Those amp gauges aren't really anything more than ornaments. No real meaningful measurements can be gained from them other than a load indication. If yours bounces slightly and deflects slightly in the negative direction when you turn on headlights or heater fan then it is probably working. The voltage gauges that replaced the amp gauge later on were a better indication of an operational charging system.
But if you'd like you can test the amp gauge, out of the car, with a 50mA load and get almost full needle deflection - reverse polarity and check deflection in the other direction. Use the WHITE and WHITE/RED STRIPED wires.
I have the same problem.. I might try the test RLS30 mentions.
I do get small deflection with lights on.
I put this down to either:
Wiring has been altered by previous owner .. or
Shunt impedance has changed.. in which case it would have to be replaced
(or.. the way shunts are calibrated is by altering the cross-section so that the total resistance increases.. I might get the hacksaw out and try it.. I had better stop thinking about this.. it's getting dangerous..)
There two 3 Amp fuses inside the SHUNT that feed the amp meter, so check these if your meter doesn't appear to be working when installed in the car.
It may be the connections in the amp meter circuit too. Those "tinned" sockets and pins probably have tarnished, greatly increasing resistance and altering the amp gauge measurements referenced to the constant ratio at the shunt.