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

Strange all this. This night the iMac was running with the screen on all night through (10+ hours), no problem. Without smc reset or anything, it just was more stable for no visible reason. But, VERY STRANGE: When I entered the room this morning, the screen went black on the spot in that very moment. I did neither touch the mac nor move the mouse, did not even come closer than a meter, the room´s floor does not vibrate or anything... I only turned the central heating down in the iMac´s room and will have made the air move a little ... (but no temperature change in the room temparature that fast, I can ensure you...). SO: What does that mean?! Either nothing and this was just a strange coincidence. Or: I am part of the problem... ;-)

Well, the problem can also arise while I am away, so I must not become superstituous.

BTW: I woke the screen up with the usual procedure. And: Covering the upper fan slots with one hand does not make the screen go black in our case. So even when I am assuming that something in the depths of the iMacs hard- and software seems to be in a very unstable state: I don´t know yet what triggers it.

Hello redbudgee, sorry for being late with my answer to your question: No, we did no clean install but only updated the system.

Maybe, i am to early, but I think, the problem in my case is solved.

I have done a clean Install with an USB stick made with disk maker x with yosemite 10.10.1) on saturday.

- start the Imac with the alt-key and choose the USB stick.

- When the installation program is displayed, chose the disk utitlity and format the whole harddisk.

- install osx 10.10.1 on the clean HD

First installation attempt failed, the imac froze. I really don´t know, what happened, maybe the system detected the replaced backlight board, that did not solve the issue (blackouts backlight) and did a firmware update or something. I am not sure. I had to power down the imac and start again.

The second time, the installation via USB was fully successful. Interesting: the email programm now has labels under the icons.

This wasn´t the case, when I updated from 10.6.2 to 10.6.8 and to 10.10.1

So i knew, something was changed by the clean installation of yosemite.

Because of that, the last hope: I did a long term test through the whole night until the morning with energy saving settings off and screen saver off.

My imac never was able to have a bright backlight after that, but now, it did it.

So the long term test went on. Sometimes, I did a restart or a went to standby. My girl friend used the imac even für work for hours and every evening, I used the Imac for many hours.

The blackout since saturday never appeared again.

I have no active edges or temperature monitor running.

Very strange: when I tried to do the clean installation a few days more ago, when the Imac was in the blackout stage, the backlight even went off when starting with the usb stick.

One of the following steps must have solved the issue:

1.) clean install by the usb stick

2.) the replaced backlight board in combination with 1.)

3.) the replaces v-sync cable in combination with 1.)

4.) 1.) +2.) +3.)

5.) maybe cleaning the internal parts of the Imac from dust with compressed air helped as well, because the hd temperature sensor only goes up to 51 degrees (before cleaning 57 degrees), though the Imac was´t very dusty

Just one information: after replacing the backlight driver board and the v-sync cable, the blackouts were few, but they were not gone.

Only the clean installation did it, but maybe, it wouldn´t be possible without replacing the the mentioned parts.

So maybe, the clean installation can be successful, but I don't know, because I did it, when I already replaced the parts.

Ulli, I wish you luck! Keep us informed, if anything happens...

What your solution would not explain is the fact that the same problem also can arise using bootcamp, right? Or are there some basic software parts/drivers etc. used by OS X and Windows in common, which could have been updated by your clean install? Being no specialist I have no clue.

I myself tried something completely different: Two days ago I simply removed the cover-grid from the downward opening where the memory is installed, and where the air is drawn in in by the fans. Assuming that a bigger opening would have positive influence on any temperature issues, just in case temperature is involved in our problem. And indeed: the computer is running smoothely since, for many hours, except one single black screen, hm.

I have no smc fan control installed yet, so I have no monitoring of the actual temperatures, sorry...

jrthurm,

i never used boot camp and windows on my imac.

But for both OS X and Windows, there is a so called EFI partition, loaded before the operating system (no matter OS X or Windows), that is rewritten by the clean install.

EFI is something like a newer bios to control the hardware and their firmware.

And the backlight is controlled by the SMC (system management controller), which is controlled and initialized by EFI.

So, it could be the case, that the EFI partition is corrupted due to different versions installed by different versions of OS X.

I bought my used Imac from the seller, that told me, he had just installed the OS X 10.6.2 from the dvd packed with the imac. Maybe, he did installed it without formatting the hd. Before the seller installed 10.6.2, there was of course a newer version of OS X. Then I installed the update to 10.6.8 without formatting, and then I went to 10.10.0 and 10.10.1.

Then on the hd, there were many different Versions of the EFI partition. If the installation program of one or more Versions of OS X would not write EFI cleanly to the hd, then this error could occur.

