Bonjour je suis content de voir qu'il y a des gents qui mettent les mains dans le cambouis!!!!
Tout d'abord , si vous voulez vous aventurer dans le diagnostic de cette carte, cela vous amènera dans les coins obscures de la réparation! ( et il vous faudra un multimetre minimum....je pourrais vous guider si vous le souhaiter)
FYI - From the 2009 21.5/27 inch service manual
LED Startup Sequence
'''LED #1 = Power available'''.
If no LED is visible:
• Verify AC 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 MXM video card is seated properly
• Verify logic board
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 LED backlight board
-If external display works then verify/replace LCD panel
-If external display does not work verify/replace logic board
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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 turn off only if AC power source is disconnected or power supply is faulty.
LED #2
• Indicates that logic board has detected power from power supply when computer is turned on. This LED will be ON when computer is turned on and power supply and voltage regulators are working correctly.
LED #3
• Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then video card might be installed incorrectly or need replacement. Note: LED #3 is not present on models with integrated graphics.
LED #4
• Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED 4 is ON and there is no image on display, then LCD panel, LED backlight board, or cables between might be installed incorrectly or need replacement.