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| r134a vs r12 manifold gauges........ https://slantsix.org/forum/viewtopic.php?t=21271 |
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| Author: | emsvitil [ Sat Dec 30, 2006 8:50 pm ] |
| Post subject: | r134a vs r12 manifold gauges........ |
It's been awhile since I've worked on a AC system and I have a R12 manifold set and repair manual (which isn't the current problem......) Anyway, I need to do some work checking a R134a system (designed for R134a, not a retrofit and it seems to be running funny, but I'm not starting off empty....), and don't see the need to get a new repair manual since the basics are pretty much the same.... I think it's a little low on the R134a, but need to know what the gauge readings should be so I don't overfill. So here's the question: Do the high and low side pressures differ from the R12 readings you should get, and if so how much? (percentage or fixed amount high or low) Thanks |
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| Author: | SlantSixDan [ Sat Dec 30, 2006 9:04 pm ] |
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They do differ, yes, and it's difficult to give a concrete figure or percentage you should be targeting. The pressure/temp curve of R134a is shaped differently to the R12 curve, and aside from ambient temp there are a lot of other factors that go into what you will see on an R134a-retrofitted system. What kind of condenser is installed, is it an old R12-type serpentine unit or a parallel-flow? Is its capacity high enough that you've got some subcooling or is the condenser barely keeping up with the heat load? What kind of fan is pulling air across the condenser, and how efficiently? |
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| Author: | emsvitil [ Sat Dec 30, 2006 9:20 pm ] |
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Not a retrofit... condenser is parrallel, electric fan with good pull. Cycles on and off every 15 seconds on max cool. Cycle is due to low side pressure switch. (gets down to about 20 psi, cycles off, pressure creeps up, cycles on..........) So I'm sure that it's low on R134a, but don't want to overdue it on filling up ........... (if it was R12, I'd open up my old manual and know what readings to shoot for given the ambient air temp.........) |
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| Author: | AnotherSix [ Sat Dec 30, 2006 10:17 pm ] |
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I would try to dig up some factory data on it. The new systems don't hold nearly as much and it is very easy to overcharge. There just isn't the wide margin like the old r12 systems, it's more like a couple of ounces instead of a pound or more. They tend to seem fine with a little too much and then when it heats up the pressure really climbs on the high side. Go slowly and give it more time than you would with r12 to stabilize between additions of refrigerant. There are generic articles on the web about r134, but for a factory system the factory data will be the best bet. |
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| Author: | Charrlie_S [ Sun Dec 31, 2006 4:32 am ] |
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A down and dirty method to check charge, with manual A/C and fixed displacement compressor, and orfice tube: Engine about 1500 rpm, doors and windows open, normal cooling, high blower, 70 degrees or higher ambient temp. Evaporator inlet and outlet pipes should be about the same temp, after system stablizes. I monitor the head pressure, and like to see about 200-250 on a hot humid day. Sometime will see above or below, that. If it is around 300 fix it. With R134a, it is better to be slightly under charge then over. And a couple of onces can make a measureable difference. Best way is to evacuate, and weigh in the proper amout of refegerant, as per the label in the engine compartment. |
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| Author: | Jeb [ Sun Dec 31, 2006 4:02 pm ] |
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I need to remember that. My dad has a set of R12 gauges but not a set for 134a. |
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| Author: | Charrlie_S [ Sun Dec 31, 2006 4:12 pm ] |
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Quote: I need to remember that. My dad has a set of R12 gauges but not a set for 134a.
Even if you have a set of R-12 gauges, you still need the hoses with the correct ends for the R-134a fittings.
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| Author: | Jeb [ Sun Dec 31, 2006 4:45 pm ] |
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I have already have a set of R134a hoses. |
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| Author: | emsvitil [ Sun Dec 31, 2006 10:00 pm ] |
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I did some digging and found a comparison of pressure vs temp on R12 and R134a (didn't save link, so I lost it...) Anyway on the low side R134a runs about 2 psi lower than R12 at equivalent temperatures. On the high side, R134a pressure runs about 10% higher pressure than R12 at equivalent temperatures. And don't forget the high side pressures vary with ambient temperature. |
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