Yes, it's unusual, but I have seen a few ammeters open circuit. If it's wired correctly, everything shuts down: ignition, headlights, charging system and all. That's because the "discharge" side of the ammeter also serves as the connection point for virtually all the circuits on the truck. If the upstream (battery) side of the ammeter has the load circuits connected to it, it will never show a discharge.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!