Gordo, the FSM shows a test procedure for the alt.
Remove the light green field wire from the alt.
Use your voltmeter to be sure there is 12v at the orange wire field terminal of the alt when the key is on.
Install a jumper wire from the field terminal u removed the green wire from on the alt, to ground.
Start the engine, ammeter should show full scale charge when u rev.
If it does not show full charge, the alt is bad, OR, the big wire from the alt to the ammeter is broken.
If it does show full charge, there is a problem with the regulator.

Re the suggestion to bypass the ammeter, simply remove the wire from one ammeter lug and put it on the other lug with the other wire. Once nutted down, it will be connected and secure. No need for that messy tape crapola, and easy to undo in the future.