Correct, a 100A alternator that is hooked up to the original ammeter and feeding the charge thru the old firewall connectors is not a good solution. That is the problem with questions like this, there isn't anyway for the OP to tell us all the details of the installation. All it takes is for a bad wire or a bad ground or a poor connection somewhere and nothing will work correctly. I have a small 60A alternator on my Duster and it holds 14.5 volts even at idle. But all of my wiring is new and the alternator connects directly to the battery.