No going to computer .. completely dead

brian kelley you do need to give us more information about what is actually going on with your computer. In the meantime check these things:

Start by checking your diagnostics LEDs. There are four built-in diagnostic LEDs on the logic board that can help troubleshoot the computer. LEDs are located on the front center of the logic board, above the IR board, to the left of the SMC reset button.

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.

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• First disconnect the AC cable from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since

• 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 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.

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 internal DisplayPort 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

No Power, Dead Unit

Symptoms

No Power, Dead Unit

• No power

• No fan spin

• No startup chime

• No image on external display

• No hard drive or optical drive activity

• Caps Lock LED on wired keyboard doesn’t light when pressed.

Quick Check

1. Verify power source.

2. Verify power cable.

3. Listen closely for signs of activity from system including: rotating fans, hard drive or optical drive activity, startup chime, etc.

Deep Dive

1. Power ON system. Verify if there is any indication that the system has powered up (fan rotation, hard drive or optical drive noise, backlight turns on).

Yes The system is powering up.

Go to Won’t Start Up.

No Go to step 2.

2. Disconnect AC cable Remove glass and LCD panel and locate the diagnostic LEDs in the middle of logic board. Reconnect AC cable to system. Verify if diagnostic LED #1 turns ON. You may alternately check for presence of a 12V DC signal between pin 4 and pin 1 of the logic board.

Yes Standby power voltage being supplied by power supply.

Go to step 3.

No Go to step 4.

3. Press power button while monitoring diagnostic LEDs. Verify if LED #2 turns ON and remains ON after releasing the power button. You may alternately check for a power request signal between pin 12 and pin 1 of logic board.

Yes Power supply functioning and logic board detects supply voltages and is powered on.

Go to Won’t Start Up.

No Go to step 5.

4. Disconnect AC cable. Carefully inspect the connectors between AC inlet and power supply, then DC connectors and cables between power supply and logic board, for damage or poor connections.

Yes Reseat misconnected cable(s), or replace damaged one(s).

No Replace DC power cable harness. Go to step 8.

5. Inspect power button and verify if it is properly connected to the logic board.

Yes Go to step 6.

No Reconnect power button.

6. Disconnect power button from logic board. Inspect cable and connector for damage. Using a multimeter (set as ohmmeter), verify continuity between the two pins of the power button when it is pressed.

Yes Power button is functioning correctly. Reconnect power button to logic board and go to step 7.

No Power button faulty. Replace rear housing (which includes power button).

7. Reconnect AC cable Press power button and Verify if system turns ON. You may alternately check for presence of a 12V DC signal between pin 11 and pin 1 of logic board when power button is pressed.

Yes Power supply functioning. If system still won’t boot, go to Won’t Start Up.

No Replace DC power cable harness. Go to step 8.

8. Press power button and verify if system turns ON.

Yes Issue solved with replaced DC power cable harness.

No Replace power supply.

Go to step 9.

9. Press power button and verify if system turns ON.

Yes Issue solved with replaced power supply.

No Reinstall power supply and replace logic board.

Let us know what you find out and what your diagnostics LEDs are showing.