Lenovo Thinkpad T540P - Showing at 14" inch but NOT at 15.6" inch (Backlight issue) ?

[Acnologia]

...

Thank you for it, but i found a really cheap complete assembly and i changed the entire casing + the screen alongside, and paid almost 'nothing' so i have fixed it this way !

[m1ch43lzm]

Schematics: https://www.badcaps.net/forum/troubl...12308-2-please

There's a boardview for the W540, may be similar
https://www.badcaps.net/forum/troubl...m-1-ws-12291-2
Use http://boardviewer.net/ to open it

Page 34 is the LCD section, there's not much to it, only the voltages, the eDP signals, PANEL_BKLT_CTRL which comes straight from the PCH and BACKLIGHT_ON which comes from the EC "ThinkEngine" pin 123, there should be a 10K pull up resistor to 3.3V on pin 33 (R675) near the LCD connector

No easy way to isolate PANEL_BKLT_CTRL to bodge it to +3.3v through a resistor if you don't mind full brightness...

[Acnologia]

Hello to all members of badcaps.

Another brainstorming that i have been messing with for months as a side project.
Bought a T540p , from eBay as 'doesn't display' ,so i thought it was either backlight fuse, or screen issue etc.

I got it in my hands, first thing, the machine works fine boots to windows , in external screen all diagnostic are good.
Step 1.
Change screen (FAT - LED ) 1366x768 to 2nd one, that i know 1000% that is working ,as it has been tested.
Machine > Doesn't give display it's really dim.
Step 2.
Use the same screen but try and fix the backlight fuses, checked all of them and all of them are working fine.
Step 3.
Use another screen from a 14" laptop Thinkpad ,and the screen works at 14" , so i am guessing now the 15.6" will work , i am putting on the 15.6" and the machine again gives dim light.
Step 4.
Bought 3 x different codes of cables (in case i had the same issue as with the E15 Gen 2) , again all cables don't work at 15.6" but the 14" display internally just fine.

WHAT THE HELL IS THE PROBLEM ? haha.

If it was graphical issue, or display or cable or backlight it SHOULDN'T work at 14" screen ,so why the hell it doesn't work at 15.6" ?

I don't know what else to check.

Thank you for anyone that takes some time to check and reply...