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@macnetic if it is okay with you lets start at some basics. this way we are all on the same page in the same book. Diagnostic LEDs The iMac (27-inch, Mid 2010) computer has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The LEDs are located on the bottom left edge of the logic board (under a piece of black mylar tape) and can only be seen when looking through the lower vents of the enclosure. Location of Diagnostic LEDs The LEDs are located under the mylar tape, on the bottom, left corner of the logic board. They can only be seen when looking through lower vents of rear housing
LED Functions
LED #1 • Indicates 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 • Indicates 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 • 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 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. LED Startup Sequence LED #1 = Power available. If no LED is visible: • Verify AC power source • Verify known-good AC cable is connected • Verify cable connection between AC inlet and power supply • Verify cable connection between power supply and logic board • Verify power supply LED #1 + LED #2 = Power available, and system is powered on. If second LED is not visible when power button is pressed: • Verify power button connection to logic board • Verify power button functionality • Verify cable connection between power supply and logic board • Verify power supply • Verify logic board LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found. If third LED is not visible after power on: • Verify that the MXM video card is seated properly • Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, check backup battery if necessary for proper boot up): • If POST boot chime is not heard, go to Won’t Start Up symptom flow, • If POST boot chime is heard, go to No Video symptom flow. LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found. If fourth LED is not visible after power on: • Verify cable connections between LCD panel and logic board • Inspect LCD display cables for cable damage • Verify external video functionality, and according to result check the following items: -If external display works then verify/replace the LED backlight board -If external display works then verify/replace the LCD panel -If external display does not work verify/replace the logic board Logic Board Test Points Some test points are accessible on the logic board when the LCD panel is removed. The test points are defined below.
Logic Board Test Point Function Pin 1 Ground Pin 3 5V power for Hard Drive(s). Note: If present, the 12V power will come directly from the Power supply to the Hard Drive thru the power harness cable. Pin 4 Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is off); corresponds to LED #1 Pin 6 Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup) Pin 11 12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2 Pin12 Power On Request signal (from logic board to power supply when power button is pressed) Pin 13 Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight Removing the Battery, Measuring DC Voltage
Unplug the computer. Allow several minutes for power supply to discharge.
Remove logic board.
Remove coin battery for 1-2 minutes. The coin battery is located on the back side of the logic board (see graphic below).
Measure DC voltage on battery touching battery with red probe, and grounding with black probe. If voltage is 2.7v or less, replace battery.
Reinstall the battery and logic board. This will reset the logic board.
Power on computer.
If computer boots, check for and install all software and firmware updates. After all this let us know what you found. If there is any visible damage on your logicboard. Post some images of is with your question. Again, use this guide Adding images to an existing question for that
