1981 Chevy Camaro No voltage when I hook up the compressor

Even if there is voltage going to operate the compressor, it has to complete the circuit by grounding out to operate. If you already have power going to it, it sounds as it is not completeing the circuit. That system will have either an expansion valve with a small tube going to the evaporator or a pressure switch. the pressure switch should be by the evaporator. to check this disconnect the switch electrical connector and jump it. If the compressor kicks on then there in may lie your problem. If it is an expansion valve then it will also have a connector there to do the same thing. If it has an expansion valve the system will need to be discharged to repair. If it's a switch it should have a schrader valve behind it so the system won't have to be discharged. Always be careful with a/c systems as you are usually dealing with high pressure and it should be discharged correctly and recycled. Also on these cars, make sure there is a good ground wire going from the firewall to the engine. If not it will melt your transmission cable and you will start your car in what you think is park and is actually drive.