'02 Duramax...Battery light came on and gauge shows discharge. Occasionally it will charge normally for a day or two then shows discharge again. Time for a new alternator I guess. My question is, they show two alternators... a 105 amp and a 130 amp. I don't know which one I have and would like to have the larger one. The salesman said they were different sizes. Can I use either or do I have to pull it off first and walk to the parts house? lol...help!