Not sure I can add anything here since my work on outboards is thin. Diodes have a forward conduction voltage typically in the .55 to .7 voltage range, but the reverse should be nearly infinate. Your meter on diode range is a bit more sophisticated than an ohmeter by applying bias to the pn junction. If I knew the configuration on the rectifier, schematic, I could tell ya what readings you should expect. I suspect the tach is powered on the rectifier side, but the tach signal is the ac pulse coming directly from the stator coil. Hope this helps, Michael.