Rent a manifold gauge or buy one... And a cheap vacuum pump to purge the manifold gauge.
Static PSI (system off testing, after 20mins of being off) with the A/C off and engine off has to be 45psi for the system to start. At this time the system is even on the high and low sides. This test only tells you that you have 134a in the system and enough to trigger the compressor.
Testing with it on, you have to know the outside airtemp to make uses of any gauges. Its best to put a thermometer infront of the condensor, and in the middle air duct, with A/C on max doors closed windows down. Blower on max.
So at like 70degs you should have about 30psi on the low and for the high well take 70 and times it by like 2.2 or 2.6 and that is your high PSI. like 160 or so.
Now if you see the gauge keep dropping then going back up (short cycle) your low on 134a.
When adding, get some warm water in something bigger than the can of 134a. (NOT HOT YOU COULD BLOW UP THE CAN) never turn the can upside down, dipping it in the warm water will help the 134a go in as a gas into the low. Keep an eye on the low and high. At 30PSI the evap is running at 33-34degs. Feel the air coming out the duct every so often to make sure its getting cold, the compressor should also stop turning on and off and just stay on.
If you like I can do a DIY video... A better (and correct one) vs the last one I did... Which had some incorrect info.
Me playing A/C tech hehe
