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Discussion Starter · #1 · (Edited)
2 days ago I finished up replacing the AC evaporator in the Wife's Silverado. Thought I was done. Making cool 60°F air but not cold enough coming out of the cabin vents even in recirculation. Put the gauge manifold back on it today. Found both low and high side pressure a bit high 85 low 300 hi at 80°F ambient at 2500rpm. Recovered refrigerant until the low was 57 and the high 275 again at 2500 rpm. Went further and checked pressures at different rpm's. @ 2000rpm low was 55 psi high was 275. @ 1500rpm 53 low 250 high. @1200rpm (normal cruise around town 40/50 mph in OD low 52 high 250. Cabin vent temp at all rpm 60°F.
No indication of any restrictions at condenser discharge ie condensation before the orifice etc. Obvious signs of expansion after the orifice.
My previous post 2 days ago lists all the work done recently to the AC system FYI
Any ideas.
 

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Still over charged at 80F ambient.
Shoot for low of 35 to 40 psi at 1500 rpm.

At 52 psig, saturation temp for R134a is 56F. That's why your vent temp is 60F.
 
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Discussion Starter · #3 ·
Thank you much. I will do that first thing in the morning. Figured it had to be something pretty simple.
 

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the low pressure is higher 50-60PSI and the high pressure is 275-300PSI @ 80f air ....then you have air in the refrigerant . dump out the R134 put the vacuum pump on it with the ac gauge then hook up the 134 refrig can .. turn the vac valve off then turn on the 134 refrig into the low side ..vacuum pump should work for 30 min or more then see if the vacuum leaks !!!
 

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Discussion Starter · #5 ·
Thank you much. I will do that first thing in the morning. Figured it had to be something pretty simple.
So I recovered a little more refrigerant bringing the low side to 34 psi and the high to 180 psi at 1500 rpm after warming the engine to operating temps. Evaporator temp is down to 53-55° .
Thank you j cat for the advice and information
 

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So I recovered a little more refrigerant bringing the low side to 34 psi and the high to 180 psi at 1500 rpm after warming the engine to operating temps. Evaporator temp is down to 53-55° .
Thank you j cat for the advice and information
I do the repairs on the ac systems . the GM vehicles when air temp is 80F,, rpm 1500 engine , open the windows , my pressures are 40-45 PSI LP the 275PSI is HP ...ac air is 55F ,, typically 25F drop for outside air ..fan speed to the max ..
 

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Discussion Starter · #7 ·
the low pressure is higher 50-60PSI and the high pressure is 275-300PSI @ 80f air ....then you have air in the refrigerant . dump out the R134 put the vacuum pump on it with the ac gauge then hook up the 134 refrig can .. turn the vac valve off then turn on the 134 refrig into the low side ..vacuum pump should work for 30 min or more then see if the vacuum leaks !!!
I don't think there is air as I had evacuated the system for 40 minutes, drop test revealed no vaccum loss after another 40 minutes.fresh oil installed prior to evacuating and new refrigant charge after. Recovering some refrigerant the next day to reduce low side pressure has resulted in AC output at 55° with 80° ambient for 3 days now. Thanks for the input. I just keep learning from you guys.
 

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I believe you have no air inside the system.

R134a at 27 psig has a saturation temp of 32F. If you are low on charge and system pressure go below 27psig at 1500 rpm, your evaporator will start icing up and block the airflow. To be safe, don't go below 30 psig on the low side.

With various new refrigerants nowadays due to obsolescence, looking up a refrigerant PT (pressure/temperature) chart is a must when checking system charge.
 

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Discussion Starter · #9 ·
I followed your previous advise and am happy with the AC operation now. Decided that with the low side being at 34 psi that I will just keep it there rather than trying to add just a little bit more refrigerant. I watched for icing after the orifice and at the evaporator outlet also. There is condensation but 0 icing.
I do remember a lot of general knowledge from my AC/R schools and experience in the Navy, but a lot has changed so I do like to get current knowledge when I can. Thanks again for your time and input.
 
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