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OK, if I'm reading this right you say it will charge to 15 volts when the lights are off, but drops to 12 volts when the lights are on. That means one of two things...first, there's a high resistance connection somewhere between the alternator & the point where the power for the lights is taken off, or second, the lights draw more current than the alternator is capable of producing. If its the first case, the high resistance connection could be in the wiring, or it could be in the ground path between the alternator case & the tractor chassis. Another possibility is a bad ground connection on the battery itself, but that should manifest as a cranking problem as well. Measure the output voltage of the alternator at the alternator with the lights on and the engine running. Take two measurements, one between the output terminal and the alternator case, and one between the output terminal and a good ground. If there's more than a few tenths of a volt difference, check the alternator mounts for a good electrical connection to the engine block. If there's no appreciable difference at the alternator, make the same measurements between the positive battery terminal and the alternator case, and between the battery positive terminal and a good ground. If there's a difference here, you have either a bad ground somewhere or a bad connection in the charging circuit between the alternator and the battery. In this case, measure the voltage between the positive battery terminal and the output terminal of the alternator. If there's more than a small voltage drop between the battery & the alternator, you have a bad connection in the charging circuit. If there's not, you have a bad ground somewhere. If the output at the alternator is in fact only 12 volts, check the wiring of the regulator input (assuming its not a one-wire alternator). If the regulator is wired correctly, you probably need a bigger alternator or smaller lights. Good luck, hope this helps. Keith
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