iMac 27 A1312 dark screen (not black) no image

[Just.G]

Hello guys ,

I have a strange problem with a iMac 27 A1312 2009 , maybe someone can help me . The screen is dark , non completely dead , no image ( working on external screen) . I've already checked with another logic board the lcd ,video card and backlight board , all working fine ...

 

[TwoH]

Quite an interesting look on that..
Is the 4th diagnostic light turned on when powered on?
If it is turned on - chances are it's the actual backlight itself
If it is turned off - it might be the LCD panel.

 

[Just.G]

The diagnostic lights are on , all 4 .