Chosen Solution
Happy New Year to you all. I have a friends iMac A1312 sitting here which suddenly displayed no screen and they then placed it in storage for the 6 months. I took it home a few days ago and when hooked it up, it continues to do the same thing. I can hear it start but the screen is completely black and can’t hear the Apple chime either. When I held a torch to the screen I can’t see the backlight. Over the last few days I have taken the machine completely apart and thoroughly cleaned it as it was super dirty but to no avail. Without the LCD screen attached to the iMac and turning it on to test I can see the onboard diagnostic LED’s on the logic board light up. However only 1 (from the left) and 2. Number 3 and 4 remain off. I can hear no chime/sound however it seems to me that the HDD and the Optical Drive are working. While cleaning I was unable to take the video card out as it appears riveted on one side. My guess is, its either the video card, or the LED Converter board (hope I said that correct) that’s bust? Update (01/12/2019) I solved this issues with the iMac working perfectly. I removed the video card, removed the thermal paste, baked it for 7 minutes on 200 degrees in the oven. I cooled the card, then sprayed it with electrical spray. I bought new thermal paste, also put new paste on the CPU. Put the entire iMac together and turned on the display, chimed and made its way to the desktop. :) :) Extremely happy!
Here’s a breakdown on the onboard diagnostic LEDs LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.
At this point I would connect an external display to see if the external works if it does then you know the LED Driver board has failed. If the display doesn’t work then you want to do one more test with everything connected restart the system press and hold the T key so the system goes into Target Disk Mode when you restart the system. Does your internal display show the FireWire & Thunderbolt icons on the screen? If it does then you know the dedicated GPU board has failed. Reference: Mac startup key combinations
