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

I respectfully disagree with you and think my comment is entirely relevant. I do think your comment is out of place and kind of bullying, that is, of course, unless you own the thread? Are you in charge of the thread and arbiter of all the comments? If not, then I would assume all comments that are meant to be helpful are welcomed.

Instead of shut down I should have said goes to black, or the fan stops spinning, but the Mac is still on. Okay, does that clarify it? The monitor is black, the iMac is still on. Others do have a fan noise. Or, maybe the noise is of a higher frequency than you can hear and you just don't hear the fan noise? And as I said it happens sometimes when the hard drive has been replaced OR has malfunctioned and the sensor is not working properly. So your hard drive does not have to have been replaced, but it does happen more frequently with hard drives that were replaced.

I am suggesting a product that worked for me. I did suggest earlier in this thread that people try the free fan controller first. If that worked, then fine. However, the paid fan controller will work. I have subsequently received several emails saying thank you from people for whom my suggestion worked. Have you made any suggestions on this thread that people wrote to you to thank you about? BTW I have no financial interest in what I recommended and paid for the product I use that solved my problem.

I suggest you go to the link I posted and read their indepth explanation about the issues people here seem to be having.

Wasn't meant to be bullying... You said shut down, and that was misworded, as you clarified. You can easily see how I got to my comment, so don't take it the wrong way.

I went to their site. I didn't find ANY in depth explanation about the issues I'm having. Can you link directly to this explanation?

So you're saying that the backlight is shutting off because of a temparature issue with the hard drive (If I can connect all the dots)?

Now here's something weird.

I have been having all the same issues as described by other users.

I have tried lots of various fixes short of anything mechanical (eg: replacing LED boards).

I tried SM fanworks, which seemed to help for a little while, then the problem returned.

My most recent attempt at a fix, and I know it sounds silly, was to play with the sleep and screen saver functions, putting them on the shortest time period. I also dimmed my screen to the lowest level I am comfortable with, since doing this (2 weeks ago) I have not had the issue reoccur.

I don't know if my 'fixes' were effective or whether it is just coincidence.

I don't know if the issue will return. But for those of you struggling with the same problem, it might be worth a try - maybe it will work, or buy you a few more weeks...

Does anyone think this might be a virus or malware of some kind?

Good-Luck all!

Yes, I agree, my original comment was misworded.

Here's a link: http://www.hddfancontrol.com/imac-hard-drive-replacement-fan-noise-issue/

This is a problem that a lot of people have with not only the iMacs but with some Mac laptops, particularly the Macbookpro.

Apple started using harddrives that required a temperature sensor. The sensors monitored the temperature and when the iMac (or some Macbookpros) got too hot the fans would kick on. However, Apple used some really cheap Chinese harddrives in some of their iMacs. These had a high failure rate with either a complete failure, or the temperature sensor malfunctioning. When people whose harddrive had failed completely replaced them they replaced them with hard drives that did not have a temperature sensor. This caused the fans to not run, or the mac to think it was hotter than it was and to make the screen go black, make the fan stop, etc. You could click the mouse and the iMac would start back up. But, the symptoms would appear again, fairly soon.

I made an appointment at the Genius Bar and took my iMac in. They had it for a week, but iMac did not go to black. While I was picking up my iMac one of the technicians came out to talk with someone. He was wearing a jacket. I went up and asked him why he had on a jacket and he said they kept the tech room really cold because there were so many machines on. Of course, that was why my iMac wouldn't fail even though they had it running 24 hours a day for almost a week. The room was too cold.

I took my iMac home and within hours it failed. I decided to turn off the heat in my place, bundled up, and waited. The place got really cold and my iMac didn't fail the rest of the day. It didn't fail the next day either. I turned the heat on again, and it failed. I did some research and downloaded one of the free fan controllers. It work somewhat. I discovered that my iMac would fail at 54° C and above (the fan controll listed my iMac temperature in the top menu bar). That was the cutoff. I put a fan behind my iMac and ran it and watched the temperature go down to about 49°C. It didn't fail. I turned off the fan and watched the temperature go up. It failed again at 54°C.

