HVAC - R134a A/C low side pressure

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R134a A/C low side pressure

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Category: The Garage
Forum Name: HVAC
Forum Description: air conditioning, heater and associated controls
URL: https://theamcforum.com/forum/forum_posts.asp?TID=38953
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Topic: R134a A/C low side pressure

Posted By: FSJunkie
Subject: R134a A/C low side pressure
Date Posted: Apr/22/2012 at 5:04pm

I have specs for what the low side pressure (good indicator of system charge level) should be at various ambient temperatures, but those specs were for R12.

What should the low side pressure be for a brand new 134a converted system? It runs about 15-18 PSI on an 80 degree day.

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1955 Packard
1966 Marlin
1972 Wagoneer
1973 Ambassador
1977 Hornet
1982 Concord D/L
1984 Eagle Limited

Replies:

Posted By: 6768rogues
Date Posted: Apr/22/2012 at 5:40pm

The low side pressure will vary and is not as good for diagnosis as the high side pressure. The one I installed on my 68 Rogue runs 150 psi on the high side on a hot day, blowing 38 degree air. My 67 runs a little higher pressure and will only get down to about 41 degrees. I had a non-AMC that needed to run about 220 high side to be good and cold. I usually start at about 150 high side and add refrigerant until I get air in the 40 degree range without exceeding a pressure of twice ambient temperature plus 80. I test it at a high idle with a fan blowing at the front of the car, all the vents but one closed, a thermometer in the open vent and the fan on low or medium.
If you converted an original system and know the weight of R12 that it took, you should add about 80% of that weight in R134A.
There, I said a lot and didn't answer your question.

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Located usually near Rochester, NY and sometimes central FL.

Posted By: Mr. Ed
Date Posted: Apr/23/2012 at 11:54am

The low side (suction side) of any air conditioning system should run at 45 degrees regardless of refrigerant. So an R-134a system should be around 40# SST. If it runs higher, the cabin is really hot or it probably has non-condsables (air or something other than refrigerant) in the system. You did evacuate the system after recovering the R-12, right?

Later!

Mr. Ed

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2014 Jeep Grand Cherokee Laredo "Gwendolyn."
1978 Concord Sport coupe "Mr. Black".
1982 Concord wagon. The Admiral. FOR SALE!
1976 Sportabout X, 304, auto, air. The Bronze Goddess

Posted By: Peter Marano
Date Posted: Apr/23/2012 at 9:56pm

In the early days of A/C purging was an accepted practice!

Posted By: Mr. Ed
Date Posted: Apr/24/2012 at 7:45am

I know. I did my share of purging when I did A/C&R service last century. But now it is a no-no and refrigerant should be recovered and the system evacuated. And purging doesn't get everything out of the system that is why evacuation is a must to ensure the system is clean and doesn't contain any non-condensables.

Later!

Mr. Ed

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2014 Jeep Grand Cherokee Laredo "Gwendolyn."
1978 Concord Sport coupe "Mr. Black".
1982 Concord wagon. The Admiral. FOR SALE!
1976 Sportabout X, 304, auto, air. The Bronze Goddess

Posted By: farna
Date Posted: May/27/2012 at 11:06am

Yeah, can't let ANYTHING marked/sold as refrigerant into the atmosphere, even it if IS perfectly harmless (such as Enviro-Safe, which is a propane/butane blend, and works better than R-134a in most systems, about the same as R-12). The blanket ruling makes it easier to police, can't blame the EPA much for that. Purging is also called "sweep charging" and requires some refrigerant to be vented to the atmosphere.

You can find R-134a pressure charts on the web easy enough, just search!

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Frank Swygert