This is how it works on paper: Due to voltage drop while cranking the engine, the voltage to the coil also drops. This causes a weak spark that may not be sufficient to start the engine. To remedy this, we use a lower voltage coil that will provide a nioe hot spark at cranking voltage. Once the engine starts, and the starter load on the battery is removed, voltage rises. To accomodate this, a resistor is used to drop the voltage back to the cranking voltage AT THE COIL. To accomplish this, the start button or key switch has a circuit that BYPASSES the resistor during cranking. So, when you are holding down the starter button, the resistor is bypassed and you have spark to start the engine. When you release the button, the circuit is broken, and the engine stalls because the resistor is probably burned out causing the broken circuit. Surprisingly, these resistors take a lot of stress while in operation, and are a relatively common failure, but not as common as I would have expected.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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