iMac Intel 27". EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz), EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz)

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Why is my backlight shutting off randomly?

I have a 27" late 2009 iMac, and recently the display's backlight has been randomly shutting itself off. The display is working. I have put a flashlight up to it when it goes black, and can see text on the screen. The backlight itself isn't showing any other signs of being faulty. When it works, it works perfectly. The only thing that resolves the issue is to put the computer in sleep mode, and wake it back up. I've kept an eye on the the temperature, and nothing is running abnormally hot. I'm completely stumped as to why my screen goes black at random intervals, and have no idea what to do about it.

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Joe, what are the last three digits of your computers serial number? that will help us to identify your computer and its hardware properly.

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Well, I opened it up to see if any of the cables on the backlight board were lose, and the end of the vertical sync cable is shredded. So now the whole display doesn't work now. Is the Vsync cable something that can be replaced? I couldn't get the display far enough out of the case to really see, but it looks like the other end of it is hard wired to the display.

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Joe, your model most likely has an LED backlight. Follow the trouble shooting guide from Apple:"

Deep Dive

1. Verify boot chime present and fans running when computer powered ON. (Reset SMC and clear PRAM if necessary for proper boot up.)

Yes Power on self test OK.

Go to step 2.

No Go to Won’t Start Up symptom flow.

2. Verify if image is visible on built-in LCD panel, with adjustable backlight level.

Yes Video present. Verify system functionality and return to user or go to appropriate troubleshooting flow.

No Go to step 3.

3. Connect supported external display. Verify if image appears on external display when system is booted.

Yes External display detected by system. Video circuitry on logic board functional.

Go to step 4.

No Go to No Video symptom flow.

4. Verify if LCD backlight is ON by looking for faint glow from display when viewed in darkened room with brightness set at maximum.

Yes LED backlight is functioning.

Go to step 7.

No Go to step 5.

5. Shine bright (low heat) flashlight onto front of LCD. With computer powered ON verify if a faint image is visible.

Yes LCD panel functional but backlight is missing or dim. Remove LCD panel and inspect and reseat the following cable connections:

-Backlight power cable between backlight board and LCD panel.

-Vertical sync cable between backlight board and LCD panel.

-DC power cable to backlight board and power supply.

Replace any damaged cable. Reinstall LCD panel and go tostep 6.

No No image or backlight.

Go to step 7.

6. Power ON system. Verify if image is now visible on LCD panel with correct backlight level.

Yes Issue resolved.

No Replace LED backlight board.

Retest. If issue persists, go to step 7.

7. Inspect and reseat internal DisplayPort cable (between logic board and LCD panel).

Power ON system. Verify if image is visible on built-in LCD display.

Yes Issue resolved.

No Replace internal DisplayPort cable.

If issue persists, replace LCD panel. Run Apple Service Display Utility to verify the resolution of intermittent display flickering symptoms.

It does sound like a LED driver board issue more than anything else. I would replace that. Hope this helps, good luck.

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After getting everything back together, it turns out that the display data cable was oft unplugged. After fixing that, and destroying the Vertical Sync cable, the backlight issue is gone! My guess is that it seems that the issue is because that Vsync cable is so flimsy and fragile, it can corrode quickly. and the LED board is getting mixed and garbled data from the Vsync, that it has no idea what to do and just shuts down. So, completely disabling Vsync solved the problem.

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