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iMac Intel 27" EMC 2374 Hi, i have very annoying problem with my 27" late 2009 iMac (MB953LL/A), that have been mentioned here few times already, but nothing seems to help. Basically the problem is that my screen backlight goes black from time to time (i still see content behind black screen with flashlight), it can be fine for a day and next day it can turn off 50 times per hour. After i put display to a sleep and wake it, it starts working fine again. I’ve read here that someone solved the problem by changing V-Sync cable, i ordered one, changed, but that didn’t helped The next thought was about overheating, so we cleaned everything from dust and removed heat sensor from screen and just sticked it on SSD, we also removed V-Sync cable completely. But the problem is still there. I had this problem on a completely cold computer that was waken up after night. Any further ideas or recommendations? Thanks. Update Just heads up regarding problem. After i detached led temp sensor from screen, it’s been more than 2 weeks without single blackout. So i don’t think that the problem is in LED driver board. I have new problem just right after removing vsync cable and led temp sensor: The screens blinks (dims) once in a 5 minutes randomly, to be more clear i made a gif: http://i.imgur.com/zUXXFvK.gif. This problem is somehow related to V-Sync cable or this might be more serious problem?

Have you tried replacing the LED driver board which is the power source for the LCD’s backlight? Here’s the IFIXIT guide: iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement. Update Further investigating has pointed to a high current load of the LED strip within the display which burns out the LED driver board. This also can create a momentary flicker effect going off every 30 or so seconds. Sadly, both the display and the LED driver will need to be replaced to properly fix this.

I had this issue where backlight would go off randomly, sometimes would work ok for an hour or so then go off (shining torch on the panel showed desktop). I measured the backlight board supply, PWM and BLON pins while in the fault state and these were in a state where backlight should have been on but wasnt. So in my case definitely the backlight board (inverter). The board is mostly solid state but there are 4 caps on it and it runs pretty hot, so I removed and tested the caps half way down the board and these measured ok, but the two near the power connector at the top seemed iffy. So I changed them - 270uF @35v (didnt have correct values so used some a little bigger) and its been ok since. I know theres some variations on this board so might be different fault on others, but mine was V267-604HF Hope this helps someone else. Its about £40 for new board and caps are a few pence so this is a cheap fix if you have the skills. UPDATE: This didn’t last - although did last a fair while. In the end it was the backlight strips in the LCD itself! I was fortunate enough to find them on AliExpress for very little money. A right fiddle to change though not for the faint hearted! The LEDs on the strips I removed had cracks in them when viewed with a magnifying glass even though they all worked. And measuring current draw on individual LEDs, many drew more current than others. So the backlight board is switching off due to excessive current draw.

Just use another monitor till you buy a new computer, I have my macs all hooked up to large screen tv 45 inch or better. I can sit back and work, with a wireless keyboard and mouse I can sit in a good chair and use a coffee table.

27" Imac Mid 2010 running El Capitan. Started having the same issue when I updated to Sierra last year. Reverting back to El Capitan solved the issue until this morning, where it started flickering before going completely black. The addition of “SMC Fan Control”, and operating the fan speed at higher RPM’s brought the screen back. Brightness must be 100%, otherwise it still flickers. It definitely seems like a heat dissipation issue, all things considered.