Dark Model - Possible Normal issue
 

I recently hacked together 2 models into one. As I was finishing up the model the model suddenly went dark except for 1 knee which remained fine. I looked at the normals and they all appear to be facing the right way, but when I move them around the model goes back to its original light value.

Does anybody have an idea of how to fix this?

The one on right has smooth on it, which greaty increases the polycount, which I can't do because it's for use in a HL1 mod, so the polycount is generally about 2000 not 7000+.

Thanks.