820-00165, turns on but powers off at login screenf

[Hardwood T/S]

Have an 820-00165 with water damage on both sides of the board where the SMC is. Cleaned the corrosion and board powers on w/charger, but will restart after a few moments on the login screen and will not power on w/battery at all.

Before I start poking around at rails in the affected area, would this strange behaviour correspond to a specific rail or power distruption? Since the machine fully turns on and then fully turns off, how could I measure a faulty line? Unless the machine decides to turn itself off after x seconds of improper voltage on a certain line.
Anyway, glad to hear your thoughts!

 

[2informaticos]

First check what happens starting in SMC bypass mode.

"will not power on w/battery at all."
Doesn't turn on with battery alone and also battery plus charger?
Check resistance between pins 17/18 and 27/28 at U7100.
Post PPBUS_G3H exact voltage.

Possibly corroded resistors/traces, or bad solders around SMC.