Replace the inverter. The working external monitor clears both the logic board and the GPU. Look on the board for Apple part # 612-0073, possibly 661-5537.
"I’ve had the same problem on a 2009 27” Quad Core i7 iMac.
The display comes on but the backlight will not stay illuminated unless set to a lower setting (+3 on the scale).I have found that when the light nears going off, the 3.3 volts coming from the Logic Board to the backlight inverter board has sagged to 2.9 volts and suddenly drops to 2.7 volts when the backlight goes off.When the backlight has failed, I can use keyboard commands to make it sleep and then reawaken, but the backlight will not stay illuminated unless set to the lower illumination setting of +3 or less.I have found that if the computer is unplugged overnight, the display will run at full illumination for about four hours before the faulty behavior begins to occur again.I have read elsewhere that the 3.3 volt signal to the backlight board is what commands power to the backlight or turns it off. This voltage is supposedly generated by the Video Card.Others have claimed they have severed this connections and provided an alternate 3.3 volt signal to the inverter board so it always stays on. This will be my next step to see if the problem can be remedied.
Has anyone had any experience with regard to the 3.3 volts going to the inverter board?"
"I have this issue in my mid 2010 2.93ghz i7 iMac. I just replaced the backlight board this morning and its still exactly the same. The LCD is displaying an image, but the backlight wont stay on.
Any ideas? Do i need to replace the LDVS cable now?
Update (03/11/2019)
PS. I can get the backlight to come on briefly with the shift-ctrl-eject keystroke. But it doesnt stay on for more than a couple of seconds. This is with the old backlight board and the new one. Surely the LDVS cable wouldn’t react to a keystroke like this? What else could it be?"
