AC compressor won't kick on
 

I just put in an AC compressor that I bought from a board member; I haven't recharged it yet. I started the car to see if the compressor made any screeching noises like my last one did (it didn't) and then turned the AC knob to the "on" position, but the compressor's clutch didn't engage. Will the clutch only engage if the system is pressurized, or would that have nothing to do with it?

Yes the clutch will not engage if there is not pressure in the system

add refrigerant and then try it

Yeah, I just found this on another site.
"On many systems, the compressor will not turn on if the refrigerant is low because the 'low pressure safety switch' prevents the compressor clutch from engaging if system pressure is low. This protects the compressor from possible damage caused by a lack of lubrication."
I'm such a newb. Thanks.:)