The manufacturers of aftermarket wiring harnesses don't like the liability of running battery charging current through decades-old dash ammeters. Therefore, they encourage people to use a voltmeter instead. There's no danger in using a good condition, properly-wired ammeter, especially if a shunt is used to bypass some of the current around the gauge. The shunt turns the ammeter into a charge/no charge indicator, but even when they were new, dash ammeters were notoriously inaccurate.
Jerry
