I have asked a few questions over the past couple monthes about my AC. Here is my latest findings.
MY AC is cold as ice at idle (27 psi). While watching a pressure gauge on my system my wife increased the RPMs and the clutch disengages when the pressure dropped below 20 psi. After the clutch is disengaged the pressure shoots to 65 psi. My clutch will not reengage unless i bump it. Thinking the pressure must be a little low at high rpms I added some 134. It is still doing the same thing. After repeated additions of freon and checking. I ended up adding a full (12 oz) can and now I can get to a higher RPM ( 2000 rpms) before the pressure drops below 20 psi and clutch kicks out. This is still not helping me when I go down the road at over 2000 rpms when it shifts.
Please note these PSI reading I gave are from a cheap gauge so they may not be extremely accurate.
Any ideas?

 

Note that the outside air temp will have an impact on the pressure you are reading (suction side) as well as the fan position (always evaluate a system on high fan speed, windows open). 20 psi roughly equals 20 degrees F so I think either your gauge is lying or the low pressure cutout is out of whack. With this in mind, if your low pressure cutout is out of whack, you should notice some reduced airflow due to the evaporator core icing over and restricting airflow. This is especially true if you are cruising and the engine speed is 1200+ RPM. FWIW, I like to evaluate/charge a system at 1200-1500 RPM.

I'm not sure what your problem is; is there a lack of cooling?

I charged a GM car a few weeks ago; I'll mention what I did here to fix it. Maybe this will help you. It was not cooling well on a 95 F degree and the compressor was cycling (implies undercharged).

*Hook up guages, run car at 1200-1400RPM for a few minutes. Fan on high, windows open. System was cycling. Poor cooling.

**Added R-12 until cycling stopped. Suction side @ 33PSI. No cycling (as should be on a hot day). Not good enough in my book.

***Added R-12 until pressure dropped to about 28 PSI. Ahhh, cold air from the dash. No cycling.

****Added R-12 until suction pressure began to rise a bit (29 PSI). This equals overcharged.

*****Slowly removed R-12 until I achieved the lowest suction pressure (27 PSI). I have now found the correct charge for the system .

All this time, engine speed @ 1200 -1400 RPM.

******Removed gauges, road test. Ice cold A/C on a 95 F degree day. My buddy was smiling and didn't he get hosed at a dealer/shop.

 

make sure that your evaporator fan is running and that you have air flow. does your vehicle have an air intake filter for the cabin air, or is your evap coil plugged and dirty? without good air flow across the evap, this will cause low suction psi. to properly analyse your problem, you should recover the refrigerent from your system. being you have already added some, your charge is likely incorrect. then connect a vacuum pump to remove any non condensables, then charge it with the correct amount of refrigerent as the vehicle specs, also use a good accurate set of gauges when doing so.