tooltime

My son has a 94 z-28 with a LT-1 in it. We were recharging my 83 and did the same to his trying to get his a/c cooler, or just not blowing out hot air. We charged the system with gas and cannot get the compressor to spin up. There is a three wire plug going to the high pressure switch. We were going to try jumping it an trick the compressor to engage. Not knowing what the three wires do, I didn't want to risk shorting something more important out. So, here is my question, can you check to see if the high pressure switch is bad, engage the compressor to see if that is the issue or is there something else I can try. All fuses look good and the relay is intact. Don't know how to test the relay, but I'm up for anything. Thanks for anyhelp!!!!! Matt PS...How the hell do you get to the compressor without a lift???? Tight tight tight.