Chosen Solution
The iMac powers up soon as it’s plugged in, but I can shut it down and restart it with the power button. So the button’s fine. When it’s in standby, I read 0 V on pin 4, 12 V on pin 11 and 4V on pin 13 on the logic board. On the PSU it’s 12 V on pin 4 and 4 V on pin 5. After it has powered up, all readings on PSU and Logic Board seem to be normal, except there is still 0 V on pin 4 of the Logic Board (I don’t know if that’s normal). BTW: Circuit continuity testing for Pin 4 on Logic Board and PSU is positive. Any suggestions is it a PSU or a Logic Board issue?
On your logic board is a set of diagnostics LED’s. After shutting the system down what do they tell you?
Here’s a breakdown of the onboard diagnostic LED’s: 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.
