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Old 06-12-2019, 07:54 PM   #1
jradford
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A/C System pressures

72 C20 with factory air that was converted to R134A by some shop in Ar. that says on sticker under hood IAW some standard and lists new oil type as well. Done before I got the truck. Not cooling well so hooked up a set of gauges but this being non standard setup for R134A not sure what pressures should be. Looked through the past posts but most just explain how their conversion was done. Looking to find out what kind of readings I should see to determine if I need to adjust the charge. At about 85 deg ambient and probably 65% humidity it is reading 35/310. This would normally indicate a restriction on the low side and an over charge but not sure with the current setup. There are bubbles in the sight glass but from what research I have done it is not an accurate indication of low charge. I don't want to drive the high side any higher. Normal readings on an R134A system should be 45/250 at this ambient temp. but with factory evaporator which is smaller I am not sure. Any recommendations based on what you guys are seeing?
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Old 06-12-2019, 10:46 PM   #2
Steeveedee
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Re: A/C System pressures

If you have a sight glass, I suspect that the receiver/dryer may not have been changed out for one compatible with R-134. Can you post a picture of it? Also, the AC depends to some extent on ram air flow. When I work on AC systems and the pressures go up, I put a box fan in front of the radiator to increase air flow. This will drive the high side pressure lower. In a pickup, the evaporator being small isn't necessarily a bad thing, since the cab is so small, so the efficiency issue is reduced.
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Old 06-12-2019, 11:55 PM   #3
WorkinLonghorn
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Re: A/C System pressures

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Old 06-13-2019, 01:04 AM   #4
randy500
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Re: A/C System pressures

Low side if the poa was adjusted for 134a should be about 27.5 if it wasn’t adjusted then it should be about 30(r12 setting), at 2000rpm with a big fan blowing into the Condensor.
High side will run optimally at 2.5 to 3.5 times ambient temperature.
Your not getting enough cooling because the low side pressure is too high, poa valve is not working right or adjusted wrong.
Check that your water valve is closing off the heater core properly too.
Check your vent temps with a thermometer.

Bubbles in the sight glass just means the receiver dryer is not full of liquid refrigerant. It doesn’t mean air is in the system. Your high pressure shows the pump is working and it’s somewhat charged. If it was really low on refrigerant you wouldn’t have high pressure.

You could add a little 134@, 2-4 ounces at a time, use a postal scale to weigh the can as your adding to the low side, run it a while and check pressures and vent temps, stop adding if you see better vent temps. If the vent temps don’t get better then it’s probably overcharged.
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Old 06-13-2019, 07:52 AM   #5
brown7373
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Re: A/C System pressures

I get 38-42 degrees when the truck is moving. The POA was adjusted for R34, the compressor, filter dryer, hoses and compressor are new. The system was flushed and new o-rings installed..
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Old 06-13-2019, 10:11 AM   #6
tdangle
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Re: A/C System pressures

Following as I have pretty much the same readings on my truck. All new hoses, POA adjusted for 27.5 new dryer (that is labeled R134 and has a sight glass). It does seem to be cooling fairly well especially at hwy speeds. In my research it does seem that the values listed for R134 don't seem to match up with R12 system conversions.
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Old 06-13-2019, 10:21 AM   #7
randy500
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Re: A/C System pressures

38 vent temps are not bad.

Here is a testing guide for pressures.
http://thefirstgensite.com/library/6...7S1BP91_98.pdf
It’s for a Pontiac, the only real difference is high side pressure can vary by model and year due to condensor placement and airflow.
Vent temps vary by model due to duct efficiency.
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Old 06-13-2019, 07:12 PM   #8
jradford
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Re: A/C System pressures

A little further info after doing more research and checking out some of ya'll's suggestions. Just ran it again with a fan blowing on the condensing coil. Low side 45, high side 425 and climbing. Probably is over charged. Looked up the spec SAE 1661 listed as the standard the conversion was done to according to the under hood sticker and it states for 80 and up vehicles so it was not probably the right way to convert the ac system as later models operated somewhat differently(no POA, etc.) The dryer looks to be the original(still has the old bubble sight) and the POA has no evidence of being cut apart to be modified for R134A so I guess I will just order a new dryer and send the POA in to be rebuilt and recalibrated and go from there. May be a good time to replace the O-rings and front A/C compressor seal as well to ensure the correct material to be compatible with R134A. Any other suggestions are welcome. Thanks
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Old 06-13-2019, 07:46 PM   #9
randy500
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Re: A/C System pressures

The overpressure valve at the back of the compressor opens around 400psi I think...
The poa valve is adjustable without cutting it apart, small nut inside the outlet, pressurize the evaporator at the line where the expansion valve connects, hook up the low pressure gauge to the low pressure port on the poa valve, adjust pressure to 27.5 or so for 134a using the nut, it’s an odd size.
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Old 06-13-2019, 09:10 PM   #10
jradford
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Re: A/C System pressures

May just go ahead and replace the condenser with a parallel flow while I am at it. Haven't found one for this truck yet though. May have to make one a different application work. Good to know I can adjust the POA. Wonder if can I get a rebuild kit for it as well?
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