mrvisserplein, this is a little lengthy, but see if that helps:" LED #1
• Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac (24-inch) is connected to a working AC power source. 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 source is disconnected or the power supply is faulty.
• 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.
• 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.
• Indicates that the computer and the LCD display 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 display on the panel, the panel or inverter might be installed incorrectly or need replacement.
• Indicates that the computer has power but no video signal (e.g., the computer is starting up or the display has entered Sleep mode, turning off the video signal). This LED will pulse when the entire system has entered Energy Saver mode.
No Video, No Boot Chime, White LED ON (Symptom 1)
The computer will turn on (indicated by the front LED ON), but there is NO boot chime and No Video on the display. The faint sound of the fans, hard drive, and optical drive may also be heard.
1. Follow instructions in the “General Information” chapter to reset the SMC. Does the computer display video after successfully resetting the SMC?
Yes: Problem solved.
No: Continue to next step.
2. Verify that only supported SDRAM memory has been installed and that it has been installed correctly. Unsupported and/or defective memory can prevent the computer from booting. It may be necessary to install known good memory for testing purposes and replace any unsupported or defective SO-DIMM’s identified during this process. Does the computer display video after
verifying and/or replacing the memory with known good memory?
Yes: Problem solved. Verify full system functionality before return the system to the customer.
No: Replace the logic board.
No Video, Boot Chime heard, White LED ON (Symptom 2)
The computer will turn on, the boot chime can be heard, a white LED appears on the front bezel and sounds from the fan or drive activity can be heard, but the display has no picture or color.
1. Check if the computer is sleeping. Press the space bar to wake the computer from sleep mode.
Did the computer wake from sleep?
Yes: Put the computer to sleep from the Apple menu and wake the computer again to test.
Check Energy Saver setting to see when the computer has been designated to sleep.
No: Continue to next step.
iMac (24-inch) Troubleshooting — Symptom Charts 215
2. Reset your computer’s PRAM (Parameter RAM):
If your iMac is on, turn it off by holding the power button -- until the unit powers off. You should hear the fans go quiet.
-- While the computer is off, with your left hand, hold down the Apple key, the Option key next to it, and the letter “R” key on the keyboard. When you have these keys all pressed down, push the power button with your right hand and then quickly move that hand to hold down the letter “P” on the keyboard. Keep these four keys pressed while the computer starts up.
-- Make sure you hold down the four keys (P - R - Apple - Option) while the computer is starting up. When you hear the computer’s start up chime for the second time, you can let go of the keys on the keyboard.
-- If you didn’t hear at least two startup chimes, go to the top of this section and begin again. It’s important that you hear two startup chimes when performing this procedure.
This indicates that you have successfully reset the computer’s PRAM (Parameter RAM).
3. Does the computer display video after successfully resetting the computer’s PRAM?
Yes: Restart the computer from the Apple menu and make sure the computer is now working correctly.
No: Continue to next step.
4. Make sure the machine is powered off, and remove the access door and front bezel to access the diagnostic LEDs. Plug in and start up the computer to observe the diagnostic LEDs. At this point in the Power On process, you should hear a boot chime, and LED #1 and #2 are On. Is LED #3 On or Off?
LED # 3 is On: The video card and logic board are communicating properly. Go to the next step.
LED # 3 is Off: The video card and logic board are not communicating. Replace the video card. If the issue persists, replace the main logic board.
5. LED #4 should be On when the video card and LCD display have communicated properly to produce a video image. Is LED #4 On or Off?
LED # 4 is On: The video card and LCD display have communicated to LCD video. In this case, there is a problem with the backlights in the panel or the inverter. Replace the LCD display which replaces both the backlights and the inverter.
LED # 4 is Off: The video card and logic board are unable to communicate to generate video. Replace the video card. If the issue persists, replace the LCD display.
6. Unplug the iMac and remove the SDRAM. Replace with known good SDRAM. Plug the iMac back in and power the unit on. Does the front LED go off after a few moments now?
Yes: Replace the original SDRAM and test again. If the front LED does not go off with the only the original SDRAM installed, replace the SDRAM. If the LED is now reliably going off after a few moments, but yet you still do not have any video, go to step 7.
No: If the LED remained on, the main logic board is not communicating with the LCD panel iMac (24-inch) Troubleshooting — Symptom Charts 216
to generate video. Replace the main logic board.
7. If video is displayed normally on an external display but not on the iMac internal display panel,replace the LCD display."
Hope this helps, good luck