iMac (27-inch, Mid 2010) — General Troubleshooting 30
2010-09-09
LED Functions
LED #1
• Indic ates that trickle voltage from power supply has been detected by logic board. This
LED will remain ON whenever the iMac is connected to a working AC power source. The
LED will remain ON even when computer has been shut down or put to sleep. The LED will
remain OFF only if the AC power source is missing or disconnected , if the logic board is
disconnected from the power supply, or if the power supply board is faulty.
LED #2
• Indic ates that the main logic board has detected proper power from the power supply
when the computer is turned on and that logic board voltage regulators are functioning
properly. This LED will be ON when the computer is turned on and the power supply
should be working correctly.
LED #3
• Indic ates 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
• Indic ates that the computer and the LCD 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 display, then the LCD panel, the LED backlight board, or the
cables between might be installed incorrectly or need replacement.