The LCD display on my 27" iMac (mid-2011) went dead/black. It's a common problem. These are my notes on a software-only fix.
There is a 17 page now-locked thread1 discussing various black/blank screen problems with 27" mid-2011 iMacs. I attempted to post this information there, but the thread was locked. Additionally, for some unknown reason, the Apple site would not let me create a new post -- only issuing a cryptic "content not allowed" error.
I have an iMac 27" mid-2011 quad-core 2.7GHz i5 [iMac 12,2, A1312 (EMC 2429)], 16GB RAM. It worked without problem until about mid-2017, at which time I upgraded the OS to 10.12/Sierra. Not too long afterwards, the screen started to go blank once a week or so. A reboot would make the problem go away for a week or so. I initially. wrote it off to thermal issues, but I had no concrete proof to support that suspicion.
Th discussion thread above has multiple blank display problems. It seems like people lumping them together and hoping for a single solution is causing confusion for many. In my case, the display would go entirely black -- no light of any source. Additionally, careful inspection with a bright flashlight indicated that there was no image at all on the LCD. This indicates it was not just a backlight failure. Finally, the iMac continued to work fine otherwise -- I could screen share, ssh in, and access files it was sharing with other devices. It seemed that the issue was somewhere between the graphics subsystem and the display subsystem.
I tried the the holy trinity of imac problem solving: leave it unplugged for 5 minutes, reset PRAM, reset SMC. This never had any effect.
Over time, it became worse -- to the point that it would blank out multiple times a day. I discovered that lowering screen brightness until it was at or near the lowest intensity would increase the amount of time that would pass between blackouts. The display became unusable so I semi-retired the iMac to some menial tasks. Every now and then I'd try the display again, but it would often black out within 30 seconds of use.
Over the course of six months, I visited the thread multiple times. I saw a couple of mentions of people who encountered this after OS upgrades. I even seem to recall one in which the poster mentioned downgrading to a previous version. I had the urge to sell the iMac, so I figured I'd take one more shot at solving the problem. Mountain Lion (10.8) was the earliest OS-X version that I could download from the app store, so I tried that. Of course, 10.12 won't let you install 10.8 on the system, so I had to create an install image of 10.8 on a USB stick.
I am happy to report that I installed 10.8 and that it's been running for many days straight without a problem. During that time, it's been looping a 1080p mpeg video to stress the graphics and generate plenty of heat -- zero problems.
- iMac (27-inch, Mid 2011)
- AKA iMac12,2
- 2.7GHz Intel Core i5
- 16GB 1333 MHz DDR3
- AMD Radeon HD 6770M 512MB
- 2560x1440 resolution display