No need to polarize a starter/generator charging system, as it polarizes itself the first time you turn the key to start it.
With the tractor running, temporarily ground the F terminal on the S/G. If the ammeter pegs (if equipped, you didn't say what model tractor this is) to the charge side, the problem is the wiring or voltage regulator. If it doesn't, then it's the S/G. If it doesn't have an ammeter, then look for a voltage jump across the battery when the F terminal is grounded. Don't leave it grounded for very long or you'll burn up the S/G.
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