A volt meter measures just that, voltage. An ammeter or amp gauge measures current. A volt meter will not work when connected to your old amp gauge wires, it needs a positive and a ground. The positive should be switched by the ignition key. An amp gauge works by detecting the direction and strength of current flowing thru it. It does not really have a positive and a ground but it does have polarity to show if you are charging or discharging. If current is flowing from the alternator to the battery, you are charging. If it is drawing from the battery, you are discharging. You can tie the amp gauge wires together and bypass the gauge but you should do the amp gauge bypass as described on other threads on here. I don't know the link right off but I'm sure someone will post it.