Chosen Solution

@mayer Hello I have an iMac A1312 27 and the screen is totally BLACK not even glowing. I already tried a -NEW LCD -NEW INVERTER NOTHING WORKED The only LED that doesn´t lit is LED 4. If I put an external display everything is working 100% :) BY NOW I do believe that is something related to the INVERTER. but I tried a brand new one and nothing changed. Do you think that the cable from the POWER SUPPLY to the inverter could not be working properly?

I’d start by following Deep dive suggestions from Apple but would stop at #7 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 to step 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. If the Apple Service Display Utility fails, contact Technical Service Provider Support Anything after that, you’d have to measure some voltages on your logicboard.

Try it with an external monitor. This model has issues with the GPU solder. Let us know your results.

I have the same 27 inch iMac 2009 model. I get a clean start up with chime and fans running but can not get a video image. not even with an external monitor. I know the power supply is good, the backlight board is good and the video card is good as well. Any ideas anyone?