backlight turns on for a short period

Hello guys,

I've replaced the backlight board by a brand new one, but unfortunately it didnt fix it. The sympthom is exaclty the same, i.e, right after the Mac is turned on, the backlight turns on for a short period (300ms lets say) and then turns off forever.

I suspect about some control signal. Since i already checked the ENABLE and PWM signals, the only one left is the VSync one.

I read on other forum that he mid 2011 no longer used a V-sync cable as it switched to an internal DisplayPort connection which integrated the V-sync into the main display cable.

Could a damaged vsync cable turn off the backlight?

Thanks!