Ammeter wildly fluctuating

[Vincent]

I've started the engine on my CJ3A. I only have the engine on the chassis and a wood cutout attached to the frame for three gauges, oil, temp, and amp.  The Amp gauge needle flies back and forth across the dial when the engine is running. I replaced it with a new amp gauge and it does the same thing. Because it is attached to a wood board, does the amp gauge need to be grounded, as if it were installed in the metal tub? And how wood I connect that? If not, what would make it fluctuate like that.
Thanks

[elawson]

The amp gauge does not need to be grounded in order to correctly read the current.

As for the meter "bouncing back and forth".  Are the wires between the generator and regulator connected securely?  Is there a good ground connection between the generator case and the regulator case?