r134a A/C Charging Information

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Staff member
Dec 14, 2002
New Jersey

I will not pretend to be an HVAC or Auto A/C tech or technical authority. That said, I have sufficient capability to "top off" a system in good condition, using real, US-made high/low gauges and a 30# honeywell bottle of r134a.

Problem is, all the good information that I had online I cannot find. I want to be definite with high/low pressures... Perhaps someone can provide their information/insight so that this can be a good reference thread.

I was going to top up two cars, one a 91 BMW retrofitted to R134a about 5 years ago, and blowing cold but cycling a bit much, and one, my 2004 saab 9-3 which seemed to be louder than normal when the compressor was cycling on, but always blowing nice, cold air.

So, the BMW... hasnt been used for over a day, cold soaked. Ambient temperature, 78F. I cannot find a good static pressure chart, so I pulled out my chemical engineering thermodynamics book, and looked at the chart. At 78F, it looks like my gauges should read roughly 83 psi to indicate that I have a dual-phase condition in there, and are not too terribly low... Well, I read about 75-78 psi.

So that would indicate a slight undercharge, and explain why the compressor seems to be cycling a bit more than normal, right?

On the saab, I didnt bother to read a static pressure because the vehicle was used quite a bit today, with the AC on, and then vehicle was heat-soaked.

The real issue is high/low pressures. Ive seen general information online that indicates a low pressure of 25-45 is correct, at 1500 RPM. Ive seen high pressure info that ranges from 2.2xambient temperature through "don't worry about it, just watch the low side".

On my BMW, it started at 25 psi when running the AC at idle on full fan, and would drop lower when the RPMs were increased. I filled it until I saw 30 psi on the low side at 1500 RPM, and the high side started creeping past 200 psi (which is roughly 2.5x ambient).

On my saab, it was down at about 30 when running at idle, and dropped a bit when at 1500 RPM. I added some so that I was solidly at 30 psi when at 1500 RPM. THe AC in the car blew cold before (it did in both, in fact), but it got colder when I did this... Additionally, what sounded like a roughness in the sound from the engine area when the compressor was engaged, seemed to go away! Thing with this car was, the high side never really fluctuated or varied from 175psi. I was expecting to see it creep, at least when we increased the engine speed and added some refrigerant.

So, long story short, an good information online? Any good sources of charts, data, etc? And, what is the right range to see the high and low pressures, both when at idle and when at 1200-1500 RPM?

If i understand you correctly, you took a low side reading with the engine off on the bmw? If that is the case, that number would not be helpful as to knowing the amount of R134a in the system.

I am going to read a bit on the web site Drew mentioned, but with my own cars i tend to reclaim the refrigerant and weigh in a new charge, then i know its accurate.

In the past when i would just top off the charge i would have a thermometer center vent on the car or truck, have the controls set to max a/c and watch the temperature drop till i was in the high 40's or low 50's depending on the temperature of the day i was working on the system, of course this is a subjective way of handling it.

And of course its easier for me, i already own the scale and the reclaiming tools for my day job.
The static reading at ambient temperature just gives the indication of if the vapor in the system is saturated, or if it is below saturation. If it is saturated, then excess will be in the system as a liquid - which you want. The charge, as I understand it, exists under ambient conditions as some liquid in equilibrium with its vapor.

If below the saturation pressure, it indicates that there is a charge, and maybe a usable one pressure-wise (nothing will trip), but not enough to be operating with full capacity.
To remove ANY doubt at all,, evac the system and pull vac then charge back the FACTORY amount/type,,,done....

i sugest you or anyone interested in AC work read/post here,i'm a regular member there too,they know there chit.
JHZ- The refrigerant in your a/c system, like you said, will be in a dual-phase condition (unless it's nearly empty, in which case it wouldn't work at all). So the pressure in the system (not running, assuming no contamination) will be solely dependent on temperature; the pressure will be the same regardless of the amount of refrigerant in the system, so long as there's enough refrigerant that some of it is liquid (and the rest gas). In other words, as long as there's enough charge in the system for the a/c system to function, pressure while shut off doesn't really tell you anything.

The a/c forum that Drew suggested is a good source of info- lots of experienced people there.

I agree with others here in that the ONLY reliable method of knowing exactly how much refrigerant is in the system is to recover and weigh it: A simple operation in the shop, but not feasable for the doityerselfer.

If you're familiar enough with a particular system and/or have good (and SPECIFIC) service info., you CAN top off a system- but it's an inexact science at best. You can get an idea of the state of charge of an a/c system based on the high and low pressures while running (it's best to be able to watch both)... but this is only a guess. Lots of factors like airflow, ambient temp, cab temp, sun load, dirty condensor/evaporator fins, refrigerant contamination, and half a dozen others will affect pressures.

IMO, when working on your own a/c system without a complete recovery/evac/manifold gauge/scale setup, you should avoid 'topping off', ESPECIALLY if the a/c is blowing cold. I'd suggest that you leave well enough alone in this case, as whatever slight benefit you MIGHT gain from topping off is far outweighed by the risk of an overcharge (which can easily cause a ruptured hose or a compressor failure).

But if you do want to try topping off, then you really need specific info from a service manual. I've seen high-side temperatures vary by easily 100 psi between R134A systems in different applications- and while there is a theoreticaly ideal pressure, low side pressures also vary quite a bit depending on application. Even a factory service manual will give a pretty wide 'acceptable' range.
Checking the static pressure is a good extra check.
And you are right about 80 - it is normal at 75ish ambient temps.
Running pressures vary because of heat, humidity, and air flow past the condenser.
Compressors cycle, and the low side pressure should be checked when it it ENGAGED, not a rest cycle.
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