820-3115 shutdowns with Wi-Fi connected

[Jay]

G'day folks,

Repaired a 3115 with liquid damage and got it working by cleaning up a few small spots of corrosion and replacing U6990.
As long as I leave the Wi-Fi and bluetooth disconnected (J3501) the machine runs great and passes all AHT and ASD tests, boots to a flash drive etc.
With the J3501 cable connected though, the machine shuts off at random points during the boot process. Sometimes when the apple shows on screen, sometimes at the end of the loading bar. The only time I can make it consistently shut down at the same point is when I hold down alt/option to select a boot drive.

My guess is that alt-boot immediately forces it to look for netboot servers on Wi-Fi so it shuts down.

The liquid damage was not near this part of the board, everything looks great and it has already been ultrasonically cleaned. This chassis works fine, Wi-Fi included, with other boards, just not this one.

With 34 pins on this connector and a lot of possibilities as to where this may be going wrong, I have no idea where to start looking. The machine does not stay on long enough for me to take measurements of any kind.

Any pointers would be greatly appreciated!

 

[2informaticos]

Discard a software problem first.
Disconnect internal HDD and boot USB macOS.
If still the same, try new BIOS.

PRAM reset did (2-3 times)?

 

[Jay]

PRAM resets do not resolve, disconnecting the HDD cable does not resolve. I had already tried component isolation which is how I ended up with the Wi-Fi cable being the problem. Could you explain why the BIOS would be the problem?

 

[Jay]

Could the SMC somehow be responsible for this behavior?

 

[2informaticos]

Open airport module and try to find if is Wifi, or Bluetooth problem.

 

[Jay]

It's a combo-module, no way to isolate one or the other on the module side.

 

[Jay]

I have ruled out L3504 and the circuit that comes after that component. L3504 - R3551 and everything in between is OK.
With L3504 on the board, it shuts down as soon as Wi-Fi is being accessed, without L3504 (to connect that whole circuit to the J3501 connector) there are no shutdowns. Don't know where to go from here though.

 

[2informaticos]

Check if still shuts down in SMC bypass mode.

 

[Jay]

Not sure what an SMC bypass is or how to do that. Google shows me something different every search.
SMC reset doesn't resolve the issue.

 

[Jay]

SMC bypass: hold power button down and then connect power cord?
Fan went max speed but it still shut down once the boot picker loaded.

 

[2informaticos]

I was thinking a current sensor may be triggered, when wireless is started.
Seems that is not the case, if still fails in SMC bypass.

Compare diode mode readings on J3501 lines with known good board.
You said the airport works with other board, but try other module in this board; nothing to lose, just try it.