Ac Clutch Problem On 2006 Gt

I swapped the engine out of my car and when I jump the AC clutch to run the compressor in manual to recharge the coolant my clutch is not engaging. When I jump the relay and get a reading of 10V down at the plug right where it connects to the compressor. I plug it back in, but the clutch still won't engage. What are my options for fixing this? Thanks for any help!

So after some more testing I hooked the battery directly up to the Compressor and it works! I ran it and charged the AC system back up to the correct level. I then hooked the compressor back in to the car the way it's supposed to be and nothing happens. The clutch won't engage. So 2 questions. Any idea why not? Does it hurt anything if the compressor runs all the time when the AC is on? Thanks.