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Old 05-31-2008, 11:34 PM
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Typical AC system capacity?

I was helping my buddy today with his AC in his truck. We vacuumed the system to -30 then added one can (12oz) of R134.
The problem is..One 12oz can got the low side up to 55psi and the high side up to around 200psi... One 12oz can? This is about what we were shooting for but we both thought it would take 3 or 4 cans of the stuff to get there.
Could this be right? If not why would this happen? The AC is blowing cool but not cold. We are both learning about AC systems so any help is very appreciated.
Thanks.

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Old 06-01-2008, 07:45 AM
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You can find the capacities of refrigerant and oil here: AC Capacities (PDF file, Adobe Acrobat reader required). There should also be a AC spec sticker inside the engine compartment that lists the capacity too.

Low side of 30 is ususally about right, and with R134a it's common to see high side pressure between 2.2 and 2.5 times ambient temperature. On a 80 degree day you would see between 176 and 200 PSI on the high side of an R134a system. The system operates in a specific range based on outside ambient temperature. High side pressure has a broad range relative to temperature because of heat load on the evaporator, humidity, airflow across the condenser, and engine speed. Fill with the proper amount of R134a and things should settle out.

Last edited by MAUSS; 06-01-2008 at 07:58 AM.
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Old 06-01-2008, 10:48 AM
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Hey thanks for the link Mauss!
They don't' have this exact truck. Its a rare Isuzu 4x4 diesel pup. That gives me a ball park idea though. I looked everywhere for that kind of info.
It looks like it should take 20 to 30ozs of refregerant and 5 to 6oz of oil.
Does anyone have any idea why it would only take less than half that to fill up? Could the system be full of oil? None came out during the vacuum process..
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Old 06-01-2008, 11:07 AM
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What year?
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Old 06-01-2008, 11:09 AM
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I think its an 83. Its been upgraded to R134a at some point.
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Old 06-01-2008, 11:19 AM
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I've found that a small amount of refrigerant , such as a single 12 oz can will expand to fill the space in the system, the pressures will be slightly off from spec, but it won't cool. You can keep pushing refrigerant in until you reach the specified amount, and the pressures will only change slightly, but the air from the ac duct will be much colder. Don't ask me to explain it, but I know they'll act that way.



Don't add any major amount of oil unless you drained that amount of oil out from somewhere. The oil will be dispersed throughout the system, with an ounce here and there in the lines and evaporator, etc.

You may want to change the dryer if the system has been opened , water in the system will mess up your pressures also.


Later, mikey
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Old 06-01-2008, 11:20 AM
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It sounds like a restriction in the condenser, or air flow through it. Take a garden hose and spray the condenser. If the pressures drop it has an air flow problem, possibly the fan clutch.
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Old 06-01-2008, 11:27 AM
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Well one 12oz can pushed it from -30 to 55psi. I think you want 30 to 45psi in the low side right?
Are saying keep going and the psi wont change as much as I am thinking?

Don't let me blow myself up Mikey.. I don't like blowing up at all!
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Old 06-01-2008, 11:27 AM
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Quote:
Originally Posted by carsavvycook
It sounds like a restriction in the condenser, or air flow through it. Take a garden hose and spray the condenser. If the pressures drop it has an air flow problem, possibly the fan clutch.

I've found that all systems all act like that if you put the hose on the condensor, even the ones that work properly.

Usually that's how I get the full amount of refrigerant in a system, and the little cans fully drained. By hosing down the condensor, the low side will almost always show less than 10 psi, (sometimes even show a vacuum), and it will pull all of the refrigerant from the can. I don't have one of those fancy charging pumps, and I really don't want a bunch of cans with 30 psi worth of refrigerant in them hanging around.

So hosing down the condensor and seeing a pressure drop is not always indicitive of a problem.


Later, mikey
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Old 06-01-2008, 11:33 AM
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I have found a lot of fan clutches and slow spinning worn out electric fans to be the problem causing insufficient cooling and too high of pressures. This being the only problem, without having to add any freon when doing an A/C service. You cannot discount this test.
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Old 06-01-2008, 11:37 AM
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Quote:
Originally Posted by weirdbeard
Well one 12oz can pushed it from -30 to 55psi. I think you want 30 to 45psi in the low side right?
Are saying keep going and the psi wont change as much as I am thinking?

Don't let me blow myself up Mikey.. I don't like blowing up at all!

Just don't hook the can to the high side port. The pressures will start to rise disproportionally if the system is overcharged, with really high, high side pressures. Like Mauss said, for 134a the high side should be no more than 2 x the ambient temp + 20%-50%. Put close to the specified amount of refrigerant in, if you can get your duct temps down to 40- 45 degrees you are doing good.


Later, mikey
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Old 06-01-2008, 11:44 AM
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Quote:
Originally Posted by carsavvycook
I have found a lot of fan clutches and slow spinning worn out electric fans to be the problem causing insufficient cooling and too high of pressures. This being the only problem, without having to add any freon when doing an A/C service. You cannot discount this test.

I'm not discounting that test, but just saying that running a hose over the condensor and seeing "a" pressure drop is not the sole indicator of a problem.

How much of a pressure drop? 10psi? 50psi? 150 psi?

All systems , even perfectly running ones, will show some small pressure drop if you run a hose over the condensor. It's no secret that water will carry heat away from a condensor much faster than air.

Also, he is trying to charge a system after evacuation with only a small amount of refrigerant in the system...I know the pressures will be close to normal, but minimal or no cooling will take place at the evaporator until close to the specified amount of refrigerant is added.

Put the proper amount of refrigerant in, then see if there are other problems.


Later, mikey
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Old 06-01-2008, 11:44 AM
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Ok. I have some good info to work with now.
Thanks fellas!
I will check the fan clutch. It very well may be worn slap out too.
It sure is hot around here!
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Old 06-01-2008, 11:52 AM
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If the fan clutch is worn out, spraying water on the condenser should drop the low side by 40-45 psi with only 1 can of freon added. The high side should drop to about 100 psi.
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Old 06-01-2008, 12:02 PM
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Quote:
Originally Posted by powerrodsmike
I've found that all systems all act like that if you put the hose on the condensor, even the ones that work properly.

Usually that's how I get the full amount of refrigerant in a system, and the little cans fully drained. By hosing down the condensor, the low side will almost always show less than 10 psi, (sometimes even show a vacuum), and it will pull all of the refrigerant from the can. I don't have one of those fancy charging pumps, and I really don't want a bunch of cans with 30 psi worth of refrigerant in them hanging around.

So hosing down the condensor and seeing a pressure drop is not always indicitive of a problem.


Later, mikey
The spraying of water on a fully charged, good working system, will drop the pressures about 10 degrees on the low side, and 15-20 on the high side. This is a good indicator to what your pressures will be at highway speeds during normal driving.
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