iMac 27" Black Screen?

I had these same sort of issues and tried everything. Turns out it is a known issue and Apple has a replacement program for the bad video card in many iMac models. The bad card causes grey screens, black screens, sleep issue, hard drive slowness, system spins, kernel panics, and more! Wow, all fixed at no charge at my local Apple Store!

See:

iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support

Same problem, random black screen, stated in Mavericks continues in Yosemite. Had the video card replaced (re: stbodie). Black screen returned after a week. Have tried all of the solutions mentioned in this thread, with the exception of having all of the wires re-plugged in. Appears Apple is just ignoring this issue.

I'll throw in my two cents worth, untested. A friend rang me about this problem with his 27", mid 2011 iMac. Was unaware of this thread when I paid him a visit, but will be suggesting to him that if it is a software problem and something to do with controlling screen brightness, we should be trying Brightness Control, a small program which controls brightness.

See http://www.macupdate.com/app/mac/10429/brightness-control

I used it for years as a fully adjustable brightness control, not as a fix for the dark-screen problem.

Thanks.

Has anyone tried this yet?

This problem has got me – and Apple and their technicians – intrigued because of the variety of so-called fixes. After talking to Apple (Sydney) and the local tech for about an hour, this is how I understand matters:

1. The fault is not directly software related. Nothing to do with suspect code, except if the code somehow affects (in a significant way) a physical attribute inside the computer, such as voltage, current, or electromagnetic radiation (EMR), and that change in attribute causes the fault to occur. Brightness variation is just such a change because voltage levels undergo significant variation as brightness is altered. But the code itself cannot be blamed.
2. Not directly related to heat, either. The tech stated that the fault is more likely to occur cold. As an example, he blocked up the vents while testing a particular machine, ran a processing task that consumed all the CPU power, and watched while the Processor Temperature Diode (I think) ran up to 98º – but no fault. He also runs cold tests.
3. Could be related to the amount of EMR being emitted in certain areas of the video card. He gave an example (from his radio repair days) of a copper track making a right-angle bend on a circuit board. At the sharp point, radiation will be emitted much like from an antenna. The amount of such radiation will depend on voltage, current, and frequency of signals on the track, and may affect – cause arcing – in areas close by.
4. Apple's engineers may have an inkling of what the problem is, but if they do they are not sharing the info with technicians in Australia. The particular tech I was talking with has been pushing Apple for more info, and offering them as much info as he can, but in his words: "I've pushed and pushed, but until Apple engineers find what the problem is – nothing". He is still pretty much working in the dark regarding this fault. i.e. there is no definite fix as yet that points to a definite connector/component/track/whatever. They fix mostly by changing the video card, but other fixes can also appear to work.
5. Apple may not give the go ahead for repair until the problem is seen to occur AND the problem is detected by an Apple on-line diagnostic program, something called VST on the Apple Diagnostic Server. The tech hooks up the faulty computer, and based on the serial number of the computer a specific test is run.
6. It is possible that the computer can be faulty, but the fault not be picked up by the diagnostic test. From Apple's point of view that creates a problem. They would like to be able to fix the fault (which at the moment can't reliably be detected) by a defined series of steps (do this, do that), otherwise they are inefficiently spending money on repairs. But they also want to keep their customers happy. I can understand Apple's point of view. If the problem hasn't been definitely located, it is not sensible for Apple to simply replace all video cards on the off chance the problem may be fixed.
7. The tech said he is getting better at the diagnosis, but he still has to see the fault occur and have it detected diagnostically. He can't repair until Apple gives the go ahead.
8. Supporting evidence by means of a video of the fault occurring, preferably several in short succession, is good evidence which Apple may accept in lieu of the diagnostic test.
9. Affected computers are subject to a worldwide recall, iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support

To sum up: by all means experiment with various software fixes, but none of them go to the core of the problem. The problem is in hardware, but software can appear to fix the problem if the software changes some attribute (voltage, current, EMR), in a way that may be different from another piece of software trying to do the same thing.

