Chosen Solution
Hi, I have a 2010 27" imac. It has the latest OS on it and has always worked fine. In the last week the screen has started going black randomly. If I shine a torch at the screen I can clearly see the computer is still on and working fine, just with the backlight out. However! I also have my gaming PC plugged into the iMac and have been using as my monitor for years. When I command F2 and go into target display mode with the PC, the imac works flawlessly as a monitor and the backlight stays on all the time. If I swap back to the iMac the screen will be on but normally only for a few minutes before it blacks out again. I am puzzled and hope someone out there has an idea what this could be. Thanks in advance.
I would start off doing a PRAM reset. Follow this Apple TN: How to reset NVRAM on your Mac. See if that clears it. You also may want to look at your displays settings and hot corner settings making sure you haven’t set the screen to go dark or go into sleep mode. The fact the screen is not lit within OS-X, but is lit when you use the iMac as an external display points to a software/firmware type of issue when running OS-X. Update (11/21/2016) To replace the PRAM battery on this system will require pulling the logic board out. On the back side of it you’ll see a silver quarter sized battery. Here is an image for reference: Back side of ‘10 27" iMac logic board note on the top edge the battery. I would recommend bringing the system into an Apple authorized service center as this can be tricky if you’ve never gone into an iMac this far before.
