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There are four main components of the electrical circuit involving the neutral light on a Honda CX500, GL500, CX650 or GL650 - the neutral switch itself, the starter disconnect switch, a diode and the neutral light.
The neutral switch - see here if you aren't sure where it is - is a small spring loaded switch which completes a circuit when the plunger on the end is pressed in by a bump on the gear selector when the transmission is in neutral. There is a test procedure for the neutral switch which can be performed with the engine still in the bike but if the neutral light comes on when your bike is in neutral and goes out when it is in gear then your neutral switch is working OK. There should be 12V on the loom side of the neutral switch - when the contact inside closes it grounds the circuit.
The starter disconnect switch is at the clutch lever on the handlebars and is designed to allow you to start the bike when the bike is in gear so long as the clutch lever is pulled. If the bike is in gear and the clutch lever is not pulled in the bike should not attempt to crank as a safety measure. If your bike will not attempt to crank while being in gear with the clutch pulled in then the starter disconnect switch is suspect, or the wiring to it.
The third component is a diode - this sits as part of the starter circuit and functions to prevent the neutral light from coming on every time that the clutch lever is pulled. The diode is located in the fuse box on CDI bikes. If the neutral light works but the engine won't start without pulling the clutch lever in the diode is suspect - probably open - or the wiring going to/from it. If it fails short the neutral light will still come on but it will also turn on whenever the clutch lever is pulled.
One other component worth noting is the bulb behind the neutral switch - if your neutral light won't come on but the system otherwise functions normally you may have a blown bulb or an issue with the power/grounding of the bulb!
Sidecar Bob sent in this circuit diagram to further assist:
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