Generally an alternator needs a source of battery voltage to excite the fields to start the charging process. It is common to also use the same terminal on the alternator to control a warning light by sending voltage out of the alternator when it is charging. With voltage being supplied by both the battery and the alternator on both sides of the warning light the bulb will not light. If the alternator does not provide power back through the wire the light will stay on showing a problem with the charging system. Automotive ignition switches have an accessory terminal on their switch and an ignition terminal which are separate electrically. The alternator is fed through the accessory terminal. Tractors being converted to alternators usually have just an ignition terminal. When both the ignition and the alternator are connected to that terminal the power coming back from the alternator will provide power to the ignition system so the engine will not shut off even if the ignition switch is off.
Adding a switched oil pressure sender to power the alternator excite circuit separates the ignition system from the alternator so the engine can be shut off as normal. The oil pressure switch is wired from the battery from somewhere different that the ignition side of the switch. When the engine starts and the oil pressure comes up enough to close the pressure switch contacts, power is fed to the alternator and it will start to charge. When the engine is shut down the oil pressure drops and the switch opens so the alternator will not discharge the battery through that wire.
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Today's Featured Article - A Cautionary Tale - by Ian Minshull. In the early 1950s my father bought an Allis Chalmers B and I used it for all the row crop work with the mangolds and potatoes, rolling and the haymaking on our farm. The farm and the Allis were sold and I have spent a lifetime working on farms throughout the country. I promised myself that one day I would own an Allis. That time event
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