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

Here's my final resolution, tootights.

It was indeed the back light for the LCDs, but it was resolved in a roundabout way.

I took it in, it wouldn't act up for them. So I took it home, followed all their trouble shooting tips and then it did for me again at home (surprise, surprise) and I took it back in. They ran tests and decided to replace the LED back light panels (there are two of them, on for the left half and one for the right half of the monitor). Repairs took several days, I picked it and it seemed fixed. About a week or so later I got a wonky flash and the brightness went up then down and when it settled down the left half was noticeably dimmer than the right half. So back in I go. They could see the left being visible dimmer, so this time they replaced the whole monitor assembly and all has been fine to date.

I actually don't think its the backlight, although it could be. JJ, your story leads me to believe that its the cable between the backlight and the backlight board... you know, the only one you can't replace yourself. When I replaced my LCD unit with a used one, my machine has worked fine ever since, but I saved my old panel.

I took it apart and the cable end that attached to the panel (and not the LED driver board) seemed loose and looked discolored.

I looked for just that replacement cable, but couldn't find it.... so, I think thats where these issues lie. Just my two cents anyway

jjhaffner, was this under warranty. That job had had to be over $500.

Wincy,

Yes, I had the Applecare warranty.

I have just received a brand new 21.5" iMac, as of May 25th. I am seeing the same kind of thing with this new unit. I will move the cursor to go to some file or to the menu bar and it suddenly blacks out. It's just happening now for a second or two and comes right back on but this is a little disconcerting.

RY

I had my iMac since December 2012. It just started to do this today. I found this article about zapping the PRAM (which I knew about) and reseting the "SMC" (system management controller, which I didn't know about).

http://gigaom.com/2011/07/26/when-to-reset-your-macs-pram-and-smc/

I did both (it recommends both in a row), and so far, my iMac stopped winking out. It started to do it every minute or so before I got fed up and searched for this. I don't want to have to bring it in for service, as I need it for work. I will keep you posted to see if the screen turning off comes back.

(iMac 27" i7 2.93 GHz, 12GB Ram, 2 TB, OS 10.6.8)

My early 2012 iMac has the black screen. It started a month or so ago. I saw the advice on dimming the screen. This cured it for about six weeks. It started happening again, so I dimmed again. It's been OK for anoher six weeks, but if it happens again, further dimming will make the iMac unusable.

This iMac is the first Apple that I have had that is "all-in-one" with the screen and computer in one unit. I think it will be the last. I may look at getting a cheap 27 inch screen and putting it in front of the existing screen. Then I'll save up for a MacMini to go with the new screen.

In over 30 years sing Mac products - started with a Lisa - this is the first expensive failure that I have had.

I totally agree with yvettegri. There is a problem with the LCD, Backlight bug making screen go black and there is a Heat/Sensor issue which makes iMac/10.8.3 go to sleep in a blink.

It so happens that my 24" iMac/10.6.8 worked thru daily 12 hour sessions playing & recording audio nonstop (FW audio interface) for years without a wink of a problem and suddenly signs off to sleep mode with fan noise going thru the roof in 85º C temperatures two days after I upgraded HDD & 10.8.3 and it's still going on.

I have got to believe this is a Heat/Sensor issue from HD upgrade combined (maybe) with OS Mountain Lion upgrade from Snow Leopard (ahhh the good ol' times) on previous HD. I have smcFanControl which kicks in around 58º C but I know iMac is going to hibernate as soon as I see 78º-80º C on my Menu bar. Audio interface is cutoff, fan noise reaches peak level, screen goes black and then ... silence. I wait a few seconds and I hit my keyboard for immediate wake of computer. I might get audio back with standalone apps without a restart but it's shutdown obligation and the whole restart process when this happens in the middle of a Pro Tools session.

Again, same iMac with HDD/OS upgrade. Either new WD HD overheats iMac and SMC puts it to sleep or SMC Heat sensor misreads "normal" working temperatures and shuts it down. After abandonning the audio interface as the source of the problem because of driver incompatibility with Mountain Lion , I'm choosing the Heat/Sensor/HDD option when I bring it to Apple Repair shop. Extremely frustrating situation.

Hey P -

I seems that I have the same problem you had - I've got all the symptoms; intermitant screen black, drive still spinning, etc. I believe that I have the same machine you had as well - mid 2011 27" Imac? I'm really glad to read that you found a way to fix it without handing the thing in to the bloody geniuses. I've never done a complicated repair on a Mac, much less bought parts online - would you mind sharing the part numbers, or whatever other identifying info you have handy? Or the Ebay supplier you used, and I can sort it out with them? I've looked for "backlight kit" and "LCD kit", and I'm still a little confused, I just don't want to buy the wrong thing...

