W520 will not power off, just reboots (hardware)

[Thinkfast]

Hi Experts!

I have a used W520 that's new to me. Several others in my collection from the same 2010-2011 period, but I've never had this exact issue. =

From a powered-OFF state:
Plugging in the 170W adapter powers on the laptop.
Bios setting for this is disabled - Using F9 "Setup Defaults" saved.

From a powered-ON state (AC):
While in BIOS settings screen, Pressing power button does not power down - it cycles the power and restarts.

From a powered-ON state (Batt):
Normal shutdowns, but only if you boot from battery and stay on battery (no AC).

From OS (Linux live installer boot USB, or Windows 7 on SATA):
"Shut down" or "Power off" results in same power cycling.

Effectively, the only way to power off is remove AC and hold down power button.

Other info:
System is stable otherwise - no crashing or odd behavior, running stable through a series of benchmarks and stress tests.
Have tried:
Updating BIOS (was very old 1.03, now 1.46).
Using 135W power supply from my W510.
Unplug, Remove battery, power button 10-sec.
Replaced CMOS battery


I am at a loss. Ideas?

[u666sa]

Connect lab power supply to it and see current draw during off, etc..