The first point is it aint an ammeter, its actually a voltmeter, its controlled by a " shunt wire"" in the loom this is set to a predetermined length to allow enough voltage to deflect it either way whilst it passes through the meters deflection coil. They were never brilliant from. new

and were only meant to show an indication of the voltage coming from or going to the battery. The workshop manual shows you how to test for deflection if that works your in wiv a chance of doin summat. You can open the loom and find the red/white wire looped into it get some similar cross section wire and make the loop longer. ( google resistance in parralel ) dont go mad,start with about 150mm of wire and see if the meter deflects. Now I can actually hear the electronics wizzers wincing

however without anpy technical info on the resistance of the meter coil and how much voltage it requires for full deflection or the factory set resistance of the shunt wire its all guess work anyway. Footnote for the princely sum of £5 you can buy a fully sealed digital volt meter and conceal it somewhere on the bike and it will give you a constant indication of what the battery voltage is in nice blue numbers

Keith

You can never have too many tools in your life, except the two legged variety
I'm never wrong, once I thought I was but I was
mistaken.
Phil Churchett winner 2013