Check the LVDS cable, or the DisplayPort cable, as it is called otherwise. It was the problem for me, although everything seemed ok. I had to put the cable under microscope to find out that one of the pins was missing (the first one). Although the display has a power cable for the LED of the LCD, if the LVDS cable won't connect the first and last pin, it won't light up the LEDS. I found out the hard way, after changing 2 displays and two motherboards. Check the picture
These iMacs have a dedicated slot for the SSD, so you can make your own Fusion drive, or just combine an SSD with your HDD. I did this to my iMac, but i did not want to merge them as a fusion drive. You should check the model, it is the PCie one. Check the picture
Finally i got my iMac working. The problem was that 1 pin of the LVDS cable (Or the DisplayPort Cable) was disconnected from the cable. So for anyone that might have a problem like this, with no backlight, on the iMacs 2012 or newer that have the backlight controller integrated on the motherboard, make sure that the Cable that connects the display data to the motherboard, is not damaged. I had to put it under microscope, because there was no way to find it out just looking at it, considering how small those pins are. Although the LEDs have a power connector, if the DisplayPort cable (LVDS) cable does not send the command to power on the LEDs, it will not power them. Take a look at the picture.
Elvis, ke dy zgjidhje. E para eshte te nderrosh komplet ekranin, por mund te kesh dhe problem daljen video nga logic board i iPod tend. Service qe merren me kete pune ka me bollek, ndaj nuk mendoj se do jete problem ta rikthesh ne jete perderisa funksionon.