Chosen Solution
Hi, I’ve gotten a broken iMac from 2012 and currently im trying to fix it. It was already at a local fixing store, where the employe could tell me that its most likely an issue with the logic board. My problem: The iMac restarts more or less instantly and while doing that the screen is showing a pure white picture, without any apple logo. Keyboard is not working (you can’t activate or deactivate caps lock), i’ve tried to reset nvram etc. I took the iMac already apart to check if i can see any obvious issues with the logic board, but couldn’t find any. While testing the RAM i’ve started the iMac without the screen attached and the boot loop was gone. At least i could just hear one chime sound. With the screen plugged in the boot loop started again to appear. All four indicator led’s on the logic board having light. Thanks in advance!
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.
Does a connected wired keyboard CAPS lock key light up when pressed after you boot the system with the display disconnected? If so, try the following. Disconnect the display so that the boot loop does not occur. Attach an external monitor and a wired keyboard. Power on the iMac while holding the option key on the wired keyboard. Do you see anything on the external monitor? If you see the startup manager screen showing bootable hard drives, then you can try to boot into your system and run some hardware diagnostic tests. If you don’t get anything meaningful on the external monitor it’s likely you either have an issue with your power supply, your GPU, or your logic board.