This is something, Windows PC user saw when using windows for a long time, upgrading to newer version of it without formatting the system. It leads often to problems with drivers and the stability of the system. In OS X, this could happen as well.

The Imac is an all in one computer with build in display and backlight. The backlight is controlled by the smc, not the monitor itself. So the imac is a very seldom desktop system, where a backlight error can be caused by the EFI or Software in the computer in general, that can be corrupted.

Maybe this can explain, why a clean install can solve the backlight issue in both operating systems

ive had the same prob with mine i installed caffine it stop the screen shutting down i have since found out that as soon as i install flash player it shuts the screen down so now i do not use flash player hope this helps

Imac just running fine without any error or screen blackout since the clean install.

No need of doing anything against the blackouts any more. No caffeine, no temperature / fan control or opening the memory slot for more air. The Flash players is installed und no problem. No active edges for the monitor standby and wakeup. Just nothing and it works.

My Imac is used very often and long. Display brightness at about 80%, it is the brightness level, where I got very often a blackout before the clean install.

Same issue here. iMac late 2009. Did a clean install of Yosemite yesterday, hoping that would do it, but it didn't work. Changing brightness is the only way I can keep the screen on or reproduce the issue. Change brightness to be > 50% and it goes black. At 20%-30%, it stays on (2+ hours so far).

Hello jdvelasquez,

sorry to hear that.

My Imac still is perfect after the clean install with USB stick after formatting the whole hardisk (important to format not just one partition).

So in may case, maybe because of the replacement of the backlight driver board or the v-sync cable in combination with the clean install the issue was solved.

The replacement of the parts without the clean install didn't work, too.

Did you really format the whole harddisk (the first entry in the disk utility)?

Thanks, Ulli. Yes. I formatted the entire disk. Just went through the same painful process again (with a flashlight on my screen), just to make sure. 😉

Same issue, unfortunately. It works ok with low brightness (25% at most), but goes black above that. Glad to hear yours is still running well! Next try might be the backlight driver board. But it drives me crazy, as I suspect this is really a software issue.

Hi jdvelasquez,

yes, this issues has driven me crazy as well. And after all, I can´t say exactly, if it was a hard- or software or a combination. I just did everything, that could help and in the end, it works until now. So, i really would like to help if I could, but at least i have my freedom with my Imac. Before the issue was solved, it would have been a pleasuare to throw the Imac out of the window. I just was one minute to go to a repair company when I thought, I have to format the whole harddisk because of my privat data on it. The fresh installation saved time and money for something, even the repair company probably would´t have solved. Good luck!!

Thanks, Ulli. Please don't feel obligated to respond. Just posting here as I continue testing things in an effort to understand what's going on.

One more reason I think this is primarily software related: Last night I did a clean install of the old Snow Leopard (10.6) and the screen works beautifully. Left it on at full brightness, w/o going to sleep, for almost 24 hours now, and no problems. Oddly, the brightness control slider (which existed in 10.5) is missing from the Display system prefs, and the brightness control keys on my keyboard didn't work either. Upgrading to 10.6.8 fixes these and all come back.

I just attempted to do a clean install of Yosemite again, now with a bright screen that was working with Snow Leopard. After loading the Yosemite image, just about to do the formatting of the disk, the screen went dark. Same issue. Is it a problem with kext files? What is loaded by the OS X installer at pre-install that could do this?

Hey guys,

*This Might Work*

I have the Late 2009 model.

I been having the same issues with screen going black for about 3 months now. I tried many fixes but noting seemed to work. The only way it would work if I had it set at the lowest brightness. Once I start adjusting the brightness, BLACK SCREEN! So I ordered the LED Driver board - installed it - and still the

black screen! Finally I tried to unplug the V-Sync cable (see below) and start up my Imac. BAM! no black screen, although there will be a high pitched whining noise if your brightness is set >50%. So my problems seems to be with a bad V-Sync cable. It works for now, I can deal with the noise until my new cable arrives.

Thanks, davidpak. Decided to take the plunge and do it. Ordered a new v-sync cable. The old one wasn't bad at all, actually, but replaced it nonetheless. Reinstalled yosemite from scratch, again. Had a brief hour or so, where I could have the brightness at a high level, but then it went black again. So no solution on the v-sync cable, at least for me.

Hey yvettegri, how I can reach you?

About the thread still running on Apples's forum, I trusted ur point of view abou the HDD Fun Control, I bought it and yeah, we came through the same way in this issue (having the screen turning to black constantly while the system still goes on, like iTunes keep playing music, etc). I replaced my HD and I do believe the screen goes black due to a misscontrol of something related to that.

Can you help me with HDD Fun Control setups? I'm running a iMac 2,66 i5 with the 10.9.4 osx.

Thanks in advance,

Cicero