Delco 12SI draws 2-3 amps on the excite-field circuit. I've never blown a pair of 3 amp diodes in parallel yet and I've got 8 tractors wired that way (with Delcos or Hitachis).
Biggest problem I've had when using resistors instead of diodes is run-on with diesels. Many diesels with 12 volt electric shut-offs will keep going even with only 3 volts. Diode at less then $2 is kind of a no-brainer. Works with gas and diesels and always allows shut-off.
Another way to wire a Delco SI series with no resistor or diode is with a generic automotive type ignition switch. The kind that has separate ACC and IGN terminals. Wire the ignition to the IGN term, and the alternator excite wire to the ACC term, and you're all set. The two are isolated when you turn the key off.
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Today's Featured Article - My Ford Golden Jubilee - by Troy Estes. This article is about my '53 Ford Jubilee and a story that starts with taking the tractor to my brother's Starter/Alternator Rebuilding shop for a wiring fix. The generator was shot as well as all the wiring. I dropped off the tractor expecting a transformation from a 6 volt to a 12 volt system utilizing the original generator housing, and a total rewiring of the whole tractor. The front end center pin bushing was worn also so I ask that they replace it if they had time. Well, that’s wha
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