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How to check if system is Charged Correctly when converting to R134

sgeer on Thu September 12, 2013 4:26 PM User is offline

Year: 1988
Make: Dodge
Model: B250 Van
Engine Size: 318
Refrigerant Type: R134A
Ambient Temp: 67
Pressure Low: 27psi
Pressure High: 163psi
Country of Origin: United States

I recently converted a R-12 system to R134a. The van which I own was built in 1988 so it originally had R-12.

I have learned a lot going through this experience and I wanted to share it with others as I feel it may help them as they consider converting an old R-12 system to R-134A.

If I have a misconception about what I have learned, I welcome people to correct me as I am learning and felt this would be a good place to get feedback as well. Hence my purpose of posting this is not only to help others but to also find out where I may have some misconceptions about how this should be done.

First a little background on me. I am a DIY Mechanic and I have been working on cars for almost my entire life, over 30 years. I have built a few engines from scratch and worked on all makes an models of cars.

In the old days, you could by R-12 over the counter, 14 ounces for 89 cents. That is when I first began maintaining AC systems in my cars.

Recently I converted a R-12 system to R-134a and I learned a lot in the process. Whenever approaching these projects I prefer to have the correct tools, and will spend the necessary funds to do it right rather then try and cut corners because of budget constraints. I am extremely anal about everything being as perfect as possible. Working on a AC system is not an exact science as pressures vary based on many factors which I will go into detail later.

First of all, converting from R-12 to a less efficient R-134 system means that your AC system is going to run higher pressures than it did before. That means that the R-12 system will not be a perfect match for 134 as it was designed to work with a different refrigerant.

But that is not necessarily a bad thing, there are things you can do to improve the efficiency of the R-12 system so that it works better with R-134.

For example, a more efficient condenser can make a big difference. Many of the R-12 systems used large tubes that ran through a grill material. This single tube winds back and forth until until the refrigerant exits on the other side of the condenser. The purpose of the condenser is to remove heat from the refrigerant. This is the part that goes near your radiator in the front of the car. Because R-12 ran at lower pressures it was more efficient at transferring heat from the Evaporator (part inside the car that gets cold) to the outside air with a condenser that had large tubes would work efficiently enough. However, with all modern AC systems, since about 1996 being converted over to R134A, equipment changed a little bit to compensate for the less efficient R134.

First and foremost, they not only changed the design and type of condensers being used, but they also often put pusher or puller fans on their condensers to assist with heat exchange, whereas R-12 systems usually did not have this extra fan. Also, I purchased a Condenser for my Van when I converted it over, which is of the serpentine type, which works better when using R-134 at removing heat from the system. The serpentine condensers are more efficient because as the refrigerant goes through them the path begins large and is small at the end.

A serpentine condenser will start by flowing the refrigerant through 3 small tubes at the top, which eventually turn into 2 and then 1 tube before it exits. This allows the condenser to expose the hottest and highest pressure refrigerant to more area quickly in a larger surface area. The reason for reducing the paths is that eventually, at the end it is assumed that the refrigerant, in a perfect system, will be primarily liquid and no longer in a gas state, so it does not need as large a path to go through.

I believe that this change improved my system by as much as 15 to 20 percent just by changing the condenser alone to this new design.

My system uses an expansion valve. The expansion valve operating pressure is different for R-12 and R-134. So when converting over, you need to make sure you get one made for R-134. How is it different you may ask?? The purpose of the expansion valve is to release the refrigerant at exactly the right pressure so that the evaporator is just above freezing. This is done by controlling the amount of pressure based on the temperature of the evaporator. The low side pressures for R-134 to be just above freezing are lower than R-12 because it is not as efficient and needs to run at a lower pressure to reach the same temperature. Therefore the low pressure cut off switch should be changed as pressures will go slightly lower then R-12 and as a result the expansion valve needs to be changed as well.

If we look at R-12, at approx 30.1psi it is at 32f degrees. However, R-134 needs to be at 27.8psi to be 32f. That means that your compressor has to work harder to bring down the pressure lower to get the same results. But since changing the compressor is difficult, it is easier to look for more efficient condensers if you want to have better results with the conversion.

Since I bought a new compressor, evaporator and hoses, I did not need to flush the system out, but if you keep these items the same you will need to disassemble the entire system and flush everything. Replace the drier and the expansion valve or orifice tube, but the rest of the components can be used again.

The system is working fine. I get about, depending on humidity, 30 to 42 degrees cooler then the outside temperature.

According to the specifications for even R-12 this is withing good operating range. You can never get an exact as there are so many factors.


If anyone has any questions about how I did this conversion please feel free to ask as I want to help others.