A lot of people on some other forums figured out what was happening and came up with various fan controllers to solve the problem. Most are free. I've tried them all and they worked sorta. But, my iMac was still going to black occasionally. So, I broke down and bought the HDD because people on another forum said it worked for them when the free ones didn't. And it worked for me, too.

Hmmm... I'm fairly positive that some of us are having the same issues you were having. If you can still see an image on the display by holding a flashlight up to it (like I can) then the machine is still running and the display is still "on".... the backlight is literally the only thing that is off. I've had a hard drive temp sensor cause a temporary sleep on my G5 before, but this isn't like that. Everything is running normally. Even an external display still works fine.

Here are some other reasons why I don't think its temperature related

1. Sometimes it happens when I turn my machine on after being off all day. Ambient air temp of 20 degrees celsius.

2. My graphics heatsink and processor diodes are currently (as I'm typing this) running at 65 degrees celsius. The HD sensor is reading at 57 degrees celsius. Currently, the backlight is functioning normally.

3. The block diagram from Apple's iMac (Late 2009) Technician Guide shows that the only pathways to the LCD are the power supply, backlight board and then back to logic board (I've attached an image below).

4. External display still works and computer performs and functions PERFECTLY fine other than no backlight.

5. You can still see an image on the LCD by holding a bright light up the screen at a 30 degree angle or so.

So, yeah.... I'm not sure we are having the same issue. In your case, I think your machine was going to sleep to preserve the system. In a lot of our cases, the machine is still running like normal (playing music, videos, etc), just no backlight. Hope this clarifies it.

No, I really think its hardware related due to a short somewhere. Either in the LCD/Backlight connection itself or on the driver board. It may be getting too much power from the powersupply.

If my new display/backlight doesn't fix the issue, then I will replace the power supply next.

Hi there!

I started to have this problem few days ago, but unlike most people comment here, when it firstly started to happen, it already became impossible to work with. Now, if I turn on the screen goes off after the start sound.

If I wait  a few time i can put login and then i can put the itunes playing...

At the moment wont be easy to support the apple care costs, so when you have any info about the results post!

Thanks!

Yeah, while my screen never went half illuminated, it did do the completely off thing every once in a while.

While I replaced the LED Driver board (like I stated above) that didn't fix the issue, as it soon came back.

Yesterday I installed a new (to me, so it was used) LCD panel/backlight, and so far so good. No issues.

Hi,

can you tell how you replaced it? I managed to get the screen out, but the LCD is in some kind of frame (which the replacement screen doesn't have) and I don't seem to be able to get the LCD out of that frame. I removed the 8 screws holding it but still no luck.

Thanks for any advice you might have.

Markus

Took a picture:

Hi Markus,

You ran into the same issue as me... the frame is glued onto the LCD. You have to pry it off. It looks like it'll hurt, and it does take some force, but it does come off.

Good luck! My new screen is functioning normally (knock on wood). I also replaced my LED Driver board at the same time.

sooooo i have Mid2011 27inch iMac and Ive been experiencing this problem for about 3 months now.  I tried all the workarounds, fan control, switchresX,  etc.  I dont have applecare and I live in a small town in Hawaii and theres no Apple store/Genius Bar.. there is only a little shop that sells/works with apple products.

I finally took it last week and they have been trying to diagnose the problem for a few days.  The tech at the store called and said he opened it up and redid the connections blahblahblah he hasnt gotten any blackouts since so i picked it up, paid their diagnostic fee, came home and plugged it in.  SAME PROBLEM!  Today, i switched the display to a lower resolution and I havent gotten any blackouts yet..kindof annoying dealing with the black bars on both sides of the screen but its been wroking fine.. fingers crossed!

Dang it. Switching to a lower resolution here too "solved" the problem on my 27in 2009 iMac.

What does that mean?

Spoke too soon. Just blacked out on lower resolution. However when it was running for a long period the brightness was reduced, so that might be working better.

Btw I set up the bottom right corner as "put monitor to sleep" so when the screen goes black a quick trip to the bottom right and out again brings it back.