3 volt hot , rail and ground need help shorted out mobo

I am newbie I'm working on a logicboard from a macbook I have a logicboard that certainly has some sort of short in it when I connect the adapter it immediatley turns on will eventually power off after about 10 seconds almost load the o/s I guess most techies in here are pc techs but maybe this will fall under both. I had a good tech which I can't get in touch with tell me the short lies in between the PPBus_3GH (3 volt hot) rail and ground. I just trying to find out exactly where he is talking about. thanks.