Hi,

"Ambient Occlusion", as its name implies, is a technique invented to gather the occlusion of ambient lighting (and nothing else).

An ambient occlusion map should then only affect ambient lighting, like 'Ambient' and 'IBL', but certainly not the diffuse lighting produced by Lights.

Here is an illustration, a character with Skin_Head channel without AO (well with the map but with strength at 0):
 

And now with the Ambient Occlusion map and strength 100, again on the Skin_Head channel:


The result is completely black !?

This should not be, only the Ambient and IBL lighting should be affected by the black AO map, we should still see the Key and Rim lights diffuse lighting on the head.

--
guy rabiller | GOETZIWOOD STUDIOS
"N.O.E." (Nations Of Earth) Sci-Fi TV Show, Showrunner.