"when large load the warning sounds."
sounds more like under charging
to me. as voltage normally drops as more load is put onto the batteries. your alt might be dying and not keeping up any more.
have you measured voltage at batteries and at alt. posts? or are you just listening to the warning buzzer and guessing?
measure both of these with small loads and big load.
also measure the field voltage.
where is the battery
sensor wire connected to? battery or alt?