Macbook Pro 2012 black screen after multiple sudden reboots, is my logic board dead?

With the internal display cable disconnected try powering on the board to see if you get any video signal on an external display just in case the built-in display is bad. Use one of your original Apple memory modules during this test to be safe. I would connect the fan as well to see if the fan spins up at all. It is Ok to do this with the Logic Board removed using just an Apple memory module, fan, DC-In Board, and external display and charger. There are a set of power on pads located on the Logic Board. See this article for details (you will need to scroll down to the correct section since the links on that page don't go directly to the correct section -- it ends up at the 2009/10 model instead):

https://beetstech.com/blog/where-are-macbook-pro-unibody-power-on-pads-located

Make sure to try a single Apple memory module in both slots just in case the one slot is bad (install only one stick leaving the other slot empty). Usually the slot nearest the Bottom Case will go bad due to cracked solder joints, but I have seen the other slot go bad once in a while. Not all memory failures include the beeps.

If you don't get any video signal on the external display with this minimal configuration, then most likely the Logic Board is bad assuming the DC-In Board is good. You can use a DC-In Board from any other Mac which fits the connector on the Logic Board to test the Logic Board outside of the case (I've done this between multiple laptops without any problems).

I notice a small white bit on the keyboard connector that you will want to gently remove. I'm not sure about the discolored spots. I have seen the discoloration before, but it is a bit hard to see in the pictures. I don't really see any signs of liquid residue or corrosion. It is hard to say whether the discoloration means anything here.

You can only use a Logic Board made specifically for a MBPro 13" (mid-2012) model. Any Logic Board for this model will fit as the differences are only with the CPU. Even though a Logic Board from a 2011 model may appear to fit, there is a very slight difference in the placement of mounting holes and posts (one post/hole is only off by about a couple of millimeters). I know this because a client had purchased an incorrect Top Case/Keyboard Assembly for the earlier model intending to do the work himself, but changed his mind so I made it fit with some case modifications as I felt bad for him. Besides you don't want to downgrade the Mac since your memory won't be compatible. I'm guessing since the Beetstech link I provided links those two 820-xxxx-X numbers together that they are compatible.

Good luck.

Nesphyd wrote:

I had the same idea of trying to power the board and see if I got any response a small while before reading your answer, .... Alas, I got no video or any other signs of life on an external monitor; oddly enough, though, the fan would start spinning really fast for no apparent reason.

When you disconnect other items you also disconnect the temperature sensors included on those items. Whenever a Mac cannot detect a temperature sensor the fan(s) will run at high speed as a safety precaution to keep the system running cool.

So the issue was a dead logic board. I almost wish it had been bad RAM, quite possibly a cheaper replacement. ;)

Glad you found the problem.

As a side consideration, when the board died I was using the CPU pretty heavily, could that have had anything to do with it, I wonder?

It is hard to say. Things can just fail. There is no way to know what failed without someone taking the time to use a schematic to check out where the failure is located. Not many people know how to properly troubleshoot a Mac at that level. I follow a guy online who gets lots of Apple Logic Boards where others have attempted to fix them incorrectly by not understanding how to interpret the schematics.

I did forget to mention that the hard drive SATA Cable on the 13" model has an extremely high rate of failure and can prevent a Mac from booting although the laptop should power on even if it doesn't see a bootable internal drive.