Cap off ground or common?

[Bill wiring]

I moved into a house where the owner left an air compressor. He started wiring it before he left, said it works but needs wiring finished. It has a 230 volt 15 amp motor which has 4 wires coming out of it 2- H, 1-com. and 1 ground going to a 3 wire plug which he has capped the com. From there he has a 3 wire plug in on wall (2- H and 1 ground) that runs 3- 12ga. 25 ft. to the breaker box that has a dual 20 amp breaker there ready for it (not wired up yet). This is an older home and for the life of me I can't find where there is any kind of actual ground wire coming into this breaker box from outside. Just 2 hot and 1 common. Also on both ground strips he has both common and ground wires tied to the same screw on both strips coming in from the shop lights and plugins. Grounds and commons are not separated at all on either stip, I'm assuming making the common and ground hot most of the time. Is that right? So here's my main question, on the air compressor with 4 wires, he has the common capped off at the plug. Should I leave it capped and just run the ground to the box like he has it? Or tie the common to the ground at plug and still just run the 3 wires to box or run a 4th wire over to box for common and put both com. and ground to same screw in box on ground strips like all other wiring is. I'm at a loss. No I'm not an electrician, just trying to make chicken salad out of chicken ---- and hope it works. Lol!!!
Thanks for any help you can give me..