Chosen Solution
The indicator light will also go on for a split second, and seems to show that the motherboard responds to the different startup combinations (I’ve tried them all). It still charges the battery. Previously I had issues with overheating, so I used SMC fancontrol to run fans at full speed most of the time. Then, for a while it would run fans at full speed during boot but never get past a black screen. Now, even less action. Other things I’ve tried:
- Disconnecting hardware from the board, such as HD, RAM, screen, etc.
- try to boot using only battery, only the charger, and both. Even tried disconnecting the onboard DC from motherboard and boot from battery.
- taking everything apart and putting it back together
Look on the logic board around the heat sink and see if you see any oily type looking residue. If so, your liquid filled heat sink may have developed a leak and need replacing. MacBook Pro 15" Unibody Late 2011 Heat Sink Replacement MacBook Pro 15" Unibody (Early 2011-Mid 2012) Heat Sink
Sounds like a problem related to the current sensing, possibly some component went bad, maybe due to prolonged excessive heat or just because it happens. You probably need a skillful repair lab who deals with component level repairs.
