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The chart on top is what I found more commonly on the internet, but the chart at the bottom is used by a YouTube mechanic I like. The numbers on these two charts deviate quite significantly. For example

at 85 Fahrenheit, the top chart says 45-55 psi on the low side. 225- 250 psi on the high side. the bottom chart says 27-42 psi on the low side. 128-181 psi on the high side.

At 105 Fahrenheit, the top chart says 50 - 55 psi on the low side. 330- 335 psi on the high side. the bottom chart says 37 - 52 psi on the low side. 196-246 psi on the high side.

Which one should I trust if I decide to recharge my refrigerant?

It's the summer, it's really hot without a working AC. Your help is much appreciated!

jeffrey
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2 Answers2

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P/t charts can be confusing if unfamiliar with refrigeration fundamentals. Do not use the first p/t chart. Why it is posted from google search is a mystery. Google cannot verify all info and one must do research to use correct info if service manuals aren't provided. A simple guide, from experience, would be examining your gauges, if they're good ones and have correct p/t references marked. To begin, remember that ac vehicles are supposed to cool interiors to between 35F to 4F. Examine your low side gauge for the corresponding pressures. If anyone expects their ac to cool down to 35F, then low side pressures should be around 30 psi. If you were to use the wrong chart, this would show incorrect pressure at 40-55 psi with temps between 45F-59F. If you evacuate a system then check to be sure zero leak back occurs, and refill with the exact amount of r134a, operating pressures will be displayed in comparison to ambient temps. Remember, you're attempting to restore factory ac when all procedures are performed correctly. You'll see low side pressures, and center vent temps correspond to expected cooling temps. If you see low side pressures between 40-59 psi, gauge temps and center vent temps should show 45F to 53F, (not cooling at all for incorrect chart reference) either the cooling fans aren't blowing, more refrigerant was put in than specified or incorrect repairs weren't followed. Refrigerant, oil and dye are the only things in a system. Absolutely no sealer as this simply contaminates a system and can create major issues for a very expensive rebuild. High side pressures will vary in proportion to ambient temps and humidity. To gain some experience, spray a water mist into the ac condenser and observe both gauges. You should see high side pressures drop; this is cooling the hot compressor discharging refrigerant into the condenser coils and water absorbs heat better than air. enter image description here

F Dryer
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I believe the difference between the two is that the top chart is for a fixed displacement compressor and the bottom is for a variable displacement compressor as shown here: R134a OPERATING PRESSURES TABLE

This is from Nissens and can be found here: https://nissens.com/Admin/Public/Download.aspx?file=Files%2FFiles%2FAuto%2FPosters%2FNissens_AC_System_Diagnostics_Poster_98x68+-+Operating+Pressures+R134A+-+ENG.pdf

Thanks! Chris

Chris W
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