the ammeter will tell you it's charging. the volt meter won't tell you as much.
difference between 12 and 14 on the average jittery tractor running voltmeter is near impossible to read. now.. lets say the alt goes south and you are running off the battery. by the time the needle on the VM is low enough for you to figure that out.. the battery may already be at 10.5-11.5v and for a lead acid battery.. that is effectively 'discharged'.. thus possibly leaving you stranded with a no start condition.
an ammter setup as center 0 net charge discharge shows charge and discharge deflection.
for an ammter like that to read correctly, you need charge line and all loads on one side of the ammeter, and the side ONLY connected to battery.
better to fix what you have.. IMHO.
again.. your machine. you might have better luck wiring a volt meter even if you can't wire an ammeter.
of course.. it's possible the ammter is bad.. in which case.. I'd sepnd the money on a new one.. vs a volt meter.
if you think the ammeter is bad.. reverse the wires.. see if it reads sligth charge instead of the discharge.. if so.. you know it is reading current both ways and the real problem is in your wireing or charge circuit.