The GM trucks late 60,s early 70,s used the harness itsself for a shunt. They had 2 small black wires with inline fuses fastened at 2 different places in the main feed wires. The ammeter registerred off this. I prefer a voltmeter. Once you learn to interpret it & have an idea of volts & amps, it will tell you more. I have seen many breakdowns from burned out ammeters & damaged wires but never a fire.
