Lightmapping and normal map issue

I'm trying to lightmap a ver simple object, it is just a pile of dirt it has diffuse and normal maps. The thing is that when I run the light mapping process my normal map is gone, the only solution I found was using the Legacy light mapped bumped diffuse shader, but I don't know why it somehow increases its intensity and my dirt looks a lot brighter and the normal maps is not as clean and visible as before....anybody knows whats happening? or how could I get this thing to work properly?