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 - New Life for an Old Allis - by Tyler Woods. My friend Jon, has an old '39 Allis Chalmers B. He thought it a marginal tractor that had long since served its time. She smoked terribly and never had much power but he couldn't afford another so he was limping along with what he had. Jon's Allis has a small front loader and though it doesn't carry much, it serves his needs. It was the hard starting and low power that made him think it was time to replace the old girl. Jon called me to help him discover why his tractor wouldn't start
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