Right Aaron, but there's more. The oil pressure switch. "excites" the alternator on those Masseys. The external regulated alternator found on the 100 series is totally different than a "3 wire" alternator. While you'll find a FIELD terminal and a SENSE terminal on the alternator, they don't use both. The pressure switch gets 12V from the regulator and once it senses oil pressure (your "run indicator") the switch sends power to the "F" (field) terminal of the alternator. That excites te alternator. This wiring scheme causes the alternator to charge as soon as the engine is running instead of having a cut in point determined by RPMs like a 3-wire alternator. There is no "sense wire" used.
Also, the pressure switch sends 12V to the fuel gauge so that the fuel guage is powered only when the engine is running.
Years ago I spent a week chasing a NO CHARGE on a 135 Massey thinking I had a bad regulator or alternator or wiring, only to find a bad PRESSURE SWITCH. Last thing in the world I suspected was something to do with the oil pressure gauge.
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