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Can a bad CPU prevent it from switching to 20V? this had liquid damage on many CPU mosfets but resistance to ground values were high so replaced all of them before working on the U3100/3200 area
You have battery connected, forget the charger; even you keep it connected too.
The board is powered from battery at this moment.
Check for pulse at the big coils...
With battery connected, check if SMC_ONOFF_L stays high?
If yes, then trigger it (as above described) and check for pulse at PM_RSMRST_L and PM_PWRBTN_L.