iMac 2011 black screen - External display works

[Nihilius]

I have iMac 2011 model which has worked fine until today.

iMacs own screen posts no picture at all - only black screen. I have external Apple LED Cinema Display connected which works without problem. I can actually use the computer with external display as normal except iMac itself doesn't post any video. It seems iMac itself and all connected devices work. iMac just doesn't show any video. Unplugging DisplayPort cable also does not make picture appear on iMac screen.

When I go to Settings > Displays only LED Cinema Display is shown. It seems like iMac doesn't even recognise its own display.

Any suggestions? Already tried PRAM reset.

 

[matreya]

That it drives an external monitor fine means the GPU is OK, but it sounds like the main logic board has developed a fault, and it's way out of warranty, so you're going to have to live with it the way it is or replace the iMac. :/

 

[Nihilius]

Unbelievable.

I just ran Disk utility software. First I ran it for SSD which is my OS drive. Utility returned an error which said something like "this drive needs to be repaired with recovery partition" (might not be literal translation since my OSX isn't in English)

I just ignored that and then ran the same utility for my HDD which is used pretty much only for storage. At one point the screen blinked and BOOM - I had picture again on both displays Disk utility detected no problems with this drive.

I have no idea what happened. This might have been just a coincidence but no matter what I got my iMac working again

 

[Nihilius]

Okay the problem started again. This is getting really weird.

This morning I woke my iMac from sleep and again the main display is black but external display works fine. Naturally I tried using Disk utility again but it didn't work this time.

When I first woke the iMac I could actually see both displays in Settings > Displays. I could arrange the screens and when I moved mouse pointer to the edge of the external screen it clearly moved to main display. I mean the cursor disappears and it can move further away but naturally I can't see it since the display is black. After experimenting I restarted and now I can't even see main display in Settings or move mouse pointer to it.

Really getting weird. Doesn't really sound like hard drive problem to me now.

 

[JohannusNL]

5 years later since the first post at this thread, and just for the record; I got an old iMac 2011 27” from an only marketplace with the same issue. No video on Internal LCD, no issues on external videoport.
Replaced the GPU; issue solved.
Since it was a ‘known’ problematic AMD HD GPU I guess some of the internal, but not all, connections are loosened which might cause a partial malfunction of the GPU.