Very weird lighting problem

[Brainer]

Also something that can make faces black is if it is considered single sided, as this will ignore lighting calculation on backside. I think some glMaterial or related functions solve that (some GL_FRONT_AND_BACK thing...). Normally every model face should be 1 sided anyway with glEnable(GL_CULL_FACE).

Yip, that solves the problem. I tried my code on a cube and everything worked properly. Thanks a lot.