using MIH81R Great Bear motherboard in non dell case (originally inspiron 3847 or 3647 or Vostro 3800 or 3900)

For anyone wishing to use the motherboard of the 3847 in a generic case:

(it is labeled MIH81R Great Bear 13040-1M and is also apparently used for inspiron 3647 and Vostro 3800 and 3900)

refer to this post on Tom's hardware:
https://forums.tomshardware.com/threads/dell-inspiron-3847-where-does-the-power-sw-reset-sw-hdd-led-power-led-and-go-on-the-motherboard.2876355/

with this diagram:

https://goo.gl/Ncroik

This almost worked for me at first try. Make sure you set + and - of your two wired pwr sw case connector on the correct side (if it fails you can try inverting them - I made that mistake). One thing that seems to have made a difference for me was to insert RAM, I don't think it will POST if you don't have any memory sticks in.

You may also need an extra system fan to remove the fan error at boot.

Hope this helps!