Compressor clutch won't kick in after I charged it.Pressure low before charge

What is your question?

Mechanic charged it two years ago. Had a little problem getting the clutch to engage. He fiddled around a bit on the compressor and it worked. Getting the same symptoms now after I charged it with the kit that I bought. Does the clutch have a separate relay to engage. The compressor turns okay. Did a little reading online would like to perform some tests to see if I have to change the compressor. Where do I find the clutch relay If there is one and the low pressure switch . Which could be the problem. As it stands right now is blowing warm air with all controls at full throttle.