Broken iMac screen: need to mirror to external monitor.

What kind of cable are you using? Mini-dvi on the mac side but what about the LCD? If the LCD is VGA maybe you need a mini-dvi to VGA connection?

The default behavior is to mirror when a unknown monitor is attached. From the sounds of it I am thinking that the problem isn’t actually the screen but the video card. Unfortunately the video card is embedded into the motherboard which makes this really expensive to have fixed. If this was the case it would appear to work as you describe but obviously, no video.

The cable I’m using is a standard dvi cable from the external LCD to a mini-dvi to dvi adapter on the back of the iMac

@johnpowell, when I I say pitch black I mean to clarify that the backlight in the LCD itself wont even turn on, would this change your hypothesis at all or does the GPU control the actual power going to the display?

And have you tried letting it sit for a few hours? It could be that it is overheating. If that gets the screen to come back we can tweak the fan settings. Maybe the temperature sensor died.

Not sure if the video card controls the backlighting. It is possible that it might.

edit :: And are you sure the DVI cable works? I have spent hours dealing with this stuff when it was just a bad cable.

@johnpowell, I have left the machine unplugged for days trying to turn it back on to see if the problem went away, it did not it the screen was still black.

Would there be anyway perhaps to connect another Mac via Ethernet and tweak the desktop mirroring settings remotely?

If you can SSH into the iMac you can enable screen sharing on the iMac by running this in /Library/Prererences. This assumes that you can SSH into it. All I have right now to SSH into is a AppleTV but it would look similar except it would be ssh username@IP.of.IMAC

$cd /Library/Preferences/
$echo -n enabled > com.apple.ScreenSharing.launchd

If you haven’t opened it up yet, I’ll suggest to do that and find the connectors to the display parts and to the CFLs (flourescent backlight tubes) and reseat those cables. That might just bring it back to life.
Otherwise It’s likely that the power supply to the CFLs has toasted. Probably not replaceable by itself, but possibly replacing the whole power supply would work.
If you have a volt meter you could tell whether the CFLs are getting power (careful! high voltage – low current but high voltage).
Also if you have the case open you might be able to shine a bright light at some side angles and see a faint image on the screen. If you can see a faint normal image on the screen when you do this then it’s almost definitely the backlight CFLs &/or their power supply.

The video card driving the internal monitor went kaput; to detect the external, shut down, then boot while holding “option + Command + P + R” then wait until the computer makes its startup chime sound twice. Then release the keys. This should cause the computer to pass video to the active display (the external in this case).

i have almost the same problem . i get the wall on my on my monitor but thats it . i would like to turn the mirror but can get to icons . can anyone help . i’m new here i don’t know if this where the answers or if i can post a question

OK I had the same problem – my backing light was dead on my laptop and I had to work out how to set up mirroring so I could use a monitor. My solution in the end was to use a flashlight app on my phone, shine it at my screen, and that way I could just about make out what was on the screen for long enough to change the settings :) ingenious, I know.

Well stated, @Nadval, and the same (phone, flashlight, lamp, sunlit window) will work for any backlit panel whose backlight went out.
That’s resouceful thinking to use the phone which was at hand.

Welcome to the fluther !