I was able to change the graphic card without unplugging the 4 cables behind the motherboard. It was a bit difficult because I only have 2 hands and had to hold the MB while unscrewing the GC but it’s possible. Thanks a lot for the trick about rotating the MB. It was useful.
Maybe you forgot to plug back some temperature sensor plug. Or maybe you damaged one of its little cables.
Great instructions ! I completed your step by step with some instructions about the same process regarding an early 2008 24" iMac with a Nvidia 8800GS (iMac8,1 - A1225 - 2211). I baked the videocard and everything is working fine now ! I'd like to add that, as I took my time to avoid damaging anything, this operation took me more than 5 hours.
On my iMac 24" (early 2008, Nvidia 8800GS), I had to unscrew 4 screws to release the graphic card from the cooler plate (don't be afraid, it's easy). Then I had to clean all the thermal paste from the upside of the graphic processor using small amount of 90º alcohol (holding the graphic card upside down so that alcohol doesn't pour inside the chips), unglue all the small stickers, then bake the graphic card during 7 min at 200ºC (graphic chip, up), then, after cooling down and reaching ambient temperature, apply small amount of thermal paste (a 2 mm nut) on the graphic chip, then reassemble everything. Voilà ! It's alive ! Infinite thanks to Jeff for his invaluable help !
On my iMac 24" (early 2008) equipped with a Nvidia 8800GS, there are 3 screws too, located at the exact same places than Jeff 's indications. Great job, Jeff ! Thanks again for your help !
RED: On my iMac 24" (early 2008), the motherboard has just one more long screw located in the middle of the motherboard, below the top one indicated by Jeff in his picture. All the other screws are like those indicated by Jeff.
ORANGE: On my iMac 24" (early 2008), the main power cord is marked «MLB» and is located in the wifi cables area. My iMac motherboard is a bit different than Jeff's one. But the differences are small. For instance, in my case, there is a small plastic fin above the memory slots that you'll have to bend a little to take out the power and left speaker cables. There is also a small cooler plate in the area above the memory slots.
On my iMac 24" (early 2008), this cable and the power supply one are covered by a plastic fin that you'll have to bend a little to extract the two cables underneath (left speaker and power supply).
RED: On my iMac 24" (early 2008), the plug for the optical drive fan is located just on the right side of the SATA bus plug. I'd like to publish a picture of it but I don't know if it's possible here.
ORANGE: On my iMac 24" (early 2008), the SATA plug is tighten by two little screws of the same size than those of the screen data plug. They are easy to remove. All the connectors are different so that it's impossible to mix them up. As said before, some of them could be a bit hard to remove but take your time, they will loosen.
I followed your indications with great success to revive my old iMac 24" (early 2008) by baking its burned-out Nvidia 8800GS !
This is a great step-by-step guide. Thank you very much for publishing it ! I'd like to add some commentaries : the 3 connectors (red point) are rather hard to unplug. Don't loose patience ! You have to pull them slowly with your nails and tweezers to avoid damaging the tiny wires. Don't be afraid, just take your time, because as indicated perfectly by Jeff, each connector has a different size.