Chosen Solution
I was recently looking for a new Magic Keyboard and came across a lady who was selling the mouse, keyboard, and the iMac itself. She had the listing up only for the keyboard and mouse the iMac was free with it because it didn’t work, she told me she used it one time it worked and then went to use it the next and it didn’t. I got it home plugged it in, and I hear the chime noise along with hard drive and fans like normal just no image. So I pulled off the display and noticed that upon start up LEDs 1, 2 and 3 light up in normal sequence and then LED 4 will come on dimly, so I connected it to an external display and it displays normally. my question is if #4 LED is for the logic board communicating with the LCD why is It still coming on dimly with or with no display? Is the screen fried or is the logic board bad? To add if I leave it idle on the Lock Screen it does go to sleep and wake up like normal and at no point does the #4 LED on the logic board have full brightness
The fourth LED will be off or dim as there can be enough leakage in the logic to light it. In any case this is expected as the display is not connected. I use an extender cable to allow the display to be hooked up when I need to diagnose a display problem like what I suspect is your issue. The fact an external display is working tells us the GPU logic is OK. So that focuses onto the display assembly it’s self. So why is it not working? It could be a few things, bad display cable, burnt backlight LED string or the LED driver logic has a problem within the logic board or lastly the display cable connector on the logic board has a problem and needs replacing. I would carefully reconnect the display and make sure not the tug on the cable as that tends to damage the logic board connector. Using some tape to hold the display on, restart the system and using a flashlight placed on the display aiming its light at a sharp angle do you see the faint image of the desktop icons? If you do then you know the issue is within the backlight. Now shut the system down and carefully remove the display. Referencing this guide Step 19 iMac 27" 2017 Display Replacement this is the backlight power cable. Using a DVM check for power at the logic board connector. Reference the schematics and board view drawings to ID the pins. If you have power then the issue is within the display assembly or the cable.