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Home Mechanic

sgeer on Thu September 12, 2013 4:53 PM User is offline

Finally, I wanted to ask, as I know there are a lot of knowledgeable people out there.

How can you tell if your system is charged if you have converted to R-134A. Since the old system often has different components, as in this case I had to change the condenser, and I have read that R-134fA does not require as much refrigerant because it is less efficient, how do we know what full is?

I have a theory which I am going to test here:

while it is true that if a system has a static pressure we cannot assume that this means the system is correctly charged because even if we can consistently measure the temperature of the components we could still have air, water, oil, etc that all affect the amount of charge in the system.

Therefore, the first thing that needs to be done is the system has to be completely evacuated and flushed out. When the system is reassembled using the correct amount of oil and evacuated to -28psi for at least one hour and left to sit for at least an hour without pressure change we now know there should be no air or additional materials in the system.

This is where getting the correct charge is possible, in my opinion. First we introduce R-134A to the system, making sure not to introduce even the smallest amount of air into the system. Once the system has enough static pressure we start the vehicle and let the compressor cycle as the low pressure switch turns off the compressor. We continue to add R-134 to the system until this cycling stops watching the manifold gauges until they are at the low end of operating pressure, on the low side. So for example, if the low side is expected to be, at a certain temperature, (check a chart for R-134), 30 to 37psi, then put in just enough freon to get it around 28psi, below the lowest psi. Once this has been accomplished, turn off the vehicle and let it sit until the whole car is ambient temperature.

Now, because the inside of the car, in the sunlight is always hotter then the outside, if we measure static pressure during the day, it will be be an accurate measurement of an equal consistent freon pressure. That is because inside the car, where the evaporator is, the temperature is going to be much hotter than the outside where the condenser is. So if you measure this static pressure, even if the engine and car have been sitting outside for some time, it will not be accurate because freon in your Evap is at a much higher pressure than freon in your condenser. So any static pressure readings would be the combination of both these areas and not a consistent. What we need is a way to measure static pressure which we know is equal throughout the entire system. In this way, as we know R-134 to be at a specific pressure at a specific temperature we will know if the system is fully charged.

But how do we get this consistent temperature, how do we know that the pressure of the freon doesn't vary more than 1 to 2 degrees through the entire system?

Simple, take measurements at night. Part your vehicle in an area where there is no wind and little humidity with the windows open. At 3am in the morning, when there is no sun to heat the inside of the car, I think we can assume that the temperature of all the AC components is consistent. Note: Make sure you have not ran your car for at least 24 hours, as the slightest engine temperature difference will make the compressor warmer.

So assuming the car has been sitting for over a day, the windows are open, and it is at night, we can no take our temperature readings. To get the most accurate readings use an infrared heat gun, the more accurate the better, and preferably with a laser pointer light.

Point the gun at the various components of the AC system and read the temperature. For example, take a reading off of the condenser, the expansion valve or orifice tube, the evaporator, the compressor high and low side lines. Preferably you will not see more than a 4 degree difference in all these components.

Take into consideration, if some components are a bit higher in temperature how much volume of the freon they hold. If it is a small volume, then in many cases I think you can leave them out of the equation, but if it is a big volume, like the condenser or evaporator you might want to see what you can do to get these to a closer temperature if possible to the other components. Sometimes this simply means leaving the hood of the car open in the night air.

I took about 5 or 6 readings from various components. I added them all together and divided this by the number of components to get the average temperature of the system.

I then went to this site:

http://www.csgnetwork.com/r134apresstempconv.html

typed in the temperature and calculated the expected static pressure based on this average temperature reading.

So for example, if the average temperature from my readings was 66f, then my pressure rounded is 65.124psig. I hook up my manifold set and with a can I pressurize the system to that pressure, or slightly more as I will have to compensate for the fact that the cans need to be a little warmer in order for the freon to get into the system.

This is because we know that you may have to slightly heat up a can, which means it will introduce higher pressures to the system. But if we go to the expected pressure, let the system cool and take another reading we should be able to get pretty close to 65.124 with a good gauge set.

Here is my question, if we evacuated the system, and we assume nothing else has been introduced, can we not assume that this means the system is charged correctly?

Anyone have any comments?

This seems to have worked well for me, but I would like to know if there is anyone else that might question this process. Again, I agree that if we know the recommend kilograms or weight that needs to be in the system it is best. However, when you convert a R12 system to 134A, we do not know the exact weight as we are not only changing refrigerant but also some system components, such as a condenser. So I think this is a good way to estimate a full charge.

Anyone have any comments?



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Home Mechanic

TRB on Thu September 12, 2013 5:32 PM User is offlineView users profile

Plain and simple, charge as close to the R12 specification as possible. Too little refrigerant will cause premature compressor failures.

