Max Beck, here is what Apple suggest in how to test the components:
Backlight Issue/No Backlight
• No image
• Partial backlight failure
• Backlight flickers
• Display dim
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.
Check Result Action Code
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
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
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.
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:
-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 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 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
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
Thanks for all your comments! Can anyone explain how to check the Pin voltages on the board? I have a Multimeter and I would just check them. If there is some low voltage would that mean that my power suppy is not producing enough power anymore or what would be the implication?
I just don't want to spend a large amount on something (LCD-Panel, power supply or inverter board without beeing sure if it really is this part...
por Max Beck
Hi Max, sorry this is 3 years late, but you should get an a/c voltage at the pins, 0.012 volts.
por Jack Voorbij
Jack, it may be three years late for Max, but right on time for me. Thanks.
por Boyd Waters
and again now for me two more years on
por Stu Blair
and me.. but partially the lower right part backlight is not.working, what can i do?
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