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Hi, I have an iMac 2009, 21,5" which has been working well until last week. First there was some flickering and sometimes a black display. Since yesterday the display seems not to work anymore. I have found the apple technicians guide for this iMac online and tried some of the solutions described there. I did a SMC/PVRAM Reset, but this did not solve the problem. To determine if it’s the gpu I connected an external monitor -> everyhing working fine on the external monitor. It seems that the LED backlight isn’t working anymore, because if you use a flashlight you can see everything on the internal screen. I asked at a local repair store but they told me (without looking at the device…) that it was either the LCD panel, which would cost almost 500€/650$ to replace or the SATA/Board (Logic board?) which would even be more expensive. My question is whether it’s possible to determine if it is the logic board, ta cable connection, the Backlight controller or the LCD-Panel. Thanks fpr your answers, Max

Max Beck, here is what Apple suggest in how to test the components: Quick Check Symptoms Backlight Issue/No Backlight • No image • Partial backlight failure • Backlight flickers • Display dim Quick Check

  1. Check for Software Update.
  2. Verify that System Preferences > Displays >Brightness control setting is above minimum.
  3. If no image on built-in display, connect external display to verify if iMac video circuitry is functioning. If no image visible on external display, go to No Video symptom flow. Deep Dive Check Result Action Code
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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: -Output cable between lower end of LED backlight board and lower end of LCD panel. -Vertical sync cable between upper end of LED backlight board and LCD panel. -DC power cable between upper right of LED backlight board and power supply. Replace any damaged cable. Reinstall LCD panel and go to step 6. No No image or backlight. Go to step 7.
  9. 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.
  10. Inspect and reseat LVDS cable between LCD panel and logic board. Power ON system. Verify if image is visible on built-in LCD display. Yes Issue resolved. No Replace LVDS cable. If issue persists, replace LCD panel. Retest. You can also check on Logic Board Test Point pin 6 to see what voltages you get there. Pin 6 tests Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup). Use a multimeter on the DC scale for that. To find the proper backlight board, it would help to get the last three digits of your computers serial number. Here is the guide showing you how to replace the backlight board. Hope this helps, good luck

I would suggest never going back to the repair shop that did not even open the iMac and told you it was the LCD or the MLB. I am an Apple Consultant and also repair only Apple computers, so I have been in quite a few iMacs. Usually ends up being the backlight board but always check the cables because I have had 2 that I can remember where another repair shop and even the Apple Store had done a repair not related to video issues but messed up the v-sync cable. The v-sync cable is very delicate and you can easily mess the pins up or crimp the cable causing intermittent or total loss of backlight. I know you have solved your issues but thought I would add this for anyone else searching on how to fix this problem.

Replace the inverter. The working external monitor clears both the logic board and the GPU. Look on the board for Apple part # 612-0073, possibly 661-5537.

Problem solved: Bought used LED Board and replaced the old one following the online guide. Seems that this was the reason for the black display ;) Thanks to all the helpful members who helped me a lot with finding out which part was broken!

I’ve had the same problem on a 2009 27” Quad Core i7 iMac. The display comes on but the backlight will not stay illuminated unless set to a lower setting (+3 on the scale).I have found that when the light nears going off, the 3.3 volts coming from the Logic Board to the backlight inverter board has sagged to 2.9 volts and suddenly drops to 2.7 volts when the backlight goes off.When the backlight has failed, I can use keyboard commands to make it sleep and then reawaken, but the backlight will not stay illuminated unless set to the lower illumination setting of +3 or less.I have found that if the computer is unplugged overnight, the display will run at full illumination for about four hours before the faulty behavior begins to occur again.I have read elsewhere that the 3.3 volt signal to the backlight board is what commands power to the backlight or turns it off. This voltage is supposedly generated by the Video Card.Others have claimed they have severed this connections and provided an alternate 3.3 volt signal to the inverter board so it always stays on. This will be my next step to see if the problem can be remedied. Has anyone had any experience with regard to the 3.3 volts going to the inverter board?

I have this issue in my mid 2010 2.93ghz i7 iMac. I just replaced the backlight board this morning and its still exactly the same. The LCD is displaying an image, but the backlight wont stay on. Any ideas? Do i need to replace the LDVS cable now? Update (03/11/2019) PS. I can get the backlight to come on briefly with the shift-ctrl-eject keystroke. But it doesnt stay on for more than a couple of seconds. This is with the old backlight board and the new one. Surely the LDVS cable wouldn’t react to a keystroke like this? What else could it be?

Solved! After almost 3 years, due to the fact I was unable to stop and take my Imac for Repair since I work with it to make my incomes. —— Basically the issue happens because the L9000 Circuit burns if you misconnect the LCD Display cable. I followed all the hints to check the LED Start sequence as you can see on this LCD Backlight not working = solved to this problem. I noticed that LED4 was not blinking on the startup sequence. —— I connected an external display using Displayport and the Mac was running fine on it. —— The next step was to buy the circuit from Digi-key, as mentioned by Yako on this Damaged display power header whilst replacing hard disk drive. You can buy the 220 OHM Ferrite Circuit here. —- Next, I took my iMac to an electronics specialist who confirmed that the Circuit was burned. He welded it again and TADA, LCD working again! —— SOME TIPS: —— a) The L9000 Circuit is below the GPU. So you will need to remove the logic board and the GPU (the whole thing with the cooler, etc.) Check the picture below to see where the Circuit is located

—— b) I Paid something like $10 for 5 220 OMH Ferrite Circuit on Digi-key. Link above. Here are some pictures after the burned Circuit was removed and the new circuit Welded.

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—— c) I paid $200 for the welding service and repair. —— If you’re facing the same issue and you’re in Brazil, contact RG Telecom. —- Thanks, everyone for the tips. Thanks a lot, everyone here for the tips and help.