There are some installations, not necessarily on LS or GM engines, that use isolating rubber bushes in the alternator case mount positions to minimize vibrations.....
In these cases a separate ground strap between the alternator case and the engine block is necessary for the system to charge....
I have seen these mainly on industrial equipment engines and diesels....
I think I remember seeing a ground placement screw on some GM alternators...but I am too lazy to go look in the shop !
