no startup sound and no display

David Anderson,here are a couple of things you can check.

LED Functions

LED #1

• Indicates that the trickle voltage from power supply has been detected by main logic board.

This LED will turn ON when you connect the iMac to a working AC power source. The LED will remain ON as long as the computer is ON or asleep.

• When computer has been correctly shutdown, the LED#1 behavior may differ:

• - If a startup event is scheduled in System Preferences/Energy saver, LED#1 will stay ON after a correct shutdown,

• - If no startup event is scheduled in System Preferences/Energy saver, LED#1 will turn OFF and will stay OFF as long as AC cable is kept connected and AC power source is present.

Disconnecting the AC cable and plugging it back will turn back this LED ON , even if computer is still off.

• After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty.

LED #2

• Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) 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 the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement.

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 the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement.

'''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, reseat or 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.'''

Symptoms

Won’t Start Up

• No startup chime.

• Error tones during startup.

• Grey screen with fan noise, or other noise.

• Will not progress beyond Apple logo or spinning gear.

Quick Check

1. Isolate OS by starting up from original install media for this computer, from a same-model computer in Target Disk Mode, or from a compatible known-good OS on an external drive.

Both AirPort and Bluetooth services are available when booted from the Install disk.

2. Reset SMC and PRAM to clear any stored,corrupted information.

3. Start up in Safe Mode by holding shift key down during startup to load only required kernel extensions and disable all startup and login items. See kBase #HT1564: Mac OS X: What is Safe Boot, Safe Mode?

4. If system generates error tones there may be an issue with the SDRAM. See kBase #HT2341: Intel based Mac: Power On Self Test RAM error codes

5. Identifying when in the startup process the computer hangs can help isolate the issue. See kBase #HT2674: Intel-based Mac: Startup sequence and error codes, symbols for information on the Macintosh startup sequence, and error codes and symbols used.

6. Run latest available service utilities

Deep Dive

Check

1. Boot computer to Apple Hardware Test on the internal hard drive or an inserted Install DVD by holding down the D key at startup. Verify if system boots up from any of these volumes.

Yes Run the extended tests and proceed with results. If AHT passes or boots with a

memory error, go to step 2.

No Go to step 2.

2. Remove installed SDRAM, and test with known-good SDRAM.Verify if computer starts up properly now

Yes SDRAM issue. Reinstall one of user’s SDRAM modules and retest. Proceed one by

one to find the faulty SDRAM module, and replace it.

No Go to step 3.

3. Wait 15 seconds after computer was powered down then disconnect AirPort card from logic board. Verify if computer starts up properly now.

Yes Go to AirPort Card Kernel Panic.

No Go to step 4.

4. Startup with Alt key to select and start from the Install DVD inserted in the optical drive, or from an external bootable volume connected. Verify if computer starts up properly from alternate bootable volume.

Yes Go to Hard Drive Not Recognized.

No Go to step 5.

5. Disconnect optical drive cable and retest. Verify if computer starts up properly now.

Yes Reconnect optical drive and go to Optical Drive Not Recognized.

No Go to step 6.

6. Remove coin battery on back of logic board (you will need to completely remove logic board to do this), and leave out for approximately 1 minute. Then reinstall battery. This will reset logic board. Verify if computer starts up properly now.

Yes Issue resolved by logic board reset. Measure DC voltage on battery touching battery with red probe, and grounding with black probe.

No Replace logic board.