240Z alternator? External voltage regulator? The earlies don't use a shunt (like the 260-280) to the ammeter. So power to/from the battery goes directly through the ammeter (via large wire). The ammeter measures current flow, not voltage - even though it's important to have a proper and stable voltage from the alternator and regulator. What could be causing disruption or variation of current flow? Does it stay in the positive, or is it swinging from positive to negative current flow? The battery discharges at start cranking and then is recharged by a properly working alternator. This recharge should show on the ammeter as a significant flow in the positive direction at first, but taper off as the battery is charged, ending with the ammeter showing a slightly positive value for the rest of the drive (or engine operation). The ammeter should also show changes in electrical use - like lighting, headlights, A/C compressor, electric fan, and etc. being turned on and off as more (or less) current is being drawn during their use. (Ya, I blabbed on for all those youngsters who didn't grow up with an ammeter...)