble method?  Is it just connect a can and look at the drier (I haven't yet confirmed there's a sight glass on it - where is it on a 1991 Miata)?  I've seen run at 1500 rpm - is that correct?  Some seem to say can upright or can upside down - sounds like gaseous vs. liquid but what does that really mean?

Thanks guys for all the help.  I hate not knowing how to do something.  For this topic, it's kinda hard to wade through all the anecdotal/opinion stuff to get to real information.  

Check to see if your system has any pressure at all.  If it doesn't, you have a leak, and there's little value in refilling the system until the leak has been sealed.

If you need to correct leaks, you might as well invest in the few additional dollars to convert to R134A, but even if you don't you're going to need a high-vacuum pump, and a new dryer.  The pump can sometimes be "borrowed" from an auto parts store.  You'll need to pull a high-vacuum (or more accurately very, very, low pressure) for a specified length of time to remove all moisture from the system, and then your system will need to be capable of maintaining that vacuum without further pumping for an additional specified length of time.

Next, the proper lubricant and coolant will need to be pumped in without introducing outside air, preferably by weight, so you'll need an accurate scale and preferably one that measures in grams.  I use my wife's kitchen scale.

All of this can be done by anyone who's willing to put in the time and effort to plan and execute the work. It's more complicated and sensitive than changing your oil, and perhaps even more so than changing a Miata top, but less difficult than swapping a 1.6 for a 1.8 engine.  

The easy way is to find a good auto shop that specializes in classic cars from the 50's through 70's.  If they are a good shop they'll have a lot of experience working on and converting R12 a/c systems, which is becoming more and more of a lost skill.  They'll also have the right tools.  Anyway, a good shop can test your system, correct your leaks, purge all moisture, install the correct quantity and type of coolant and compressor lubricant, and do so reasonably affordability.  With everything sorted, your system should cool well for many years.

If the clutch activates (turning the pump) then you definitely have pressure.  The clutch won't activate under a certain pressure.  Make sure it's actually turning the pump though since the pulley will turn without the clutch activated.

The truly easiest way would be to top it of with a blend (while noting the warning above about now shop may not work on it after that).  Without a gauge, or knowing how much is in it already, you would just need to slowly add refrigerant (while running ar 1500 rpm plus) and check the outlet temps.  Add until it's as cold as you dare.  A slight over charge will just result in it blowing warmer.

Unless you are really short on money, or don't mind buying some tools, it might be easier to just have a shop top it off and add some stop leak.  You might want to get a quote on that because I have no idea what a shop will charge you for a can of R12, they will likely suggest a full R134 conversation.

Did a little more investigating.  Clutch isn't engaging, but engine speed changes when turned on, so at least the switch itself must be working.  There is at least a bit of pressure release when pushing on the larger Schrader valve, but it certainly isn't high.  Some pressure indicates there isn't a leak, so I'm going to try to add some R12 or the Enviro-safe recommended above.  It'll take a bit to get something delivered to try.