FBPersimmon

Hey all,

I was planning to run a pressure test on the AC system, which was converted to R134a by the PO. I haven't found any R12 specs and I know R134a runs at a higher pressure, so anyone happen to know what sort of Hi/Lo side pressures I should see for static and running for R134a conversion for a 13B? My AC works, but it only "cools" the air, so I'm thinking it may need a little top up, but haven't found any specs for the OEM system, let alone R134a conversions. I tried a search but didn't see any threads on this, but let me know if I missed it.

Thanks!

LongDuck

I know you're asking for actual pressures, but speaking from practical experience, I converted the fill nozzle on my 84SE to 134a and top off every summer with one of the auto store gauge sets that show green / blue / yellow / red ranges and it works just fine once I'm in the blue range. You can roll down the drivers side window and put your hand at the vent located there (*close the lever which draws air from outside at the cowl), and feel the temperature drop as you charge it. I typically also rev the engine a bit to gump up compressor speed, as this helps to draw the refrigerant from the container. When the container gets ice cold, its empty.

This method works every summer, as the old seals just don't hold the system pressure 100% from year to year. I also live in the Sonoran Desert in Arizona - so I know a thing or two about A/C efficiency. The stock system cools adequately well here in temps up to 115-118F, but I usually choose another vehicle if I have a choice on super hot days (~120+). Yes, it does get that hot here,

FBPersimmon

Thanks! Looks like the range is around 35-45 psi (70-80F ambient) based on some photos I found of those handheld chargers, so I'll try to get it to around 40. I borrowed one of those high and low pressure gauge sets from a friend, which doesn't have any indicators on it in terms of full/low/high. Great to know it works for you even in those temps. I live in Sacramento, so while it doesn't get nearly as hot, we get some toasty summers here as well.

Toruki

This is a really good thread on the subject. Although this talks a lot about R152 instead of 134 the ideas should be the same, specifically about the pressures and behavior of the system as you charge it.

https://www.rx7club.com/1st-generati...enser-1137402/

yeti

iTrader: (1)

The charge pressures are also dependent on the ambient air temperature, or that's how I understand the chart.

t_g_farrell

iTrader: (1)

I fill by temp, watch the temp of the air exiting the vents when they bottom out, let a bit out and bobs your uncle!

peejay

You don't check air conditioning systems by pressure, you check them by weight. Pressure in a running system is highly dependent on ambient air temperature, humidity, airspeed across the condenser, airspeed across the evaporator, engine speed, etc.

That said, the pressures are actually really similar to R-12. I did an R-134a conversion on my '80 in 2000, when R-12 was at its most expensive (like $120/pound, in '00 dollars) and IIRC low side was around 30-35 like normal and high side was around 150-200psi. That is a general rule of thumb, high side should be at least 5 times what the low side is, but there is a lot of leeway in that, it is not a hard and fast rule! The high side is going to be screaming high on a 95 degree day at 90% humidity.

One thing to bear in mind, A/C systems generally are only concerned with the low side pressure. It cannot get so low that moisture freezes on the evaporator. The high side goes to whatever it needs to go to in order for the low side to get low. This is where system pressures are important, BUT. Not sure about GSL-SE, but the A/C system in my '80 did not have a low side pressure switch to turn the compressor off, it had a evaporator temperature switch. Meaning, it was controlling the compressor clutch by actually measuring the temperature of the evaporator. This made a conversion that much closer to R-12 performance because you didn't have to change to an R-134a specific low-side switch. (Some cars, like Fords, were adjustable)

System capacity is determined by having as much refrigerant as possible in the system while still getting 100% of it to condense to a liquid in the condenser and evaporate to a gas in the evaporator. The pressures will be remarkably similar between a system that is correctly charged, and a system that is down 50%. The pressure is only related to the temperature of the refrigerant, it has nothing to do with the volume.


Reference: I did a LOT of R-134a conversions. Where I was working, we did it as a matter of course on any R-12 car that came in for A/C service.

Toruki

When I first converted from R12 to R134a in my 83 with the Sanden compressor, I calculated the correct weight of the R134 to be 1.8 lbs. The oil capacity best I could figure is 5.0 oz.

FBPersimmon

Thanks for all the replies! The easiest method for me will just be via low side pressure. Will report back what the results are. Guess it's also an excuse to by a thermometer for the garage cause I'm curious how much the actual temps will change

peejay

Low side pressure will always be about 30-35psi until you have the system way overcharged.

Filling by pressure is very much an acquired skill. On a cycling system, you can add until the compressor is cycling "just right".
If there is a sight glass, and I'm pretty sure there is, you can watch it for bubbles or lack thereof.

FBPersimmon

Got a can of pure R134a in there and it seems much, much happier. Low side is around 35-40psi, high side was around 180-200psi at 75F ambient. Air at vents went from around 60F to mid-40s. Thanks for all the help and info!

peejay

My '81 with the factory installed A/C is very happy with 1.2lb of refrigerant. Am getting 38-45F from the vents as the compressor cycles going down the road.

At idle, incidentally, the compressor does not cycle.

Toruki

Hey peejay, what refrigerant are you using?

peejay

R-134a.

There's no underhood A/C sticker, and there is no information for '81 in Alldata, but I did find that the GSL-SE specified 1.5-1.7lb of R12 and 7.8 ounces of oil, and I knew that GSL-SE all had factory installed air, so I went with that.

When I used to do conversions, we always just added the ester oil and didn't bother trying to flush out the old oil. We figured that any leftover mineral oil would find its way to the receiver/dryer and stay there.

I DO want to replace it because technically you're supposed to, the dessicant isn't compatible with R-134a and everything made since 1993 or so has the correct dessicant. But I wanted to make sure the A/C functioned before I threw parts at it. I tell myself that running the A/C for a while like this will transport any leftover mineral oil up there before I change it

Although when I converted my '80, I don't think I even added oil, just evacuated, stuck the conversion fittings on, and went with an 80% charge. Memory is kinda fuzzy. That was longer ago than I was alive back then.

Toruki

It's amazing how quickly that happens. I like to tell people my car is older than they are.

peejay

It's weird to think that I was 20 when I got my first RX-7, and I have had at least one in my life since then, which was like 23 years ago.

My car was built 10/80. SWMBO was built in 1985. This is a point I bring up sometimes, too

(I have an '81 RX-7, an '83 GTI, an '84 RX-7, and an '86 VW Quantum. All of which were built very early in their production life. Also? No Life till Leather, Kill 'Em All, Ride the Lightning, Master of Puppets.)