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  #1  
Old 08-04-2002, 01:30 PM
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Question R134 high side pressure

Realizing that most of you AC experts don't believe there is a proper R12 conversion, but if there were, what should the high side pressure be on an 85 degree day? My gage only has scales for R22, R12 and R503, so I need the number from one of those.
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  #2  
Old 08-04-2002, 02:09 PM
RSH RSH is offline
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R-134a pressure

If you no what the R-12 hide side pressure was supposed to be, R-134a should be within 10/15% of the established R-12 pressure
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  #3  
Old 08-04-2002, 02:16 PM
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I don't know what that number is either.
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  #4  
Old 08-04-2002, 04:27 PM
RSH RSH is offline
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R-134a pressure

Another basic pressure setting rule is to set the high side 2.2 to 2.4 times the ambient temp since there are to many variables for an exact pressure.
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  #5  
Old 08-04-2002, 04:30 PM
LarryBible
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You should not be using a set of R12 gauges on 134. Cross contamination is NOT healthy for either system. Converting gauges is a very bad idea.

The pressures on the gauges are the same however. Pressure is pressure.

There is no exact proper pressure for a converted system. You will have to experiment a little to get it so it will cool well.

There are many here who think it's not a good idea to convert a 124 car to 134. It's not because it's a bad idea, it's just that some cars convert well and some don't. The 124 car is one that does NOT convert well. That said, if you don't live in Houston or Miami, you might get by with it just fine. Also, if you're driving is mostly constant speed highway driving it might also work for you.

The important thing in a conversion is that you evacuate the system and use oil for 134. The death kits that you buy at Wal Mart or Pep Boys give instructions for not evacuating. This approach does not have the "death" connotation attached to it for nothing. It will eventually kill your a/c system. If you want to convert, you should, at a minimum, replace the filter drier, put in 6 oz. of Ester oil and evacuate. If you do that, you will have a reasonable chance of success, it may not cool as well as R12, but at least you won't kill the system. Cross contaminating with the R12 gauges will do nothing for ensuring your success either. A 134 gauge set and connectors can be had for about $100 or less now. That's peanuts compared to the cost of all the components that you stand to ruin.

Sorry for the lecture, I just want you to be successful.

Best of luck,
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  #6  
Old 08-04-2002, 10:42 PM
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I'll first suggest you search under my user name for a thread within the past 2 months. I just did this project and provided lots of info on the topic in that thread. Furthermore, I will second what the others have said: gauges read the same pressures no matter what, and 2.2 - 2.4 times ambient temperature high side is a good way to measure. But sometimes an even more important pressure is the low side pressure (which is what produces your cool, in cabin air). You are looking for a charge that produces 25psi low side. Remember, all these pressures are read with the a/c on max at and the engine turning 1500rpm.

Good luck.

Greg
'84 300D
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  #7  
Old 08-05-2002, 12:16 AM
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Hold your fire!!

The system was converted two years ago by a certified AC shop using a new (not rebuilt) Nippondenso compressor and new dryer. The pressure hose was replaced, as it was the source of the leak. It worked well even in the 100+f / 99% humidity summers of south GA. Not as fast, but well enough. I had to break the system when the engine was pulled due to a rounded Allen head bolt on the fuel cooler. I am simply recharging what is an already converted system. I have had good results converting other vehicles just blowing half a can of R134 through the system, adding ester and recharging. My 89 Honda for example. Converted five years ago and still cools well enough to suit me. My Toyota truck was converted in 98 and works well too. "Cooling well" is a subjective term. The only way to measure a system is the temp of the output air and I can live with 40 degrees. My Honda will go to 35. Both those vehicles are "disposable" so great if they work, so what if they don't. This MB is another story. I don't want to have to buy another Nippondenso!!
So, with that said, I should charge to around 200 on the high side? That sounds high. And, pressure may be pressure, but the scales read differently for each gas.
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  #8  
Old 08-05-2002, 12:43 AM
RSH RSH is offline
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R-134a pressure

If you use the multiple method, useing the ambient temp you gave in your initial post (85'F) X 2.2 would equal 187 which would be a safe place to start and get you in the ballpark.
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  #9  
Old 08-05-2002, 01:46 AM
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Whew! It's been hot in here

Thanks HSH! Larry; I PROMISE I will flush those hoses and guages with an environmentally friendly solvent ! The last thing in the world I would want is to be party to a contamination, especially a cross contamination!! I am afraid that I am condemned to a life of "available resources" and must make do with what I have.
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