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

I'm reminded of "2001, A Space Odyssey" and their Failure Mode Analysis: Run it until it fails, then do an autopsy. If it's usually a week between blackouts and machine has been fine for well over a month, you _might_ have solved it. Not very helpful, I know.

That's the irritating nature of intermittent problems - absolutely the worst. If it's repeatable/predictable, it might suggest a solution. It took months to track this down because it only rarely happened at first. When it became more frequent and somewhat predictable, we substituted/replaced every video component between display and logic board - short of replacing display (with new cable/connectors attached). Was only desperation and the process of elimination that finally led to those connectors in my case. You might well be dealing with something completely different. Frustrating, to say the least.

Did you try wiping the hard drive and installing back to the factory disks? Evidently not. (Sorry but it's a questions that has to be asked here.)

Is anyone out there still dealing with this problem? Have you tried this possibility?

Since my last post (and quite a few hours setting up Windows on my iMac27 with this issue), The Screen Went Black Again, so MM appears to be correct here. I won't waste any more time with this. Off to get the monitor replaced. (I'm not about to attempt the procedure he suggested, so the replacement I believe is the required action, no?)

So I too have the same issue with a 27 inch Mid 2010 iMac. I have the Radeon 5750 and would have loved (and expected) to see a recall extended for this issue due to the large number of common complaints. My unit started showing the problem 4 months out of warranty. After reading this entire thread have to admit the more logical explanation is a software / firmware glitch in the display itself and not the connectors, GPU or other components which have all been intermittent patches based on the posts. I know folks that get their problem solved often don't repost the success stories and hope if anyone here does find a real solution shouts it loudly for all to hear. I have a background in troubleshooting complex electronic mainframe sized systems with millions of components. I'm not familiar with Mac hardware anymore but my first job was repairing old mac classics and the late 80's early 90's vintage units :-) Apple has always had the approach of replacing components until the problem goes away so there probably aren't many technicians capable of actually debugging this problem.

Here's my thinking. Like many other posts on this thread....no other issues observed when screen goes blank. Sleep and restart often works for a few minutes or seconds but rarely much longer.

So I booted and held down the D key to start the Apple HW test. I feel this was informative though inconclusive. I ran the short test a handful of times - no issues found. (note, I did this after having issues with black screen every 1-2 seconds after leaving the sleep/hibernate state). I then ran the extended test twice in a row allowing the second pass to go overnight.

Mind you the screen intensity is very bright during the apple HW test and didn't adjust with the keyboard controls. The screen never blanked on me for HOURS while running near full brightness and was still on the next morning. The unit was VERY VERY hot though. So I wouldn't recommend leaving the unit on overnight as I did...might lead to other issues. Another take away from this is the FAN was not even though the unit was tremendously hot. The self test includes a FAN test and I could hear the fan spin up and working. So very surprised it wasn't on and guess the folks that wrote the apple HW test disabled the fan intentionally.

I'm using the unit today with no black screens though I'm convinced it will come back. I'm keeping brightness as low as I can stand it as it seems a logical way to reduce stress on the display...but I have no reason at this point to believe it makes a big difference.

My main take away is this....

The obvious hardware replacements aren't working.

The problem has existed for some users for years and for others of us - only more recently. The software involved is probably a VERY flaky race condition in some firmware of the display circuit in the LED panel or LED driver that is going to be extremely difficult to debug even for apple engineers. What causes it to trip for each of us is probably very different depending on an infinite combination of hardware and software settings which is why changes that folks make seem to work for a period of time but are not "fixing" the underlying issue.

I was about to reinstall the old OS as others had suggested but I'm thinking it also isn't worth a bother. I downloaded the SMC fan control and will give that a try as a workaround. I haven't used it yet.

WHat I do think helps is to not HIBERNATE the unit. In all the debugging I did, the common action that seems to have had the best results is to actually SHUTDOWN each time I'm done with the unit and restart every time I sit down. I'll continue to do this, I'll try the fan software whenever I get black screens to see if it helps restore stability (but doubt it will help much). I also made sure my backups are intact in case the whole system outright dies. Don't forget the basics!

I welcome any similar comments and encourage folks to keep voicing their complaints. Eventually someones bound to listen to either the fact there is a real issue that needs to be addressed or to sales numbers for iMACs that will drop as word of mouth about the reliability spreads. These are very expensive units. Very pretty....but hard to maintain. Unless we hear some success stories posted...I wouldn't pay to bring these units in for repair though that was my plan until this week. If i have to spend any money at this point, it will probably be to buy a desktop unit since those are going to be much cheaper and easier to repair out of warranty.

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 :-)