Chosen Solution
I’ve got a 2009 27” iMac 1312 I get no video either externally or on the main screen. But if I unplug the ribbon cable from the main screen the external video works! Any ideas what it could be? Maybe logic board? The LCD was replaced 3 years ago.
With the display removed locate the onboard diagnostic LED’s located at the bottom left edge near the RAM bay. There is a piece of black tape covering it. Carefully re-connect the display making sure its not shorting anything out. Re-connect the power and turn on the system what do the LED’s tell you? Now disconnect the internal display and plug in your external what does the LED’s tell you?
