iMac screen goes black, but system runs fine in background

I've had this issue for about two months, and like for many here, nothing worked. I was about to buy a new computer, when a friend sent me SMC RESET INSTRUCTIONS THAT WERE DIFFERENT, AND IT WORKED! All the instructions I'd seen before didn't mention you need to remove all peripherals, and holding down the power button after you've done so. This probably won't work for all, but I had to post in case it works for some. Here's the complete instructions:

SMC reset

1. Shut down the computer
2. Unplug the computer’s power cord and all peripherals (anything in the USB port etc. EVERYTHING)
3. Press and hold the power button for 5 seconds
4. Release the power button
5. Attach the computer’s power cable
6. Press the power button to turn on the computer

Then,

PRAM Reset

1. Shut down the computer
2. Locate the following keys: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4
3. Turn on the computer
4. Press and hold the Command - Option - P - R keys. You must press this key combination before the gray screen appears
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time
6. Release the keys

I hope this works for some of you as it did for me!

A sad update to this. This worked for an entire evening. Put the thing to sleep. When I got up this morning it went back to its old self. Tried the resets again, and it didn't work.

The one thing I'm hung up on is that when I did the PRAM Rest (when it did work) I never got to the second startup sound. No idea what that means.

I was experiencing the same sporadic behavior with my 2009 27" iMac before taking the machine to the Apple store to be checked. Ultimately, they had to replace the graphics card in the machine to solve the problem with the screen going Black.

Work around that has been working for me: I have a small fan blowing across the rear of my 2008 24" iMac which keeps the video on. When I forget to turn the fan on, the screens (both monitors) go dark after the iMac gets hot.

Thanks. I have an early 2009 24" iMac and a lat 2009 27" iMac. I have a fan behind them. I put the fan on them 6 months ago after the 27 started going dark.

I think if I had put the fan on them years ago, they would not be having a new problem every month. I do think an external fan is all to the good. CERTAINLY cannot hurt.

Just yesterday, after the 27 monitor working well for 4 months, the right side of it went darker. Always something.

Thought i would give everyone an update regarding my 2010 black screen problem.I have had this problem for the past 3 years,my brightness has to be set to around 4 squares in order to avoid the black screen.I have connected a second display via Display port to DVI cable.My Imac still black screens,however the external monitor stays on.

I have found this oddity also. My late 2009 27 iMac screen kept going black.However, the iMac would drive an external monitor.

For about four months, there was a very odd situation. The 27 would function as an external monitor to my iMac 24, and the 27 would drive an external monitor, but the 27 would never drive its own monitor.

My new 27 should be delivered tomorrow.

Hello! I have a problem. I have a Imac at my home and I turned on the power and everything is on, except for the monitor screen. The monitor screen won't turn on. I have been trying this several times. How can I get the monitor screen on my Imac to turn on?

Are you sure about this? I've done this twice.

Same for me, SMC reset, PRAM reset, different keyboard, no joy.

Mid-2011 27" iMac, 4 years no problems, last night the screen suddenly went dark. A bright torch held against the screen reveals that the mac is still running.

Off to the Apple Store.

<sigh />

Yesterday my Mac was also doing like that I have tried everything like pram vram smc and then with frustration removed battery and opened its back cover with screw driver. Unplugged all tiny mini cables n given some heat with heat gun replugged all cables fixed its back cover and now my MacBook is running perfectly. Remember guys i suggest u to do all these things very carefully because we aren't i repairing field

Hello everyone,

just wanted to add my own experience which may be useful to someone.

I'm on a late 2009 27" iMac which worked perfectly until a couple of month a go when suddenly every once in a while the screen started going black randomly. Tried EVERYTHING. multiple SMC \ PRAM resets with no avail. switched different keyboards, running fans at full speed. Nothing really solved the issue.

The only (partially) working solution has been to lower down screen darkness to 1/3 which worked pretty fine but that's not a solution as i'm a designer and that's definitively not an ideal solution.

Then a couple of days ago i switched some of my USB inputs and guess what? no more black screen (it's been more than 24hrs now, fingers crossed).

So, to add a new option to all those listed in this topic i would suggest this:

Switch your USB inputs, in my case seems like there's one USB port failing hence sending the screen in protection mode (off). Switch your USB inputs leaving one empty, try all and see if this helps.

Sorry for my poor english, hope this helps someone as this issue totally drove me mad.

I also have the same screen turned off issue on my iMac 2008. However, the apple authorized dealer told me that there is no parts available for 2008 iMac including video cards or led backlight board? They said that these parts are out of order and apple don't make it anymore.. ? Is this true ? I am very surprised no parts issue. any thought on this?

I have a late 2009 27iMac and over a year ago this black screen thing occurred. I tried everything, read every discussion I could find, took it to Apple repair and another repair guy three times.

The ONLY thing I have found that works is what is said a couple of times in these discussions. "Open it, disconnect all the wires you can find, and connect them back."

I also have had this issue with my mid 2010 27"iMac. After reading all the posts here, I think my problem has been resolved by installing the SMC fan control software. My iMac was at 40 degrees C (104 F) but is now running at 91F after setting the 3 fans to the middle setting. I also adjusted the brightness to about 60% from about 80%. I had previously tried the SMC reset & Pram reset to no avail.

This issue didn't start until I upgraded to the latest ver of Yosemite. I had no issue with Mavericks or the earlier Yosemite versions. Based on the poor reviews for ElCapitan, I haven't upgraded. I believe there is a video driver issue for the older iMacs (mine has the ATI 5750) and Apple hasn't addressed it.

Thank you to all that took the time to post. I will provide an update later if I find the black screen issue re-occurs.