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wptski on Thu September 12, 2013 7:06 PM User is offline

Quote
Originally posted by: TRB
Plain and simple, charge as close to the R12 specification as possible. Too little refrigerant will cause premature compressor failures.
Can you detail the BOLD words?

GM Tech on Thu September 12, 2013 9:24 PM User is offline

Bottom line- does it cool? Do you want to get out of vehicle? Or are you happy inside on a hot day?

bottom bottom line- would your wife be happy in it?



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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Olds442 on Fri September 13, 2013 1:14 AM User is offline

Okay, Sgeer you never answered the # 1 question. Why didn't you simply fix the leak and re-charge with R-12 ? Why buy a bunch of expensive parts that you didn't have to and do all that experimentation ?

mk378 on Fri September 13, 2013 9:29 AM User is offline

The newer design condensers which are basically traditional tube and fin construction, but with some of the tubes connected in parallel, are called "piccolo" or "6mm" units. A serpentine condenser has a single flat tube.

A TXV does not regulate to a specific temperature. They keep the refrigerant flow optimal for proper cooling based on evaporator temperature and pressure. Other controls are necessary to prevent evaporator freeze-up.

Static pressure is meaningless for evaluating charge level. Consider two cylinders of refrigerant, one new and full and another with only one drop of liquid remaining. Both will have the same static pressure.

Conversions should be charged to the same amount (weight) of 134a as was specified for R-12, unless that causes the high side pressure to go too high. In that case you may get better results using less.



Edited: Fri September 13, 2013 at 9:32 AM by mk378

sgeer on Fri September 13, 2013 9:39 AM User is offline

Quote
Originally posted by: Olds442
Okay, Sgeer you never answered the # 1 question. Why didn't you simply fix the leak and re-charge with R-12 ? Why buy a bunch of expensive parts that you didn't have to and do all that experimentation ?

The R12 system was not working due to a leak so I wanted to start fresh with a brand new system and not find out the next component failure I would have to fix, as the Van is over 25 years old..

I think the extra expense is worth this piece of mind, knowing that the refrigerant is going to be tougher on the system too.

Hope that answers your question.


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Home Mechanic

sgeer on Fri September 13, 2013 10:01 AM User is offline

Quote
Originally posted by: mk378
The newer design condensers which are basically traditional tube and fin construction, but with some of the tubes connected in parallel, are called "piccolo" or "6mm" units. A serpentine condenser has a single flat tube.

Thanks for the correction, yes that is the type I am using not serpentine, got them mixed up.


Quote

A TXV does not regulate to a specific temperature. They keep the refrigerant flow optimal for proper cooling based on evaporator temperature and pressure. Other controls are necessary to prevent evaporator freeze-up.

Thanks, that makes more sense.


Quote

Static pressure is meaningless for evaluating charge level. Consider two cylinders of refrigerant, one new and full and another with only one drop of liquid remaining. Both will have the same static pressure.

I respectfully disagree. If we vacuum the cylinders to -28psi and pu refrigerent in them until they both have 40 psi, the only thing that is creating this 40 psi is the refrigerant because the system was vacuumed, so we assume there is no air or water in the system, just the refrigerent was introduced. Therefore, if we know the refrigerent has a spific pressure at a specific temperature wouldn't it stand to reason that the same amount of refrigerent is in both cylinders?

In your example you would be CORRECT if we did not evacuate the system, as then you are correct, we could have pressurized air and a drop of refrigerant, the key is in evacuating it. This is why ai agree, there is no way to tell how charged a system is unless you empty out all oils and flush and evac, which is what ai did. It is also critical, that all areas of the system have exactly the same temperature, otherwise densities will be different.

But if we assume the densisty of the refrigerant is the same if nothing else exists in the system, and temperatures are constant, then the system as a container should have the right amount of refrigerant based on static pressure?




Quote

Conversions should be charged to the same amount (weight) of 134a as was specified for R-12, unless that causes the high side pressure to go too high. In that case you may get better results using less.

What if the condensor has been changed? The new condensor has much less vloume because of its design then the one ai replaced. Otherwise I agree, same weight, slightly less because of R134



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Home Mechanic

wptski on Fri September 13, 2013 10:19 AM User is offline

Even if your talking about a system, connect gauges and see 20psi on both sides, doesn't that tell you that your low on charge?

sgeer on Sat September 14, 2013 4:59 AM User is offline

I believe that you cannot know for sure based on pressure unless you evacuate the system.

