It sounds like the sensing coil of the ammeter has given up. Only a small fraction of the current passes through the needle circuit- - - -most of it bypasses the needle through a shunt. I like to add a second shunt made from a piece of stainless steel hose clamp material shaped like an "Ohms" symbol in a wiring diagram to keep the ammeter from pegging out right after startup. It's quick, simple to make, and adds a little protection to the fragile needle-deflecting circuit. Use the solid part of the clamp, not the area where the worm gear engages it. You probably need a new ammeter now, though.
For the ammeter to work properly, all the wires except the one from the starter should attach to one post of the meter, including the one from the alternator, and the fuse block. That allows the meter needle to deflect in the (-) direction when the engine is not running and the headlights are turned on, for instance, and indicate a charge when the engine is running. Good luck!
Jerry
