10si exciter wire

A diode is used in the alternator excitation wire.  The exciter wire should not be connected directly to the battery.  It should be powered from a switched source.  The diode is to prevent back feed from the exciter terminal to the ignition switch when the engine is shut off.  Depending on how it is connected to the switch and what else is connected to that same switch terminal, that back feed from the exciter terminal can prevent the engine from shutting off.  A 2 to 5 amp, 50-volt diode will work in the exciter wire.  If a universal ignition switch, with an accessory terminal is used it is possible to wire the excitation wire to the accessory terminal (best if it alone is on that terminal) without using a diode, the switch design will prevent the back feed issue.   Another method is to use an indicator light wired into the exciter wire. You need an insulated base lamp holder (2 wires) as the lamp holder must be wired into the excitation wire in series.  And an incandescent bulb is required, LEDs are not chosen for this task.  You could route the excitation wire through a normally open oil pressure switch, that will cut power to the alternator excitation terminal when the engine is stopped. A number of Massey Fergusons used an oil pressure switch to control the alternator excitation and gauges on the diesels where there were no ignition switches.  The oil pressure switches were fed direct from a battery power source.  

Yes, you can run an alternator with a wire to the excitation terminal... just like a 1 wire alternator....Yes, the main purpose of a diode is to stop reverse flow.. Putting a diode in the line may make the alternator "kick in" at a somewhat higher RPM compared to a resistor... similar to a single wire alternator ???????    Just a thought..