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

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.

Great observation, thank you, redbudgee. But Yosemite is already running on our iMac 27/mid2011, and the issue we all decribe arose while using Yosemite. It also arises with bootcamp. It even has arisen (on bad days) during the startup process even before the apple appears on the screen. So: It is probably not Yosemite itself that fixes the problem. But maybe reinstalling the system or installing a new system has a similar effect like SMC?

So, I opened the Imac to see, if any Dust is in there und there was some, but not that much, I was hoping to find. Anyway, I cleaned him with a can of compressed air. I put the power cable for the backlight driver out and in again. Every board in the Imac i blowed, of course the fan´s as well and the cooling rips. When opening, I had to pull out the V-Sync cable, which was not in a good shape, but I hadn´t a new one. When closing the Mac, I tried to put the V.Sync Cable very deep in the slot, which succeeded hopefully without further damage,

Results:

1.) The HDD temperature sensor now increases only to max of 51 degrees, when using the Imac for hours. Before, it went to 57 degrees.

2.) Until now, there was no screen blackout, but i didn't stressed the system that much, only normal use of Safari and so on for many hours.

I hope, I will never see the black screen again and try to post the long term results in one or two weeks.

btw: I don´t really think, we have a software issue, in my opinion, it is clearly a hardware issue. I use yosemite 10.10.1, when the black screen appeared.

Thanks for your reply jrhurm,

Thats a shame. Out of interest. Did you do a complete wipe of your hard drive and install Yosemite? Im just listing all the details as it might help define the issue. In my case, thinking back this might have started when I installed Mountain Lion and continued after updating to Mavericks. For Yosemite I did a full reformat and install from USB. Im only mentioning this as maybe (and take this with a grain of salt as im not an expert at all) whatever .kext file or driver is helping this is only replaced on a full reinstall. Is it possible that the hardware still references some OSX /Apple drivers or software even when bootcamp is running Windows? Again a long shot as this is well past my knowledge.

I used the iMac all day today again with photoshop, indesign and illustrator and had no black screen. Windows black screen indicates it is hardware. Im certainly not convinced it is fixed though but it is most definitely improved. Previously about 10minutes was the best I would get.

Maybe there are multiple hardware issues here which would at least explain why this is so hard to pin down to one component. I will continue to update anyway.

Yesterday, it happened again, black screen.

- screen got bright aber waking up via active edge for only about 2 Seconds max

- restarting and shutting down the Imac didn´t help, black screen in 2 Seconds after start

- screen was alive when the backlight was set to the two or three lowest brightness levels and waking up via active edge

- resetting the SMC did help! After the reset and startup of yosemite, problem was gone

What does that mean?

- SMC defective?

- defective kext (device driver) for the SMC in Yosemite or because of corruptes kext by upgrading from snow leopard?

- defective kext (device driver) for the SMC in Yosemite, because Yosemite´s kext ist faulty?

When the Imac is in the problematic situation of black screen, you get always a bright screen when waking up or when the system starts (even, when ist lasts only für 2 Seconds, but there is always brightness for one moment)

If this would be a hardware-fault in the screen or the backlight in the screen, that wouldn´t happen this way.

I even don´t think, in this case, I have a faulty backlight driver board.

In my case obviously, there is something wrong with the SMC ot the SMC-Driver, that manages the backlight.

I started with the original snow leopard DVD in the situation of black screen and I saw with a flash light the installation screen.

So it used the old kext for the SMC from snow leopard.

This would mean, when the problem is with SMC kext from 10.6 and 10.10, then it is not a kext fault.

It may be a hardware SMC problem. I don´t know, why the SMC sometimes must be resetted. Maybe, it is to old, or the data ist gone. Is there a battery on the logic-board, that is faulty, so that the SMC memory is lost?

Otherwise, SMC maybe can´t be replaced with a new one, only the complete logicboard should be replaced.

Not easy to find the fault. Maybe I just reset the SMC for the future, wenn it is nescessary. Like bump starting an old Volkswagen

Ulli, thank you, precise description, and i find everything exactly the same with our iMac 27/mid2011. I did the SMC reset a couple of days ago (four resets in a row, even). While that had made the problem disappear successfully for more than 48 hours running time with screen on, when I did it two weeks ago, it did not help that long this time. The screen always goes dark again after a certain while (but not after seconds, as experienced before).

I have a solution that worked for me!! But first some background.

I have been dealing with the screen on my iMac 27" (Mid 2011) for several months now. In the beginning it would randomly just go dark and I could not get it to come alive without shutting it down and restarting. I soon realized that it was only the screen that was shutting down, since certain key strokes produced a sound (e.g., command-shift-escape). So then I found that if I clicked the power button a few times, then I would get the logging on screen. I reset the PRAM, etc, but no help. I called Apple support and they recommended fixing everything in Disk Utilities (permissions, repair disk), but that did not improve things. I was then using Mountain Lion and they strongly recommended updating to Yosemite. I eventually did but things actually got worse. I changed my security setting to "require password after 1 minute" so that I would not have to log in after every dark screen episode.

After reading many blogs, there was much discussion about heat being the cause. While the screen was bright, I put my hand over the vents and found that every time I put my hand over the middle part of the screen (back of the iMac, near the top), the screen would go black within a few seconds. Clearly, blocking the airflow raised the temperature sufficiently that it caused the system to shut down. You should try this!

Solution: As suggested by someone in one of the blogs, I installed "smc Fan Control", a freeware utility that allows you to increase the default fan speed for the three fans that are evidently in the iMac. The utility showed that the internal temperature of my mac was 49 °C (120 °F), which seems quite hot. I set the speeds of the three fans to ca. 2000 and within 15 minutes the temperature had come down to 39 °C (102 °F), still warm, but much better. Since doing this, the computer has not gone dark on me. I should note that even at this temperature, if I block the central part of the vent slot with my hand, the computer screen will go black within a few seconds. So, as long as I don't do this, I can now use my computer without the dreaded black screen.

Again black screen:

What I found out:

- in black screen situation even a clean install from USB leads to a black screen, so no kext problem

- in the situation, the Preference Pane for Monitor has no brightness control (saw via vnc on a Macbook)

- this time, the SMC reset did NOT work. Maybe heat makes the black screen effect worse and the SMC Reset did only cool down the iMac a little, so that the effect did not appear afterwards.

Now I try the fan Control again and turn the fans to 1500 rpm.

Maybe, this helps

The Question is: what exactly is damaged? The answer helps find out the costs and if it is worth. Logic board and backlight a few hundred euro. But, if this did not help? Apple itself would probably don´t find the exact solution and is much too expensive.

Very very difficult.

Hello,

did one ever cooked a backlight driver board in the oven, just like some do with their graphics cards to repair?

Maybe, this could help?!

I bought a used backlight driver board.

I will tell you the results in a month or so.

Just one short information:

since some hours, i have the black screen situation and i can work with low level of brightness.

When increasing the brightness to more than 4 blocks, the screen blackouts after a few seconds every time. PRAM reset, SMC reset, Shutdowns or Restarts do not help.

In system preferences in the monitor settings, for testing purposes, I just put the brightness slider very very fast (almost like crazy :-) in all possible positions for about 30 seconds to bump the backlight driver board a little.

Now, I have the brightness at about 65% for some minutes, which wasn´t possible before. Of course, because of the hardware issue, it will blackout again, but maybe, this is an new or helpful information for someone. I don't know, if this behavior is reproducible, I will see. This is of course no real solution, i wait for my used backlight board. I will replace the v-sync cable as well.

Sync cable and backlight driver board replaced.

and voilá

Black screen again.

******