On several tractors of various brands, the starter switch is mechanical and has no terminal for a bypass to the coil. Others have a relay to the starter that has no 4th terminal for bypassing (Ford cars/trucks had the 4th terminal). If a vehicle had a ground to engage starter relay (many old Ford tractors) again no source for 12v for starting. Wiring the bypass to the battery side of any of the above, caused the ignition to be on all the time. If wired to the starter motor side of the relay or switch the bypass will work (briefly) while starting, but the ignition system will try to operate the starter motor. (the smoke rolls) The diode is a easy answer to provide 10 or so volts (cranking voltage drop) to the coil for nice hot temporary ignition "encouragement" and preventing back feeding the starter motor which has about 0.15 ohms. Jim
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Today's Featured Article - Restoration Story: Fordson Major - by Anthony West. George bought his Fordson Major from a an implement sale about 18 years ago for £200.00 (UK). There is no known history regarding its origins or what service it had done, but the following work was undertaken alone to bring it up to show standard. From the engine number, it was found that this Major was produced late 1946. It was almost complete but had various parts that would definitely need replacing.
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