CasualSpeed

How good is the air conditioning supposed to be in a 1991 Miata? I had a system installed in my car because I'm in GA and deal with 95+ degree heat regularly. After the install, I was expecting to be comfortable in my own car with the hardtop on. Well, I'm not. I can tell the A/C is on, but my back is still sweating and I'm not at a comfortable temp even at full blast with recirculating on. Is this everyone's experience? What can I do so that I can be comfortable in my car in the summer? Do I just go back to not using it and take the top off? This doesn't work in traffic or city driving, nor does it leave me in a presentable state if I'm going somewhere.

gooflophaze

Also in georgia. 91 should have R12. My 93 used an r12 alternative and it was fantastic until it leaked out. If you did a r134 conversion without changing the condenser core, performance will suffer greatly. Also, if you're intercooled / have something blocking the condenser / aren't running an undertray / are missing any air ducting, performance will suffer.

Gee Emm

Air con is good, when working properly. Can't speak for the refrigerants, no experience with them, but the condenser needs to see lots of air, and if you have an FMIC good ducting is essential.

If you have no FMIC I'd suggest you get an aircon shop to check the system is functioning as it is supposed to. If you do have an FMIC, ducting, ducting, ducting ... every last molecule of air coming in the nose can only get out via the radiator core. Don't just look around the radiator, there are plenty of other places air can escape without doing cooling duty. Plenty of info on here, search is your friend.

nickalltogether

My 90 model was converted to r134 before I got it. I do have an intercooler, but even non-intercooled I wasn't terribly overwhelmed by the performance. In legit southern heat it needs to be on 3 or 4 on the blower speed, and on recirculation. I haven't done it yet, but I feel like some window tint would make a big difference for me.

For comparison, my stock 94 model is significantly better.

CasualSpeed

It's R134. I installed it a little while ago so it was never R12. It sounds like I made a huge mistake when I bought a system off of another '91 and should have bought a system off of an NA8 which would have solved this problem. ****, but at least I can fix this.

I don't have a FMIC on my car yet, but I will soon. I've still got the stock undertray.

DNMakinson

iTrader: (3)

Another help when idling: With MS3 I run my idle up by, IIRC, 300 RPM with A/C on. So, instead of 800RPM, it is running 1100.

CasualSpeed

Ah yeah, I think I fixed my A/C idle up. My big problem with tuning that was when it would "idle down" it would sometimes cause my idle to shut off if the conditions were right (well, "wrong"), but I got past that with some testing. I idle at 850, and idle up to 1k.

DNMakinson

iTrader: (3)

I was only referring to increased cooling capacity in my above comment. Get that compressor to spinning.

CasualSpeed

Got it, but these past few times that it has been very frustrating to me has been on long car trips. I actually cannot run my A/C at idle because I've still got the stock radiator from my originally A/C-less car and I'll overheat if I'm running the A/C in stop and go traffic in hot weather. It's not pleasant. I do have a crossflow radiator that I need to go ahead and install so I can solve this with a little effort.

gooflophaze

So you're missing the A/C fan on the radiator?

technicalninja

Without a condenser fan you will overheat the engine in 3-5 minutes.
You will overheat the AC system in 15 seconds.
High side pressures go from less than 150 (moving above 30 mph) to 450+ in seconds.
This is HARD on compressors, condensers, and high side lines, especially if they are old.
R12 systems commonly did not have high side pressure cutouts (a pressure switch that breaks the clutch signal somewhere above 400 psi).
If you change to R134A you are supposed to add a high pressure cut out.
The final safety on the system is either a blow out plug or a high pressure relief valve which vents refrigerant when pressures exceed 500 psi.
I've never seen a relief valve that vented actually seal again and a blow out plug vents it all.
Both of these parts are no longer available separately now days, you get to buy a compressor

BAD, BAD idea to run AC systems below 30 mph with no fan action...
Above 30 you have so much CFM across the condenser than fans don't matter.

A properly converted R12 system will run sub 40 degree vent temps on low blower in 105 degree Texas heat.
Sometimes it will require a modern condenser.
This is what I do for a living...

DNMakinson

iTrader: (3)

@technicalninja While you're here, what should the minimum time before re-engagement be set to? So the pressure can equalize before powering the clutch again. Stock '99 system.

And thanks for coming in. I should have pinged you earlier on this thread.

CasualSpeed

I am NOT missing a condensor fan. I pulled the complete set of parts necessary from the old Mazda manual that goes over installing A/C into an A/C-less car and made sure to get them all, and had them installed by a shop. It's just my radiator is the thin one found on manual transmission A/C-less cars. It sounds like the problem I'm having is related to using R134 refrigerant in an R12 system.

Honestly, at this point it sounds like I wasted my money and should buy https://treasurecoastmiata.com/i-188...te-ac-kit.html (when in stock) and just re-do the entire thing.

huesmann

iTrader: (1)

So CasualSpeed, can you clarify—did you install an NA6 A/C system in a car that didn't come with A/C?

