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I'm rewiring my '68 project car with the ISIS aftermarket wiring system that "sends" a ground signal to a master controller, rather than running 12 volts through the various switches. The one function that won't work with the ISIS system is gauge light dimming. As a result, I have to run the gauge lights through the dimmer on the stock lightswitch.

So far as I can tell with my trusty multimeter, the stock switch simply adds resistance in series with the gauge lights to dim them. Is that correct?
 

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Yes, the OE switch has a large wattage rheostat to drop (waste in heat) the leftover current from the selected brightness.

I suspect the ISIS system has the dash lights, read resistance, as part of the emitter circuit of a transistor driver and you control the amount of conduction, read control the amount of current flow of the driver, to light the bulbs, hence, both ends of the light need to be connected to the control unit.
 

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Yes, the OE switch has a large wattage rheostat to drop (waste in heat) the leftover current from the selected brightness.

I suspect the ISIS system has the dash lights, read resistance, as part of the emitter circuit of a transistor driver and you control the amount of conduction, read control the amount of current flow of the driver, to light the bulbs, hence, both ends of the light need to be connected to the control unit.
Thanks for the confirmation of the resistance.

The ISIS system allows a variable, center tapped 5V input which can be used to provide dimming - with the 0 - 12v output tracking the 0 - 5v input. Unfortunately I'd have to add a completely separate potentiometer to do so, so I'll keep it simple and run the 12V through the OEM switch.
 

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The ISIS system allows a variable, center tapped 5V input which can be used to provide dimming - with the 0 - 12v output tracking the 0 - 5v input. Unfortunately I'd have to add a completely separate potentiometer to do so, so I'll keep it simple and run the 12V through the OEM switch.
The rheostat is too low of resistance to provide the 0-5V?
Does ISIS give a value of variable resistance?
 

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Discussion Starter #5
The rheostat is too low of resistance to provide the 0-5V?
Does ISIS give a value of variable resistance?
The ISIS master cell can't take more than 5V, and then on only two particular inputs. It's not a matter of resistance, it's creating a 0V - 5V input; but if you give it too much (12V say) you fry a portion of the master cell. I need to provide 5V for the Viper VSS, so I'll have it in the car, but the only way to provide the 0V to 5V "signal" is a center tapped potentiometer, with one side grounded and the other at 5V, and the signal coming off the tap. I'm just not sure I want to add another "switch" to the dash.
 

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For 5v I would use a little 5v regulation IC on a small heat sink, either fixed or there are variable VR's.
 
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