Q: iMac 27" Screen goes black repeatedly, but then settles for a while?

I want to share with my experience with you guys.

I am having iMac 27" 2011 and it started to black out since April of 2014. I've tried many different ways, including controlling the fan speed to reduce the heat, replacing the power unit and backlight board. None worded. There is one guy in the forum suggested that it is related to a SHUTDOWN signal sent from logic board that caused this problem, so I unplugged the vsync cable. At that time OS X 10.10 came out, and I installed the beta version immediately, and since then my iMac only went black once or twice. When it happens, it was just resumed from sleep or beginning phrase of usage, and not under the pressure of overheating or power overload.

My take is that it is a combination of hardware and software problem. When the voltage of the LED light is unstable, the driver tries to correct it. However the driver may adjusts the voltage in the wrong pace and cause the LED light to be off. I analyzed the .kext file from OS X 10.10 DP version 1-7. And it seems that there is a file called AppleBacklightExpert.kext that has been changed in most of DP versions. And the black screen issue happened during version 2-4. Currently the screen is very stable, haven't went black for weeks.

So, the final configuration that keeps my 3 year old iMac working is:

use Mac Fan Control to keep the PSU under 50C, GPU die under 60C

install 10.10 DP7

while the vsync cable is unplugged.

I am not an expert in EE, so I don't know exactly which one really saved my mac, or the combination stabilized the LED lighting, but it worked! I hope my experiments could solicit some great discoveries and solve this problem from the root.

i joined the club few days ago. iMac 27' mid 2010. It's not the heat issue. Reduce brightness and use ctrl shift eject comb. Everything else works. Did the test with Drive Genius, TechTool pro, DiskWarrior, ect. All tests are negative, RAM ok, video memory, ect. No errors! If I turn brightness to max., it goes black.

how much does it cost to get it fixed?

I am using iStat fan control and do not have an issue for days. I don't think that resloved the problem, but at least I guess it is a software glitch. Updated OS at 10.9.5.

I just installed TG pro app (Trial version) to see the temperature of my iMac 27In. What are the temperature levels that I should be aiming for?  It appears that when I dim the screen my computer isn't blacking out, but honestly, it's hard to see the screen when it's so dim.  This just doesn't seem like a good idea.

I installed Yosemite on my 2009 27in iMac on Friday morning and it hasn't blacked out since (Sunday noon now). Let's hope it continues that way. Anyone else with similar experiences?

Hello Michael

I have the imac 27 late 2010 and started to experience problems last week.

I did the fix you suggested, it is to soon to tel how well it worked but im typing this message on my imac wit the brightness at 80%. ill keep you informed, this fix took me les than 30 minutes so if it works that would be awesome. and it was indeed an LG panel.

Fingers crossed X

The connector fix didn't work for me, it stil goes black at random times, increasing the brighnes slightly seems to trigger it, increase the brightness a lot ironically seems to stop it for a while.

Late 2009 iMac Core i7 2.8ghz 27"

I have had all of the problems stated above: screen backlight would not stay on past half brightness level. I've had to keep my screen on the lowest brightness setting in order to have a functioning computer. It would almost instantly cut the backlight and i would have to prompt it to wake up from sleep (control+shift+eject).

After changing the backlight board it did not cure the problem.

Although i have just removed the vsync cable from the display to the backlight board and everything is now fully functioning. My brightness has been on maximum and I have had no problems since. Although I've yet to replace the vsync cable. I did see some of the contact points were damaged.

Hi Markus I have a 2009 27" iMac that's had the backlight board replaced but after a week in now goes to black screen again! Before I install Yosemite on it id yours still working! Cheers

Update:

My 27in iMac late 2009 could go three days without going black or black out every few seconds. I worked around it by having a hotspot in the lower right to trigger the "screen off".

It went black only once after upgrading to Yosemite in October but has not gone black at all since the upgrade to 10.10.1.on the 18th of November.

