Is your coolant full? Seems silly but since it is water cooled.... On mine it seems if the coolant is a bit low (yah I have a valley gasket leak) the alternator gets too hot and the battery light comes on. I assume it is not putting out the right amps anymore. I top up the coolant and it works fine again. The alternator may also be on it's last leg. Who knows. I haven't had this truck long.