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Backlight randomly turning off

Hi, i have very annoying problem with my 27" late 2009 iMac (MB953LL/A). Basically the problem is that my screen backlight goes black from time to time (i still see content behind black screen with flashlight), it can be fine for a day and next day it can turn off 50 times per hour. After i put display to a sleep and wake it, it starts working fine again. I've read here that someone solved the problem by changing V-Sync cable, i ordered one, changed, but that didn't helped. The next thought was about overheating, so we cleaned everything from dust and removed heat sensor from screen and just sticked it on SSD, we also removed V-Sync cable completely. But the problem is still there. Besides, I had this problem on a completely cold computer that was waken up after night and with 100% running fans in a cold room.

Any further ideas or recommendations? Thanks.

yes i did smc and pram reset

iMac, OS X Mavericks (10.9.2), null

Posted on Apr 16, 2015 5:27 AM

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16 replies

Apr 16, 2015 6:36 AM in response to helgidub

I've got the same computer, and have had the same issues for over a year. Year before last Apple switched out my HD, installed a new logic board, new graphics card, and I have the issue. I have since switched out my LED driver board, as well as my v sync cable.


My iMac runs perfectly as long as I keep the screen brightness at 2 or 1 bar when using the F2 key, or F1 key to adjust. I have taken it in to our Apple store early this year, and they ran hardware testing overnight and could not duplicate my issues, yet when I brought it home, the next day it began intermittently, yet regularly going black. the reboot and waking from sleep lets me turn my screen down to the above mentioned bars, but that is my only solution thus far. I also have Fan Control, as well as Temperature Control on to keep track of my heat dissipation, as the top rear corner of my screen runs very hot at times, again, sporadically. There are times when I'm playing games when it stays cool, yet later, when surfing the web the fans will kick on.


I'm at the same loss as you are, and am almost ready to order a new LED screen and install it.

Apr 16, 2015 6:38 AM in response to John Travitzky

I've noticed that i had less 'blackouts' with reduced screen brightness, but i'm working with graphics, so this isn't a solution for me. My LCD is pretty hot too, especially bottom corners, it was something around 75°C at the time when we tested it. I also use smcFanControl, but it doesn't help, even if i run all fans on 100% speed. I think that this can be broken temp sensor or there is overheating problems with screen. iMac Screen aren't that expensive, even with shipping, i've paid like 2,5 times more for new GPU + install last year, so maybe this could be an option, i'm just scared of buying it and having same problem again. I could just buy a new iMac, but after SSD upgrade it works just perfect, so i'm still ready to figure out the problem. Thanks for your information John!

Apr 17, 2015 2:12 PM in response to helgidub

I purchased and exchanged the LED board and the cable as well, as there were signs of heat corrosion at the solder, (slight burning, and signs of melted solder) as well as a bent copper connector, of which there are 4. My model did not have the ribbon connector. I did this after the repairs were done by Apple, and the problems began. The actual repair initially was for the recall of the HD's on my model, in which Apple offered to replace it for free, under the recall. Once they had my computer, they replaced the motherboard, the radeon card, and the HD. about 5 days after the repairs were done, my fans started acting up, so I installed the Fan Control as well as the temperature gauge app. I called Apple, and they said it probably had nothing to do with the repairs, and that my warranty was for 30 days after on their repairs, so I let it ride. about 6 weeks later is when my screen began blacking out, requiring me to dim it down. At the time, I only had to take the indicator down about 2 bars rather than what I have to do now, which is 14 bars down. It has incrementally gone down with time. My iMac was out of warranty by then, and the repair warranty written on my receipt from Apple had expired, so I just lived with it for about a year, then, since my screen had become increasingly darker by necessity, I opened up my iMac and found the bent connectors as well as the solder burns. I used to do circuit work for an engineering company, printing Intel boards and cards, so I'm familiar with issues that can arise, so I ordered the LED board, and a new v-sync cable, that plugs into the base of the LED board and installed them. that was about 6 months ago. I still have the issues, but no more fan issues. That's about it, which is why I'm thinking of a new LED screen itself. Not much left to replace, unless of course, the heating from the initial repairs caused another issue. Either way, I have since taken it down for diagnostics, and Apple said they could find no blacking out, nor any hardware issue according to their testing. Since my iMac is 6 years old now, I've got plenty of use from it, and I'm very happy with it. But I still enjoy the tinkering from time to time, which is why I may pop for the new screen just to see what happens.

Apr 17, 2015 2:27 PM in response to John Travitzky

Thanks for the information John, i will place on hold my LED board purchase. Anyway, i think that this may be overheating problem, after i removed heat sensor from screen and sticked it on SDD, i got only few black outs and after increased my fans on CPU blackouts stopped at all. It's too early too say anything, but i will report on that after a week in this thread. I think you should try to do the same with your led heat sensor.

Apr 24, 2015 7:18 AM in response to helgidub

Hello Helgidub!

I'm curious as to your success on the moving of the heat sensor on your LED to another area; have you had any more black outs, and are you running at full or almost full brightness?

Also, do you keep your iMac on 24/7 or use sleep? I use display sleep after 15 minutes, and put HD to sleep after 20 minutes.

For 2.5 years my iMac was on 24/7, running 1500 desktop images, random every 5 seconds, with no sleep, so I'm certain that wore it down a bit.

I've got some spare time this week end, and would tackle this if you have positive results to share. I'm hoping your idea has worked, for both of us.

Apr 24, 2015 7:58 AM in response to John Travitzky

Hello John. The results are quite good: No blackouts since I moved led heat sensor from screen. Yes, I run full brightness, just as always. I got new problem though: screen blinks once in a while (once in 10 min or less), just dims brightness a bit, i made a gif to be more clear:

User uploaded file

Not sure whats the deal with that, but I think thats because I removed V-Sync cable, will try to put it back next month. There was no ‘blinks’ before removing led heat sensor and v-sync cable, so it might be related to that.


I use sleep at night, but since i’m working at home, my iMac runs more than 14hrs per day non-stop, I don’t use screen/hdd sleep at all.


Hope this helps!

Jul 2, 2015 1:21 PM in response to helgidub

Well, I was installing a new cable, my son was holding the glass while i was disconnecting the lcd cable, and i inadvertently pulled the solder from the connector on my logic board, (which I was going to replace anyways) rendering the imac useless unless in regards to video. I am going to order a complete new logic board soon, but meanwhile i have the new 27 inch 3.2 gzh turboboost to 3.9 ghz, with 32 gb of ram, and since I always have backups, (7 TB; 2 each 3TB, and one 1TB portable ) I lost only a couple hours installing apps. Wish i could tell you some different news, but I went to expeditious way and just bought the new iMac. I'm not rushing the old one now, as it's summer, and we're having too much fun with the grand baby.

Backlight randomly turning off

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