The purpose of the warning light is to: 1 - Warn if the ignition is "on" and the alternator is NOT charging, and 2 - Limit the backflow current from the alternator when the alternator is charging and the ignition switch is opened. (Without the lamp the backflow current is sufficient to continue to fire the coil when the ign is switch opened. End result is the engine will continue to run after you shut the ignition off.)
The diode serves the same purpose as #2 - it prevents backflow current from the alternator to the coil when the ignition switch is opened. On the VAC the manual switch accomplishes the same thing.
On your MD the oil pressure switch together with the manifold serves a surrogate for ignition switch on a gas tractor.
A ballast resistor is not required if the coil is a "self-ballasted" (ie. 12 volt) coil. It has the required resistance incorporated within the coil primary winding.
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Today's Featured Article - Identifying Tractor Noises - by Curtis Von Fange. Listening To Your Tractor : Part 3 - In this series we are continuing to learn the fine art of listening to our tractor in hopes of keeping it running longer. One particularly important facet is to hear and identify the particular noises that our
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