How to Top off a Car's A/C System Using a Manifold Gauge 

Now have you tried jumping your AC relay? The compressor should turn on. If the compressor doesn't come on. You have a bad AC fuse under the hood., connection to the compressor or the compressors bad or open? Regardless, how much freon the compressor should turn on. Now keep in mind when you do this. It's by passing all the safety switches to run the compressor so you don't want to run it very long. We just want to see if the compressor will engage. If the compressor still doesn't engage, then you'll have to go at the connector at the compressor and see if you've got 12 V or system voltage and a good ground.

@@NobleAutomotive So I did Jump the Low Pressure Switch, and the A/C compressor did come on. However upon further inspection I found metal shavings in my orifice tube so I decided it’s 2023 the A/C system is 23 years old now on my 2000 Silverado so I replaced the Compressor, Accumulator and the Condenser. I flushed the Evaporator and lines with A/C flush and put everything back together… when pulling a vaccum I could only achieve 25” mhg… I thought that was odd, but I went ahead and pulled a vac for an hour, then let it sit 1 hour. It held the vaccum for the resting hour so I assumed no leaks. And after charging with 2 12 OZ cans then 10 OZ of a 3rd (2LBS) the compressor runs continuously and doesn’t short cycle and the vents read 38 Fahrenheit… if something happens to it before to long I will update, still uncertain on why I could only achieve 25 “ vac tho… but thanks for the troubleshooting info!!!