It's hard to understand electricity because you can't see it unless it's lightning. You can see water so it's easier to understand thinking of electricity as water.
Current is volume, voltage is pressure, resistance is a water valve, wires are pipes.
Your battery is supply (water tank). Anything needing lots of current needs a large wire (large pipe) like the starter motor. Volume.
Anything needing lots of voltage needs a small gauge wire (small pipe but thick wall) like a spark plug wire. Pressure.
Ohms Law is the relationship between voltage, current and resistance.
If the ammeter is wired correctly then engine not running (alt/gen not turning), anything you turn on, lights, etc., ammeter needle should move left (-- discharge). Engine running ammeter needle should move right (+ charge).
I would wire everything back stock the way it was. It's been that way nearly 50 years working fine. Back when that tractor was designed all you really cared about was engine running above idle ammeter shows charging. Running needle moving left discharge means a problem with alt/gen, regulator or wiring problems.
Meters on tractors like that years ago were better because at a glance even in bright sunlight you could see if everything was ok.