An update, after trying everything I've read about this issue online, I finally solved it by installing the Owly app. Works beautifully, no black screen (hope I didn't just jinx myself) for 3 weeks.

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 had not gone dark on me. I should note that even at this temperature, if I block the central part of the vent slot, the computer will go dark within a few seconds. So, as long as I don't do this, I can now use my computer without the dreaded black screen.

What about just pointing an 8 inch fan on the back of the computer. It seems that would have the same effect. I tried that but nothing.

Now, my screen stays on for only seconds before going black, no matter how low the dimmer settings.

I'm using an external monitor in mirror display to use my late 2009 iMac 27.

Well this is strange, and may be good. I was minutes away from going to shell out $500 to get the LED display on my late 2009 27 iMac replaced, and I decided to give it one last try.

I hooked up the 27 as an external monitor to my 24 iMac via the mini displayport cable.

After being black for 4 months, the darn 27 monitor works! It works as an external display. Which means the monitor is probably not defective.

What does this mean? Thanks.

Says to me: faulty internal connection somewhere.

I've had my friend's mid-2011 27" iMac for a week now, trying to pinpoint the problem. It's subject to the recall, but the local Apple tech and I agreed that until we can reliably trigger the fault, it's best not to send it for repair just yet.

Another interesting thing: when this particular machine goes black screen, it can always be fixed by Opt-Cmd-Eject which puts the machine to sleep. Now, sometimes after about 5 seconds the machine automatically comes out of sleep, all by itself, and the screen is okay. But other times, I have to Opt-Cmd-Eject a second time.

The tech couldn't explain this behaviour. What could be the explanation of a machine in sleep mode, coming out of sleep by itself?

Also, I'm thinking of starting a new thread: not how to fix the fault, but how to induce it. If the fault can be reliably induced, it can be fixed.

I have used the Opt-Cmd-Eject method and the screen comes back on,when my Mac was at its worse i would Opt-Cmd-Eject and then within a second it would black screen again.

As i explained the other day mine is now fine as long as i keep the brightness on 3 blocks,anymore and it black screens instantly.

Not forgetting i am running Mavericks which is very stable,i tried to upgrade to Yosemite last week and after a while of using it,the black screens came back big time!!!

My monitor had that exact same problem.

If brightness went above 50% it went black. Then 40%. Then 30%. Then it would come on for only seconds and go black.

Now, it works just fine as an external monitor.

2 months ago I took it to the Apple repair shop and then another place. They both checked or changed something that cost about $200 (graphics card? blah blah panel, board?) , and returned the Mac to me, but both times the black monitor returned.

But the monitor DOES WORK as an external display.

And this just occurred to me.

My 27 iMac works as a monitor for my 24 iMac. But it does not work as a monitor for itself.

However, and oddly. The 27 iMac will send it signal to an external monitor.

The brains of the 27 will show visual to from another computer. And to another computer. But it will now show visuals on itself.

hi guys

I'm having this problem as well.. it's started this week about Tuesday

I've been told it could be "adware" and cleanmymac" and even that its my logic board battery.

so here goes.

i run SSD Fan control

Owly app

using a usb keyboard (a knackered one for the time been)

only using track and a wacom one tablet

disconnected my bluetooth mouse from my iMac in the other room and other things related

i do have all the plugs filled at the back of the iMac

will give it a go

and report back

You can add me to the list!

Since last week I too have a black screen on my 27" 2011 iMac. I have tried all the tips here; fan control, keyboard, singel user, etc. I did a clean install of Yosemite twice and also tried starting up from an external drive, but after just a few seconds my Mac goes black. Connected an extra monitor and that keeps working even if the iMac's screen goes black.

But now for the weird thing: if I start up Bootcamp, the Imac screen is working perfectly!!!

So, it looks like OSX is putting the screen to sleep for some reason, but why is it still doing that after a clean install?