Chosen Solution
I have this imac that a friend gave to me as one that won’t power up. Long story short, I ended up replacing logic board and graphics card , and someone had taken the HD so I replaced that with one that had sierra already loaded. It will now boot with external monitor just fine, but lcd will not work. All 4 LED’S light up and backlight also lights. I got another lcd and backlight inverter with same results. I have also replaced the LVDS cable. I am not sure what to do next and am pulling my hair out on this thing ,so any help is much appreciated.
At this point I’m suspecting either the display is not the correct one or the LVDS connector on either the display or the logic board is damaged (given the fact you has replaced the LVDS cable already). As you’ll need to open the system again take some tight sharp pictures of connectors looking straight down the slot and at an angle face on from both sides so we can see if the connector is damaged (both). You might want to try another cable to make sure it didn’t get damaged somehow. Adding images to an existing question
