iMac doesn't start, but led 1 is on, what's the problem?

Alright, first , it started with vertical lines on the entire screen, making the mac impossible to use (but it was still starting, booting, hearing the sound ....), witch let me think to a graphic card issue.

So i took everything appart, helped by this great website, build it up again with all the same pieces), and then the mac just didn't start at all (no fans, no sound).

So i took off the screen again, and saw that the led 1 was lighting. But, if i press the on/off button, the led 1 just immediatly turn off and nothing happen. If i plug off the AC cord and plug it again, then the led 1 lights again, but still can't go further.

Time for voltmeter now. The pin 4 of the MB is showing 14.5 V (shouldn't it be 12 V ? Here in france we are using 220v AC...), nothing on the pin 11, and 4,5 on the pin 12. If i press the on/off button, led 1 turns off, but pin 4 is still showing 14.5, 0 on pin 11 and still 4.5 on pin 12.

Doc , i need your help !

i wish to believe that the MB is still alright and that the problem comes from the graphic card... Any helps anyone ?

Thanks in advance

Answered! View the answer I have this problem too

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LED 1 is always on when mains power is present. Go over all you connections again to ensure that you do not have one that is not correct. Many times on a second look a connector was found to be loose, not snapped in, ribbon cable not fully inserted and cam locked.

Follow some of the links at right to see how to further diagnose. From your description the GPU or internal display chain was acting up. The DIY test would have been to first attach an external monitor to see if the same behavior exists (if it did the problem would be in the GPU) if it didn't then the problem would be in the display chain… display most likely.

If you can get the unit back up and running start with the external monitor test, and observe all the LED lights.

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I've been checking all these connections since 3 days and nights, since i took it appart many times and rebuild it again, each plug looks fine. Actually i even tried to run the unit with everything unplugged ( at least with PCU and on/off button plugged), it didn't work.

Definitly, i should have use an external monitor first. And i should not live in the countryside too where you can't get a display wire within a week...

Thanks machead3, but I still can't find a solution to get the unit back.

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Actually, i rode something saying that the MB has a kind of security that won't let the unit start if the GPU is dead, can it be confirmed ?

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Damien will be eternally grateful.
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