I’m not sure, but it’s something to do with UE4 using UV layer 2 ( or 1, as it would be ) for lighting. The problem, as far as I can tell, is that even though I used UE4’s auto UV generation to make a copy of layer 0 into layer 1 ( or 2 ), it still didn’t correct this issue. Not sure why.
could be your uv’s are reversed then. I use Maya and have not had this particular issue except when my normals were facing funny directions.
FBX is an odd format, with all kinds of silliness happening.
Normally the UE4 takes the first uv channel as the lightmap layout, but sometimes those uv’s are overlapping. This will cause some problems (black spots on your mesh, entire black mesh,…). Sometimes you dont have to create the lightmap, but just when the first channel follows the rules of a lightmap (no overlapping uvs). This happens with all programs, not just blender ^^ Here you can get more information about lightmaps:
I dont think that my uv’s are reveresed, but I also dont have to create a 2nd uv channel for every mesh 
god, I knew I shouldnt have posted here now its happening to meeee >.<
But now you know the solution
So I had this issue for ages and couldn’t figure out what was wrong because my meshes weren’t lighting correctly even though I had created a secondary UV. from Epic told me how to fix it. Here’s what he said:
In the Mesh Viewer in UE4 make sure that you’re got the Destination Lightmap coordinate set to 1 instead of 0. This will be located in the details panel to the right.
Hopefully, that’s your issue.
Have you tried changing the Light Map Resolution AND changing Light Map Coordinate Index? That’s what works for me when this happens. Change the Light Map Resolution to 512 and set Light Map Coordinate Index to 1. https://learnunrealengine/fix-mesh-turning-black-issue-ue4-geometry-brush-converted-static-mesh-turns-black-building-lighting/
i tried all above but it just not work, in my case my geometry is combined multiple mesh, perhaps the geometry is too complex, i solved it by break it down to simpler models and export it one by one
I set light map 1, but when I tried to save it… I don’t know what turned it back to 0.[quote=“DavidNixon, post:28, topic:289186, full:true, username:DavidNixon”]
Have you tried changing the Light Map Resolution AND changing Light Map Coordinate Index? That’s what works for me when this happens. Change the Light Map Resolution to 512 and set Light Map Coordinate Index to 1. https://learnunrealengine/fix-mesh-turning-black-issue-ue4-geometry-brush-converted-static-mesh-turns-black-building-lighting/
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I set it to 1, but when I tried to save it… I don’t know what turned it back to 0.
Make sure you’re adjusting the Destination Lightmap Index, not the Source. Source is what it uses to auto-generate lightmap unwraps.