I received a Late '08 15" MBP that would not power on at all (no workarounds or anything) and I don't know its background.
I can connect the MagSafe and the green "charged" light comes on, and I know the battery is fully charged because the charge indicator displays all of the green dots. I also have 3.467 V on G3Hot. I opened up the machine and took out the logic board when I first got it and it had a decent amount of corrosion on the right side of the processor side of the logic board as well as at the center of the bottom edge on the same processor side of the board. I didn't see any problems on the side of the board facing the bottom plate. The thing that concerned me is that what seemed to be causing most of the corrosion was a small, button-cell looking battery near the RAM slots. When I measured that battery for power, I got .055 V (almost nothing). I pressed on, however. After a good scrubbing with some alcohol and a toothbrush, the corrosion looked gone. I let the alcohol dry and put it all back together. I did an SMC reset just out of habit and plugged in the MagSafe. The light turned green. I tested the battery with the buttons on the side. The lights all lit right up. Everything was looking great! I pushed the power button....
Aaaaaannnnddddd...........
NOTHING.
What can I possibly do other than pay for Apple to replace the board or buy a board myself and replace it?
BTW: Shorting G3Hot does not power up the machine. Also, when cleaning the MBP, I used alcohol that was labeled "Isopropyl alcohol (95%) in a glycerin solution (5%)." Would this have messed anything up?
One last question: Does anyone have info on the "battery" or what it does? Is it like a CMOS battery on PCs? Can you replace it?