Chosen Solution
Hi guys, kinda old post but still hoping to get a reply (or even a better, a solution :D). Changed the video card on an iMac 2011 27" A1312 but it won’t power on. Tried a lot of things but does not seem to work. Diagnostic LED 1 is on, 2-4 not, and will not what ever I try. Its not turning on at all. I replaced the PSU and measured the pins on the logic board. I only get some voltage on pin 11 and 13. Pin 11 is giving me 12V and pin 13 3.97V. Its kinda weird cause pin 4 is not giving anything while LED 1 is turned on. Pin 11 is corresponding with LED 2, I’ve got 12V on that but LED 2 is not turning on… Also removed and replaced RAM. When I push the power button and remove the power cable and put it back and then release the power button, all the fans start spinning, so the power button is okey. When I remeasure in this state I get the following results: Pin 2-5 nothingPin 6 - 3.3VPin 7 - nothingPin 8 - 12VPin 9 - 12VPin 10 - 12 VPin 11 - 12VPin 12-13 nothingPin 14 3.3V There seems to be more pins with power but doesn’t help me any further. Hopefully some of you can help me out. Thanks in advance.
After a lot of testing, trail and failure, I finally pinpointed the problem to a short circuit. As soon as I tighten the screw from the heatsink GPU I get a short circuit and the whole system shuts down. As long as I don’t put in that screw everything works fine and runs perfectly :O Has anyone been in this weird situation before?
