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Measure the coil resistance. If its still a 6 volt truck, the resistance should be around 1.5 ohms, without a ballast resistor. If its been converted to 12v , with the same 1.5 ohm coil, you need a 1.5 ohm ballast resistor in series with it, or buy a new coil with a 3 ohm resistance and not use a ballast.