2001 Volvo S60 alternator

I fixed an alternator in my car, twice. But from scrap. Even that doesn't work. And while my process I took out alternator wire that makes the dash meter off while the car start. The battery light didn't gone. Later on I start car fixed back whatever I popup. And think to check by driving. But still the charge in idel position is 12. Something and after sometimes charge goes on decrease.I run my car after sometimes (10/20 mins)i run my car later on the battery light came.I gussed the alternator is bad. But may be there's prpblems in wiring.
so let me be clear plz.I have been getting problems with this.