Air conditioning

With a new air compressor and you start car and turn air con on the compressor clicks on and off and on and off until you turn car off. Could you give me some ideas on what I should check please.it's on a friends car. I think it has rough idle when trying to work air con but I would have to dble check on the idle part I just told you.I think from memory the compressor is a 4715 j. That is what the sticker says on top.my mate said he had the system filled up. Do you only pump gas into the lowside only or both with car running and air con turned on and fan on high. He said fuses are ok to. I think he has replace one relay. A small black relay.we are trying to fix this at home. My mate said he's thinking of giving the compressor it's own power switch. So when he wants to use air-conditioning they will have to turn on the compressor with its own switch so the compressor stays on. If you could help out with a few things that we could check please. That would be great. It's a 2001 Sep , 4 door astra. Hatch I think.
Thano you
Shaneo

I hope you can help out.
shaneillingworth@hotmail.com