I'm doing a project and I'm using the Honda s2000 cluster gauge.
In the wiring schematic, there are two wires that I would like to ask a question.
There is a RED for IGNITION-KEY and RED/BLACK for ILLUMINATION.

I don't know the function of the Illumination signal - when I turn my ignition key, the gauge speedo will light up and startup to reset the tachometer (normal) and there is a light in the cluster gauge too.
Is the illumination signal wire illuminate the inside of the cluster gauge + the rest of the control switches at night?

Thank you

 

Red/Black goes to the tail light relay.

Utah

 

Like Utah says, it goes to the tail light relay. When it sees power on that wire, it dims the cluster. Notice how when you turn on your headlights, the cluster dims (obviously it needs to be brighter during the day to counter the natural light). That's the purpose of the illumination wire.