If you have a GM-10SI or equivalent alternator:
You MUST connect the negative cable (-) on the battery to frame ground, or the alternator will be instantly damaged when the battery is connected.Wire the idiot lamp's 2-leads: one to the #1 terminal on alternator, the other to the 'ON' terminal of the ignition switch (same place the coil is fed from, if battery ignition). If you have a magneto, you'll have to install another off/on switch between the load side of the Ammeter and the wire to the idiot lamp and turn it on when in use and off at shut down to prevent battery drain. Alternator #2 (sense) terminal is wired to the big output terminal as you said. One terminal of the Ammeter is wired to battery plus (+) usually on the starter switch (or solenoid) battery cable terminal. The other Ammeter terminal is wired to loads - alternator big terminal, lights fuse, ignition switch battery terminal (if battery/coil ignition), or the added switch if you have a magneto. If ammeter reads backwards, swap the wires on the terminals from one side to the other. A tractor equipped with a magneto has a grounding switch to kill the magneto. Don't try to wire battery voltage or anything else to that switch! A double-pole double throw 'DPDT' (2-circuit) switch could be substituted for a mag kill switch, wired to un-ground the mag wire and apply battery voltage to the idiot lamp circuit when 'on'.
| 
