VERY weird Elitebook 8460p dim display issue

[RD-Chatt]

I have encountered one of the strangest things.

Machine in question: HP Elitebook 8460p

Was functioning ok until two days ago, built-in display goes extremely dark when plugged into power, is OK if you un-plug power and run on battery.

If battery is not installed, built-in display is fine.

I can boot all the way into Windows on battery, then plug power in and screen immediately goes dark. If I remove power and close lid, screen gets bright.

If I boot with power in, HP splash screen is so dark you can barely see it. As splash screen is before Windows drivers load, rules out driver, leading to suspect hardware / firmware / bios.

When built-in display is dark, external monitor is fine.

I’ve tried multiple things:

Checked Windows power settings for brightness when plugged in and on battery, both set to max brightness

Tried to adjust with Fn+F10 button (also tried F9 and F11), no effect.

Removed battery, held down power button, plugged power in without battery in. That seemed to fix it for about two minutes.

Updated BIOS

Updated Video driver

Turned off Fast boot

Turned on Fast boot

Turned on Virtualization

Turned off virtualization

Turned off power monitor circuit

Turned on power monitor circuit

I am sure I’ve tried other things

I found that if the battery is not in, internal and external works on plugged-in power just fine. Thought it might be the battery and changed battery with known good 8460p, same issue.

Swapped hard drive with another 8460p, problem stayed with the notebook. So is definitely a hardware / firmware / bios issue and not a Windows driver issue.

If I boot with battery in and power plugged in, and get to Windows screen, then remove battery, internal is dim.

Any ideas? I have Googled and googled and have seen some posts and suggestions, but none have worked.