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In SMC bypass mode the board does the 3 second thing.
(Just checking, hold down power, plug in magsafe, hold power button for 10 seconds, release) Fan immediately spins but ony for a second.
I'm not 100% sure what you mena by " all big coils; especially PCH/CPU. " but this is what I've checked
L7310 - 3.2v
C1031 - 3.6v
PLT_RESET_L - no pulse
ALL_SYS_Power_Good - this was strange it might have just been my multimter, Most of the time it would pulseat 5.3v but We watched it for a while ( about 10 pulses) and it would range between 27v - 4v
I'm not sure how did you get so high pulses, especially on PCH and CPU coils.
Use SMC mode to check for voltage, if then have 1s fan spining.
Trigger SMC_ONOFF_L and check, without disconnecting power, once entered SMC bypass.
If ALL_SYS_PWRGD and mentioned coils pulse, then check at pins 7 & 3 of U1950 too.
You've said to get consistent 1s power in SMC bypass.
Once in that mode, do not disconnect charger; machine will remain in that mode when you trigger SMC_ONOFF_L and get another 1s power.
This will help you to measure something.
But, if you note to get the same 1s power in normal mode, do it then.
SMC bypass helps you note when power appears, because of fan spins high.
"How do I Trigger SMC_ONOFF_L?"
You have more than 1 year on the forum, I didn't expect such question...
I hope this "How to" collection will help our members. I will possibly modify this post in time, if updates available. Trigger SMC_ONOFF_L (PMU_ONOFF_L): It reffers to simulate power button function, when the keyboard is not connected. We need to do this when we work with the board...