That being said, in order to have 20psi pressure the amnient temperatue would be approx 22f

At 22f temperature this would be a ormal reading. However if it is 80 degrees f outside, which should be about 90psi, I would assume under these conditions that you are probanly low, which means you have a leak somewhere as closed systems should not loose refrigerant. Sometimes it is easier under these circumstances to add refrigerant.. However, it will probably leak out again until you fix the leak.

Hope that helps.





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Home Mechanic

wptski on Sat September 14, 2013 7:49 AM User is offline

Quote
Originally posted by: sgeer
I believe that you cannot know for sure based on pressure unless you evacuate the system.

That being said, in order to have 20psi pressure the amnient temperatue would be approx 22f

At 22f temperature this would be a ormal reading. However if it is 80 degrees f outside, which should be about 90psi, I would assume under these conditions that you are probanly low, which means you have a leak somewhere as closed systems should not loose refrigerant. Sometimes it is easier under these circumstances to add refrigerant.. However, it will probably leak out again until you fix the leak.

Hope that helps.
That's a static pressure of 20psi on both sides which probably won't keep the low pressure switch closed.

94RX-7 on Sat September 14, 2013 11:49 AM User is offline

Quote
Originally posted by: wptski
Even if your talking about a system, connect gauges and see 20psi on both sides, doesn't that tell you that your low on charge?

Depends on the weather.

It could be telling you that your charge is perfect and the weather outside is rather chilly.

Or it could be telling you that the weather outside is chilly and there is still some refrigerant in the system that is in a liquid state, but the charge is low.

Or it could be telling you that the weather outside is chilly and the system is overcharged.

94RX-7 on Sat September 14, 2013 12:01 PM User is offline

Quote
Originally posted by: sgeer

I respectfully disagree. If we vacuum the cylinders to -28psi and pu refrigerent in them until they both have 40 psi, the only thing that is creating this 40 psi is the refrigerant because the system was vacuumed, so we assume there is no air or water in the system, just the refrigerent was introduced. Therefore, if we know the refrigerent has a spific pressure at a specific temperature wouldn't it stand to reason that the same amount of refrigerent is in both cylinders?


Go read up on vapor pressures and saturation temperatures.

The pressure in the vessel will increase to the saturation pressure, then the refrigerant will begin to condense and turn into a liquid as more vapor is introduced.

wptski on Sat September 14, 2013 1:06 PM User is offline

Quote
Originally posted by: 94RX-7

Depends on the weather.

It could be telling you that your charge is perfect and the weather outside is rather chilly.

Or it could be telling you that the weather outside is chilly and there is still some refrigerant in the system that is in a liquid state, but the charge is low.

Or it could be telling you that the weather outside is chilly and the system is overcharged.
What if the car is 23 years old and the A/C has never been touched? Since your connecting gauges, there must be a problem, correct?

Would anyone work on a A/C system when it's cold enough to use the heater? Got to be real here.

Dougflas on Sat September 14, 2013 8:11 PM User is offline

to the original poster...you have a system that requires 3 lbs of refrigerant (system A) and one that requires 8 lbs of refrigerant (system B). Both systems are parked next to each other and have not been started for 10 hrs. If you put your gauges on each and do nothing else (both systems use the same refrigerant) {both systems contain 1 1/2 lbs of refrigerant} the guages should read exactly the same. You can not determine if the proper amount of freon is in the systems.

NickD on Sun September 15, 2013 6:44 AM User is offline

P-T curves of R-134a are higher than R-12, but the problem with R-134a is that the rate of pressure versus temperature increases drastically about about 90*F. That translates to using less R-134a than the specified R-12 weight. Also the specific heat of R-134a is greater than R-12, so need improved means to get rid of it. Like a parallel flow condenser or more fans.

Lots of promises were made by the OE's with the overnight change from R-12 to R-134a, but were never kept. No two vehicles have the same characteristics, others suggested just using 90% of the R-12 capacity that may or may not work.

Reducing refrigerant amount creates foam in the high side line that does not cool as well as solid liquid, the general rule is to attempt to keep the pressures in line with R-12. Also with reduced refrigerant, idle pressures drastically decrease causing excessive cycling. Some try to use a smaller sized orifice to correct this. You definitely have to add a HPCO valve so you don't blow up your system.

Charging by pressures requires only doing this on a 85*F day or preferably above, and you are strictly on your own in this subject. Plus have to address compatibility issues such as the dryer and barrier type hoses. Easiest way to attack this issue is to find post 94 parts and replace firewall forward.

Other issue I deal with, is the vehicle even worth this effort, my 88 Supra that I had since new with only 50K miles on it, so I just stuck with R-12, only takes 28 ounces that is another consideration. But with any 25 year old vehicle, that rubber tends to rot. Daily drivers with road salt after ten years go to the wrecking yard. Nothing left of them.

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