Black screen no chime

Dec 8, 2014
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Ok so i have a iMac 8,1 with a sensor control problem and the TMoP error (fans on full power, sensor apps don't detect cpu or gnu sensor despite being connected, stay on for about 10 mins) . After looking around it seemed like the logic board. The 8,1 had a much better screen than the 7,1 i have (7,1 was yellowish) so thought id swap it.

Well I went ahead and swapped over the screens ( and while at it the dvd drive too as the 2008 was a bit better)

So first of all I put back together the 'bad' mac (8,1 with the fans spinning and suspected bad logic board).

I fired it up and still with the fans spinning she booted up fine straight into Yosemite albeit with a slightly yellowy screen so I was happy at this point...

This is where it went wrong...

So then I reassembled my 7,1 with the odd and display from my 8,1 that id swapped...

Now it doesn't boot or chime, just the fans spin at a slow speed I only have 3 leds of the 4 lit as well

I then read online this could be a psu fault... its not as I stole that from the other one to test...

i then swapped the graphics cards between the 8,1 and 7,1 8,1 boots just fine 7,1 just has the 3 leds on it still ... im really stuck I don't know whats left except the logic board but I don't understand how I could break it by disconnecting just the lcd temp sensor and the lvds cable ?

Im at a real loss as to what to do now from having two decent imacs less than a week ago I have none any help would be great?

Now I have two imacs by the looks of it both with bad logic boards any ideas what else I could try?

Thanks in advance

Oh yea and now I have put it all back to how it started this morning and still no dice

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