Thanks so much in advance - your contribution to the thread was absolutely crucial, finally (after months and quite a bit of time and cash spent on Apple employees) I think there might be a solution...

Jerry

On my Mac it started after Apple replaced the harddisk as part of their warranty scheme (nothing was wrong with the previous harddisk, it was exchanged as a precaution). So I have to assume it is either a problem with the heat sensor on the harddisk or the interaction between harddisk and heat sensor.

I am a technician by trade (not apple) and have repaired many laptops and desktop computers including Imacs. I have seen many times where someone has brought a computer in to the Genius Bar and have gotten a wrong diagnosis. If your computer is out of warranty they tend to generalize their findings and blame a lot of the problems on a faulty logic board or other component when all it needs is a good cleaning.

I came accross a 27" Imac with the same symptoms as described in this discussion and my initial thought was it was overheating and shutting down the backlight. The Imac has a number of thermal sensors built in and when one of components reach a certain temperature it is shut down. When I opened up the Imac lo and behold the fans were caked with dust. I cleaned out all the fans and heat sinks and the problem has disappeared.

If your computer is more than a couple years old and has never been opened this is most likely the case regarding this problem. If you're daring or adventureous enough or simply have no choice because of location there are walkthroughs available on ifixit.com that will guide you through removing the display which is really as far as you'll need to go.

The 27" Imac has three fans, two of which are visible when the lcd is removed, the third is behind the logic board. All you need is a vacuum hose (preferred) or a can of spray duster. Make sure you vacuum up as much dust as you can including the heat sinks and vents. You may need the spray duster to get to the third fan via the vent on the back of the imac. I personally took the whole computer apart to get to the fans and cleaned them thoroughly but it's not really necessary.

I hope this information helps.

Hi,

I have a late 2009 27in iMac. Apple did the Samsung 1 TB harddisk replacement, but since then I have the problem that the monitor keeps going to sleep. It can be every 2 minutes, or it can be 3 days till it happens, but happen it will.

The Mac is running, I can see the desktop dimly when I shine a light on it, and by making the bottom right corner activate the screen saver I can get it back to working by moving the mouse to the bottom right corner and out again, thereby activating and deactivating the screen saver.

The problem doesn't seem to be temperature related. After reading

http://blog.macsales.com/16878-imac-hard-drive-upgradeability-expanded/comment-p age-1#comment-63865

I wonder if Apple put a "not completely compatible" drive in …

look further back in this thread, in particular the posts by pbook4g5 - his seem to be the only posts that describe the problem you (and I) are having, and go on to describe a solution that worked. His diagnosis is that a bad Backlight Driver board, or a bad LCD kit (display and backlights) is the source of the problem. He replaced both and it worked out. I have yet to execute his fix - waiting on parts. He has apparently stopped checking on on this thread, so I strongly recommend that you go back over the thread and judge for yourself.

Soon as I'm able to install the replacement parts I'll post again the results.

Hey Jerry,

Sorry for the late response. Like you've noticed, in my findings and research (from my own machine and one other where I work) I have seen no issue of overheating that has caused the backlight panel to be shut off.

I mean, I'm not ruling it out completely, but I've seen my computer's backlight shut off in the first minute of operation when the machine was still (basically) cold. So, I ruled out temperature pretty quickly.

I honestly think its either a problem with the backlight itself, or the power cable that connects the backlight/LCD to the LED driver board itself. Thats the ONLY cable you can't seem to buy on its own. So, maybe that's what is fouling these boards up. Either way, I bought my parts off of this ebay seller:

http://myworld.ebay.com/electronicmac/&_trksid=p3984.m1439.l2754

He currently has a few used LCD panels for 175 (but i made an offer at 125 and he accepted) and a few NEW driver boards. So that's where I would start.

When you're changing the LCD panel out, you have to take the metal frame off your old one and transfer it to your new one. Easier said than done... its not only held on by 6 (maybe 8) screws (can't remember), its also held on with adhesive. So, you have to pry... a lot, and with force. Hope this helps!

EDIT: My iMac has been functioning normally since the replacement, further leading me to believe temperature has nothing to do with (at least) my issue.

@pbook4g5

What exactly did you replace?