820-3115B fan spins but no chime and no display

[avlcat]

2012 Macbook Pro A1278, board number 820-3115-B, EMC 2554. 2.5GHz i5 CPU.

The macbook came dead on arrival. No visible signs of liquid damage, everything looks clean. Checking power rails I found missing PPBUS_G3H. The R7005 was bad, after I replaced it the board voltages came to norm (PPBUS is 12.53V, 3V42 is 3.447, all 5V and 3V3 rails present). The board charges the battery (light switches from green to orange).

When I connect charger the fan starts spinning (high speed without trackpad; normal speed if trackpad is connected). However the board does not chime and there is no image at all. CPU_VCORE is 1.12V almost immediately dropping to 0.9V.

SMC reset is OK (charger light blinks as it should). Unable to do PRAM reset though as it does not chime.

I reprogrammed BIOS with clean ME region (getting clean ME file for the same EMC number) - this did not help at all. Tried several clean ME region files downloaded from internet - same result. Even tried to make clean ME file myself taking original BIOS dump and cleaning the region - same result; fan spin and no chime.

Then I temporarily soldered the BIOS chip from another working 3115 board to this one. The board chimed and screen came white. I was able to run EFI ASD and it did not report issues except of missing serial number. However once I tried to boot the board from bootable USB installer - it froze immediately once apple logo was displayed. Same happened when I tried to boot off internal hard drive with known good Mac OS installed.

Also I noticed that USB mouse light will come on once the computer is powered, but will turn off when board freezes (that was all tested with BIOS chip from other board).

I put original BIOS back and it does not chime again.

Any help will be appreciated.

[avlcat]

Re: 820-3115B fan spins but no chime and no display

Also worth to mention is that it makes single beep if turned on with RAM removed.

[ktmmotocross]

Re: 820-3115B fan spins but no chime and no display

so as u have little progress with that bios, save its dump and put it to yours bios

[piernov]

Re: 820-3115B fan spins but no chime and no display

VCore droppping to 0.9V means the BIOS at least starts to run. It should most likely chime in this situation. 820-3115 is known to have some issue with RAM slot so try with different sticks and alternatively in one slot or the other.
PRAM reset not working may be because either the keyboard is bad (disconnect internal, try with external) or there's an EFI password set.

BIOS chip transfer won't work as it will have a dirty ME region, macOS will not boot in this state. You need to transfer the original ME region to the donor BIOS.