no backlight, screen functions normally when connected to other mac.

I have a macbook pro 13" mid-2017 A1708 here of a customer who dropped it off because the screen wouldn't turn on but keyboard does light up. First thing I did was hold up a flashlight to the screen and it turns out it does show image but backlight is not turning on, no matter the angle of the screen.

I just happened to have another laptop of the exact same model here of another customer that has flexgate going on. (the backlight not working when opened to 90 degrees but at 30 degrees it does turn on.)

So I took the opportunity to just switch the displays around of the two laptops.

When I connected the flexgate screen to the laptop that had no backlight at all, it too showed no backlight at all.

Connecting the screen of the laptop that had no backlight at all to the laptop with flexgate, the screen functions properly with backlight turning on in all angles.

I tried exchanging the display cable that connects the motherboard with the display pcb, still no backlight at all.

Thus I now know for certain that the issue is not the backlight itself, nor the backlight flex cables, nor the display pcb, nor the display cable.

Before bringing in the laptop, the customer had already tried out all the different key combinations to reset nvram, pram and smc. Just to be sure I also tried all the key combinations and the keypress sequence:

"Press the power button (OFF) once, bringing an invisible dialog box on the MacBook Pro black screen

1. Press the ‘S’ button - shortcut for sleep on Mac
2. Force a hard shutdown by holding the power button for 4 seconds
3. wait for about 16 seconds
4. Press the power button (OFF) to turn the MacBook Pro on again."

None of the key combinations work to bring the backlight back to life nor do the brightness function keys. Booting into safemode does nothing either. Backlight just stays off.

Any ideas as to what might be the issue here?

recap: LCD, display pcb, display cable, backlight and backlight cables, are all 100% functional but backlight won't turn on at all.

@danj

I have found the boardview and the schematic thanks to some googling.

I have been tracing out the logic and found the largest part of the logic behind the backlight power. There is no corrosion anywhere on the board, the caps in the large part of the backlight logic and those surrounding the display cable connector look to be fine, no physical defects to be seen nor is any of them shorted to ground. Inductors all look fine as well. Fuse is intact.