I hope it stays that way. Very happy camper :-)

SOLVED FOR ME!

I had exactly the same issue with my used Imac 27 late 2009 i5, I bought (used) at the weekend.

Sporadicly black screen going bright when wake up from sleep, the longer the Imac runs, the more it gets.

Used some fan control programs and pram und smc reset. Fan control was the best, but it happend again.

I tried to force the problem by stressing the hdd and the cpu by powering up to 100% load by some programs and it was going very worse so i could not realy work any more, except from my macbook via vnc. It was at the end as worse, that after wakeup the screen got black a few seconds. Even when starting the system the Screen got black after some seconds.

Then I remembered some trouble when connecting the power cable to the Imac. It was hard to put it in, so i had to press against the front, what causes the screen front glas to move a little an there was a noise, so I stopped puttting the cable completely in.

So I put the power cord in the back of the Imac aigain, and this time, it went much deeper in.

After this, the issue was completely gone. I stressed the system with 100% load for hd and cpu und now even when the hd was at 59 degrees, and the cpu at 80 degrees and the gpu also was much hotter than normal, the system went without any trouble for many hours, what wouldn´t be possible before. Then I deinstalled the fan control program and stressed again for many hours without any error, what also wouldn´t be possible before. The seller told me, there was never any issue with the imac and I believed him. So I realy believe, there wasn´t any problem except of the power cord.

I think, because the power cord was not fully in, the system wasn´t grounded und this causes the issue.

Maybe, this is helpful for you.

Good luck.

I experience the same issue with an imac 27 may 2011: black screen after a few seconds, wake up possible only after switching to sleep mode etc, as described by many of you. At the service center, where the problem could be reproduced, but no hard- or software issue be made out clearly, they assumed a problem with the screen and suggested an expensive replacement. I took the imac back home instead and was able to make the screen work again for more than 48 hours by performing a repeated NVRAM reset (p+r+command+option), did that once first, helped for a while, did it three times in a row - helped for more than 48 hours. But anyway: The problem returned again, after the next restart, I had the black screen again after an hour or so.

Did anyone apply the NVRAM reset with similar results?

And, probably more interesting: Did any of you check what console writes down the moment the screen goes black? Since I am no expert: Did it make any sense for you?

BTW.: I have the imac running the system DVD now without any problem - so I guess it is not the screen or not the screen alone that causes the problem.

And obviously this is not the machine I am using right now for posting this.

Thanks for any hints!

So, after I thought, everything is fine, yesterday the black Screen appeared again, even very often.

So I will open the Imac and observe the LED Backlight Power Cable and the Vsync Cable. Besides, I will take a look at the LED Backlicht driver Board. Maybe, I´ll see something obviously damaged. VideoCard is ok, because I see an Image when using a flashlight.

Just wanted to post and let everyone know I had the usual issue with my 2011 27" iMac.

The screen started going black. Shining a light into the screen you could still make out the screen. You could run the computer fine on an external monitor etc etc. The only solution is restart or putting it to sleep and waking up again. This started out happening every month, then week, then day then hour then minute until the iMac became unusable. Lowering the brightness delayed it. As did running SMC but eventually it would go.

I ended up buying a new iMac as I was due for a new one anyway which I am of course very happy. with but I recently installed Yosemite on my 2011 model out of interest. I had to do this with the aid of a torch so I could see the install fields and buttons .

Since installing Yosemite Today it has been running at full brightness all day with 0 blackouts. It could be that MarkusWinter and Callzhang's ideas are proving correct so far. Time will tell and I will update. I will be using the old computer for graphic design all day everyday to test it.

Im fairly staggered as I was sure it was a LED blacklight issue. I think probably this is still the case. That is a hardware issue which maybe apple been able to get around with software.

TL;DR: 2011 27' iMac. Unusable due to black screen issue. Now working for a full day at full brightness after full clean install of Yosemite.