Fast Fan Spin - No Boot Chime - No Display

A1278 2012 MB Pro 13"

OK where to start..... Unit powers up but that is it,No boot, no sound from the DVD drive, nothing, only a fast fan spin after a few seconds.

1 - Connected to power supply with the battery connected the fan starts spinning, after a few seconds at full speed. Nothing else happening (There is only a white light on at the SSD for the sleep thing).

2- Connected to power supply with the battery connected and OPT+CTRL+P+R the fan starts spinning, after a few seconds at full speed. Nothing else happening (There is only a white light on at the SSD for the sleep thing).

3 - Connected to power supply with the battery disconnected the fan starts spinning, after a few seconds at full speed. Nothing else happening (There is only a white light on at the SSD for the sleep thing).

4- Connected to power supply with the battery disconnected and OPT+CTRL+P+R the fan starts spinning, after a few seconds at full speed. Nothing else happening (There is only a white light on at the SSD for the sleep thing).

Resetting NVRAM cannot do, no effect with the battery light changing colors etc)

Tried a NEW SSD with a Fresh Install, Tried an SSD with OS installed.

Tried booting with nothing connected. Tried with external display.

Tried booting with no RAM. Pressed power button for 30 seconds for SMC reset.

Tried with 4 different batteries and 4 known working screens.

Also with 4 different shells with working keyboards and SATA cables etc.

So we can rule out all of the above for faulty components. Tried everything.

So there must be a problem on the board that is stopping the chime and display coming on. We have tested the display connector and rails all seems good.

The board looks good under the microscope, so where to test for the rail that manages the boot sequence? I have 2 boards showing similar characteristics.

Fan spins then nothing only the fan spins. Driving me insane.

OFF TOPIC to any pro fixer. There is nobody who I can trust or is capable to repair these where I am so if anyone wants to fix 7 or 8 logic boards MBP 2012 & MB Air 2014-17 that I will gladly send them to you if the price is right. Some can be fixed and some maybe not. I have far to many sidelined units & spares here for comfort.