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Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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Screen black but external works OK

I'm debating on whether this is a dead LCD display or a bad video controller. Since the external display work (VGA adapter to Dell 19").

System Profile does not detect any displays connected, though the external connection *does* work.

I've found the video adapter 661-4673 for about $190, and this is definitely the cheaper of the 2 components. A replacement LCD is about 3x that. :(

I checked the graphics ROM version and it matches the newer version mentioned in the Apple support doc DL71

Any suggestions on diagnosing this further?

This had been running 10.5 and I'm in the process of upgrading to 10.6.

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I've removed the glass and bezel and have the LCD display in-hand. With the LVDS cable disconnected and the inverter cables connected, is it correct that the CCFL bulbs should light? What voltage should I be getting from the inverter?


I measured the voltage from the inverter at 0.14 V AC. This seems wrong. I tried powering on with only the inverter cables plugged in and the bulbs did not light.

I then tried with only the LVDS connect and I was able to see the Apple logo and spinning wheel then it went blank. I can't reproduce this now, though.

Is there some sensor involved with powering the inverter when the display is detected? Could the temperature sensor be preventing the LCD from getting power and thus preventing the inverter from getting power too?

I found a 2nd graphics FW update in Apple support doc DL938 but that won't install either.

It would be good to know:

* Does the inverter get power regardless of a display connected?

* What voltage should the inverter be producing?

* What effect does the temperature sensor have?

* Could a PRAM issue be causing the display to not be detected?

* Can I force a reinstall of either of the graphics FW updates?


Some scheduled updates install firmware so I won't worry about them not installing. It's good you took the time to double check.


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If the GPU/video controller was defective you would have no external video so I doubt replacing the video controller would fix things here. You likely have either a CFL issue or the inverter has gone.

Whats odd here is Profile doesn't show your display. Are you sure you were looking under the right heading Graphics/Displays Click on the heading and it should show info for both displays iMac and External.

Have you tried running Apple Hardware Test?

Looking at your display at an angle you should see a faint image of your desktop (with laptops you can often use a flashlight to shine through the Apple logo here you can't). If you can then you have video only the lighting of the display has a problem.

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The version of "System Profiler" I looked at was from the Snow Leopard install DVD. Once I had completed the 10.6 install, the installed version of "System Profiler" does indeed show the external display.

I used the flashlight trick on a laptop before. This screen looks completely blank. I've examined it with the glass removed as well. I don't think this is a CFL issue. Is the inverter serviceable?

The AHT is not starting on boot. I'll have to dig up the install media, I guess.

Thanks for the suggestions.


I would have held off pulling the display off until I had finished checking out the simple stuff first and doing some level of software diagnosing.

If you have the system put back together I would do the following:

Give resetting the PRAM & SMC a try here's the needed steps to do them: Reset PRAM & Reset SMC.

If they don't help it sounds like you have a LCD display issue. To isolate out where you should give Apple Hardware Test a try and see if it comes back with an error/s AHT. You will need to have the external display connected to see what it finds.

Let us know what you get - Good Luck!


I can't get into the AHT and my attempts so far to reset PRAM have failed. I have a Macally keyboard, but I don't find any evidence that the keyboard "Cmd+Opt+P+R" shouldn't work. I removed the internal battery last night and left it out. Hopefully I'll be able to tell if that was enough to reset or not.

I can't get to AHT either, using the "D" key. I'm trying to locate the original media. I believe AHT would be on disk 2, correct?

Could this be as simple as a bad LVDS cable?



I found this (haven't tested it) see if you can get it to work for you


Thanks for the AHT pkg. It installed and ran fine. The 1st run reported no issues with "extended testing" selected. I have a 2nd pass running now.


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I serviced dead backlight issue last week, more on that here:

there are three types of voltages going in the inverter- 24V DC main voltage, backlight ON/OFF signal (I refer that as backlight-enable in the post on macrumors forum) that should be 2-5.5V DC to get the backlight, and Backglight-intensity voltage (on the inverter pcb marked as VBR) that should be between 0-3.3V DC. check these and tell us your measurements to trace down the problem.

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Hi Donmoron,

Hope you can help me. I've too got an imac that won't display to the lcd but works fine on an extrenal LCD.

I've read thorugh your comments on macrumors and am clear that I need to take a wire from the 3rd diagnostic LED on the imac mother board but I get lost after that. What wire on the inverter cable must I connect that to? Which is the backlight enable signal?? and also when you mention:

(note that i disconnected the backlight-enable wire from the main logic board to not to fry the GPU and left it connected only on the inverter side). working perfectly!

Could you expand on this? Do you mean you disconnected the cable from the PSU side? It's just you mention logic board above but I think you are referreing to the PSU?

Many Thanks in advance for your help.

Kind Regards,



@Charles - How about creating your own question with your details maybe we can help.


Uh, I started this thread 11/12/12. This is ancient history now.


Sorry, I thought you replaced the inverter back in Nov. Did it not work?


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I have the same issue No Display on iMac LCD , but it`s working fine on external Monitor. Already I have second one (iMac) and I swapped all the parts one by one including the LCD as well but the second one still working fine with all of the first one parts and the first one still not display on LCD. ???

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What does shining a flashlight into the screen show you? Hold the flashlight on the glass at a sharp angle can you see your desktop icons? If you do the backlight needs repairs. If not, you likely have a bad display unit or the cable.

One trick here is to use TG Pro to see if the displays thermal sensor is responding. If it is then the cable is likely OK.


Hi Dan,

Really appreciate for your comment, but I checked the LCD display with the cable on the other iMac (Same model) its working fine. Also I swapped the other on on this one and got same issue. The iMac works perfectly fine just on external Monitors. Actually I swapped the inverter, power-board, LCD, LCD cable and the graphics card as well, but still no display and no backlight issue on LCD.


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