CasualSpeed

Yes. I have attached the PDF I was referencing when assembling the parts to this reply. Most of it came from a donor car (someone who had ripped our their own A/C system), and some parts I had to hunt and find like the power steering idle bracket (my car has a manual rack). Also of note is my car is vvt engine swapped, so I ended up with an NB a/c compressor. I am not knowledgeable about A/C units or anything, and it seems like neither was my mechanic.

deezums

iTrader: (3)

Before swapping the whole kit I'd get a set of manifold gauges for less than $100 and try and verify it's got a proper charge. You have to recalculate when swapping to R134, and swapping components like the compressor isn't gonna make it one of those "nice" conversion calculations like R12*.75+.25lb=r134a or whatever that comes in the conversion kits. I think most NB compressors are the same as the NA, but I think the last years might have an even smaller volume compressor.

Like this one.
https://www.redlineautoparts.com/maz...k-miles-nb054/

I don't know anywhere near as much as Tech up there, but I do know that charging off system pressures is not an exact science. Either way, gauges would show what's going on and probably the first step before swapping pieces.

technicalninja

Sadly most high pressure cut outs re-engage at 325-350 psi. This is TOO soon for me.
I'd like to see re-engagement at 250.
If you have a black Suburban or Expedition you can cycle between cut out and cut in for 5+ minutes if it's been sitting in the sun all day and the mechanical fan clutch is weak.
Dropping the windows, keeping the recirc door on fresh and rolling down the road for 30 seconds BEFORE engaging the compressor will give you decent cooling within 3 minutes and seriously reduce the load on the system.

Deezums is correct...
Without proper diagnosis of what is going on just swapping stuff around is fruitless.

The single most common thing I see from DIY work is air in the system.
2% air will screw up the gauge readings, 5% air will make the system work poorly.

Over oiling is another problem I see a lot of. This normally won't hurt a system but the excessive oil coats the inside of the condenser and evaporator and can seriously reduce thermal efficacy.
The oil to refrigerant ratio I use is 1 to 4. A 1 lb system will use 4 oz of oil, 2 lb =8 and so on.

Not verifying recirc is another. Recirc is always worth at least 7 degrees and sometimes as much as 20.

So, if you have a bit of air, too much oil, and a recirc door that does not close and seal you end up with crappy 65 degree air.


I've been doing AC work for 38 years and do not "set" a refrigerant level with gauges alone. The gauges are useful for diagnosing problems, not setting levels.

Only accurate way is by weight. Estimating how much is left in that second can you had pierce is FAR more accurate that using the gauges.

I'll give you another tip, when things are working properly the low side line is freaking cold all the way to the compressor, like grabbing an ice cube. (late model stuff with the secondary heat exchanger in the suction line is an exception but that line will be freaking cold directly before the exchanger)
When you are a little low the compressor end of this line will be noticeably warmer.

You need a bunch of little things happening properly to make a system work. It's not rocket science but you cannot make any mistakes and expect efficacy or longevity.
Any of you guys that can make a 250 hp turbo Miata work can easily do you own AC work. With just a little bit of education you will be better than 75% of the "technicians" that do this work.

The ratio for R12 to R134 weight conversion I use is 75-80%. A 2 lb R12 system becomes a 1.5-1.6 lb R134 system

CasualSpeed

What kind of shop should I go see then? Who in my area (Atlanta GA) would be knowledgeable enough about both Miatas and A/C systems to help me out with this case? I am not confident messing around with refrigerants myself and don't want to risk making things worse by fiddling on my own.

technicalninja

Auto Cool in Atlanta sounds a lot like me. The negative reviews tell a pretty good story on what happens when things go wrong there. I have the same issues with customer here.
I also do not make "appointments" as 75% of appointments do not show up.
I also do not like long term project cars which clutter up the shop.
I also do not like MOST 20+ year old cars as they are either basket cases that need tons of stuff or parts are no longer available (this is a HUGE problem now days)
Your early Miata is very hard to get parts for. I re-did a 90 and it needed an evaporator, Nothing was available new but my customer did the leg work to find a good core.
When you are working on an old limited part availability vehicle the customer's attitude is massively important.
This looks like a shop to do the evac and recharge and you yourself do most of the parts changing and sourcing.
I also do not like "customer supplied" parts. Commonly it's cheap *** Autozone junk, Chinese compressors, pure crap and those customers EXPECT you to make their trash parts work and are pissed when they don't.

From the end of May to now I've worked 7 days a week 12+ hours a day. I took Memorial day off.
It's the only day I've had off since May...
Old "project" cars should be done in the off season. This winter I already have multiple "Classics" scheduled including a 63 Corvette and a 20K miles DeLorean. The DeLorean has almost zero parts availability, thank God it's super nice already..

I expect AutoCool runs the same way. Have your ducks in a row, your Is dotted and your Ts crossed BEFORE you take your car to them during the busy season and they most likely will work with you.
Take basket cases to them AFTER Halloween when they are slow and starving...

I've never spoken with them, I just did a quick Google search and found someone very similar to me. This is NOT a recommendation for their services.
Always trust your intuition; 99% of the time it's right. If you "feel" they're underhanded run like hell

technicalninja

Sorry, forgot link to their website

Who We Are (autocoolatlanta.com)