And as was suggested, try the SMC reset, and also PRAM. Make sure to test the RAM slots - power up with one module in and the other empty, and then with the other populated and the other empty. From what you've mentioned, your left I/O board does not sound suspicious, although it's impossible to be sure given what I know. In fact, I would leave the battery out until you have a computer that powers on, because it will only cloud the issue. I would not assume you have a good battery, since there are all kinds of things that could be wrong with it. Seems to work with the battery having some charge to it, and some reason I have to hold the power button 10 seconds with the macbook unplug with no battery and then continue to hold it for another 10 seconds when plugging in the ac adapter and then I can press the power button and it turns on, this is only if I don't have the battery in, not sure if that is normal and it does seem to run hot too.Ĭorrect - a 60W will not likely power this machine without a charge on the battery, and an 85W will power it with no battery. So, from that I looked over the I/O board and took the black like tape cover off the bottom of the I/O board and found what looks like one of chip capacitor next one of the IC's that looked burned and so I took a cap off a modem PCI card that no longer works and soldered that in replace of the burned one, and then test the voltage output from the power cable that goes to the logic board and now its outputting 12.6 volts, I went a head and put everything back together, now the green light turns orange and the battery is charging but it still doesn't turn on, waited 5 minutes and then it would turn on! 5 volts when testing all 3 gray wires being (+) and all 3 black wires being (-) on the power cable at the logic board end which didn't seem right.

I did try resetting the SMC but didn't do any good, still didn't power on, so I decided to remove the logic board and left I/O board and look for any components with physical damage and test any fuses soldered to the left I/O, but didn't see any anything, I then test the voltages from the power cable coming from the Left I/O board to the logic board, even when the magsafe plug had the green LED I was getting.

I got some life out of the macbook now and here is some trouble shooting I did to get it to now power on:

2012 macbook pro power cord