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 - Fire in the Field A hay fire is no laughing matter-well, maybe one was! And a good life-lesson, too. Following World War II many farm boys returned home both older and wiser. One such man was my employer the summer I was sixteen. He was a farmer by birth and a farmer by choice, and like many returning soldiers, he was our silent hero: without medals or decorations, but with a certain ability to survive. It was on his farm that I learned to use the combination hand clutch and brake on a John D
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