The regulator/rectifier takes A/C current produced by the permanent-magnet alternator under the flywheel, and rectifies it to DC, and holds the voltage doen to the correct level for proper charging of the battery. This happens while the engine is RUNNING. It has nothing whatever to do with "cranking" or actually, as I assume you mean starting. (I wish people would be more clear..., as in "cranking" means the engine is rotating, by means of the starter (hand or electric), and starting meaning just what it says!) The engine should start with the regulator/rectifier unplugged but there would be no recharging of the battery. Now, if your engine has no breaker points, and the "box" you are talking about is part of the igntion system, that is another matter. I believe some Onans have as many as 2 ignition modules, and they are proprietary (read expensive). For good advice, post the numbers on the engine tag over at the forum at www.perr.com. There's a couple of real knowledgeable Onan guys over there.
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