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Now that you have experienced the wonder of 12V conversions, I'm sure you understand why it's got to be the most common subject here. My 2N has a 12V conversion that uses a sidemarker light as an idiot light. I've also heard of the pushbutton, diode and resistor methods. They're all intended to provide excitation current to the alternator while preventing the alternator from backfeeding the ignition circuit. If I remember correctly, 10 ohms is the correct value for the resistor in the excitation circuit. However, there is likely supposed to be another resistor in the ignition coil circuit. If this resistor is not present, or is the wrong value, it's conceivable that the 10 ohms in the excitation circuit is not enough to prevent keeping the ignition ciruit going. I'd check the ignition circuit to make sure it's done right -- depending on which coil you have. How to tell? Well, first you need to figure out if you've got a 6V or 12V coil. Have you a good meter? If so, the 6V coil should measure about an ohm on the primary side while the 12V coil should be closer to 3 ohms. No meter? What tractor is this? (I'll assume this is a front-mount distributor on an N for now.) The ignition circuit for a 6V coil should include the OEM ballast resistor (built into the "terminal block") and an additional resistor. The voltage at the coil with the ignition switch on, the engine stopped, and the points closed should be 2.5V to 4.0V. The circuit for a 12V coil may have just one resistor (not the OEM resistor) or it may have various combinations. The voltage at the coil should be somewhere around 10V to 11V. In either case, if the voltages are higher the coil life will likely be shorter and the 10 ohm resistor in the excitation circuit may not be enough to prevent backfeeding the coil.
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