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I'm trying to wire an auxiliary light harness (KC) to my truck. I don't want to use the supplied switch, since it doesn't really look good on the dash. I have a nice unknown provenance switch which I'd like to use, but no instructions. Looks like this:

enter image description here

This switch has four terminals, and an LED that is supposed to light up when the switch is closed (on).

First, this is what I know about the switch. If I wire it like this, it works, and the LED on the switch turns on when the switch is on:

enter image description here

Also, I noticed that there are some diodes at play here, when the switch is closed (on), going from 4 to 1 and 2 to 3.

Now, I want to use this switch to control my aux lights while having the LED light on the switch work. The aux lights switch is supposed to be wired as follows, with the harness having a green, white, and brown wire.

enter image description here

How should I write the green, white, and brown from the harness to the switch? I have a 12V power supply that I can use to test without installing in the car.

(EDIT) here's from the harness documentation:enter image description here

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Consider that you've used the switch diagram to interrupt both leads to the LED and really needed only to interrupt one. To confirm this, connect the wires as shown for 1 and 3, but connect wires 2 and 4 directly, bypassing the switch.

What should happen is that the device will energize/activate.

Swap things around and connect 2 and 4 through the switch and connect 1 and 3 together. Device will energize/activate.

What you have is a double pole single throw switch. It's convenient when you have to switch two different circuits that may have different power sources. If you have two different circuits to be switched on at the same time and have the same power source, you'd simply jumper across 3 and 4 as well as connect the power to 3 (or 4).

I suspect that you have some feature with the factory switch that is not clear in the diagram. There's no need to have a ground lead independently wired to a switch, unless there's a circuit from the 12v source (white) to ground (brown) that you will not be using.

If you are satisfied that the last paragraph applies, connect your white lead to 3 and your green lead to 1 (or use 4 and 2 respectively) to accomplish your goal.

EDIT: Allowing that you also desire to have the on-board LED to illuminate, it may be that one pair provides power to the LED. Test the circuit for both options noted above. If the LED lights up for either, it's your choice. If it lights up for only one pair, well, use that one. The crossing diodes may be that which allows for illumination, regardless of the selected pair.

OP Suggested Edit: grounding in a motor vehicle is not so much an earthing of the circuit for protection but more a means of providing the second leg of the circuit. Imagine how much of a mess you would have if you had to use a common bus throughout the vehicle or if you had to have a negative lead for every device which uses 12v power. The chassis and frame become that common bus. Any ground you can use is the same ground across the entire vehicle.

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  • Thanks! I will try out and report. I have updated the post with the KC diagram from the harness documentation.
    – Will I Am
    Apr 21, 2018 at 1:00
  • Thank you, so I connected the negative directly to the multimeter and the positive interrupted by the switch, and it works fine. However I have not been able to get the switch light to work. I can get it to work if I have a negative wire (which I won't, I only have the ground to work with, However, I used the ground on my power supply, I'm not sure if the ground in the above diagram has a different meaning. Given that a car has rubber tires, is it ever really grounded? What does chassis ground mean in automotive?
    – Will I Am
    Apr 23, 2018 at 1:19
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    grounding in a motor vehicle is not so much an earthing of the circuit for protection but more a means of providing the second leg of the circuit. Imagine how much of a mess you would have if you had to use a common bus throughout the vehicle or if you had to have a negative lead for every device which uses 12v power. The chassis and frame become that common bus. Any ground you can use is the same ground across the entire vehicle.
    – fred_dot_u
    Apr 23, 2018 at 9:27
  • Thanks, this answers my question! I would suggest you copy the comment into your answer, since it was the missing part of the equation there. :)
    – Will I Am
    Apr 23, 2018 at 14:27

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