How-to: Aux Light Hookup with Indicator Light

Discussion in 'How-To Submissions' started by Scrambler82, May 13, 2012.

  1. Scrambler82

    Scrambler82 Old Guy User

    Aux Light Hookup with Indicator Light


    The schematic will work with either Off-Road, Back-Up, Headlights or any Auxiliary Lighting can be hooked up with slight variations on this schematics.
    All grounds are grounded back to a terminal strip or stud not to the body or frame, this will reduce electrical problems from poor grounding through rubber mounts, rust, corrosion, paint and grease.

    What you will need:

    - Relay(s) – 30-amp Automotive Four Pin Relay, pins numbered 85, 86, 87, and 30. If you have a relay that has pin 87a, this will work but be aware that pin 87a will be live with Battery Power when the lights are off; this is good if you are getting creative but not so if a wire or a tool hits it and things start to short out.
    - 10-ga Automotive Wire – Do-Not use Household Wire, that is found at the local Hardware Store, for automotive wiring. Buy enough Wire to run from the battery to where you want to mount the relays then on to the Lights; no real min or max on this wire length, it will all be considered short runs.
    - 16-ga Automotive Wire – buy enough to go from the inside Circuit panel to where you want to mount your switch, then to the relay under the hood.
    - Dash Switch – This is to control the Relay with a low amperage circuit, the switch should be capable of handling 12 volts minimum; a 125 or 250-volt switch will work if it is the type you want. There are too many different types of switches to mention here but for the easiest type and related to the schematic buy a SPST (Single Pole Single Throw) (On-Off) switch.
    - Fuse Tap - A connector to tap a Circuit on the Circuit Panel inside of your vehicle.
    -Fuse Holders – Two in-line fuse holders, one for a low amperage fuse (2 amps), for16-ga wire and one for a high amperage fuse (30 or 40 amps) for 10-ga wire
    - Fuses – One 2-amp for the Switch/Relay Circuit and one higher amperage fused based on the total current draw of the Main power, i.e. 2, 100 watt lights = 200 watt / 12 volts = 16+ amps, you will need a 25 or 30 amp fuse for the lights.
    The Fuse Holders can be the older Barrel glass Fuse or the newer Blade Type, your call.


    Activating Relay – Pin 85 on the Relay is used to activate it and 86 is used for the ground but both wires are required to make the Relay work. Some Relays without pin 85 but they ground through the mounting hardware. It is a good idea to ground close to the battery or directly to the negative post on the battery. (Pins 85 and 86 are interchangeable)

    Main Power from Battery - Use a minimum of a fused 12-ga wire from the battery to pin 30 and from pin 87 to the lights when hooking up 2 - 100 watt lights. It is better to use a heavier gauge wire than a smaller gauge wire, i.e. 10-ga instead of 12-ga. In my opinion there is no such thing as over kill when wiring high current in a vehicle, bigger is better.

    Fuses – The fuse is there to protect the circuit and all of the components in the circuit and needs to be rated for the smallest component in the circuit plus a small margin for initial draw. MAKE SURE TO USE FUSES.

    The Install – (Everyone does things a little differently but this is how I do it and it has worked fine for many installs)

    Step 1) Mount the Relay under the hood and the Dash Switch where you want them.

    Step 2) Using a DC Electrical Probe or Multimeter find the power type you want to control the Relay via the Dash Switch.
    a) On all of the time, so the Dash Switch will do the main power control
    b) On with the key, so the power will be controlled by the ignition via the Dash Switch.

    Hooking up the Switch with a low amperage circuit:

    Step 3) Run a 16-ga wire from the power point on the Circuit Panel (Step 2) to one pin on the switch.
    Note: Usually there are two pins on a SPST Sw, sometimes there are three pins; it is suggested to use the two pin type of SPST Switch.
    Two pin switches are on/off devices and any wire can go to either pin.
    Three pin Switches are on/on devices; although they can be used for on/off the extra pin will be live when the lights are off. If you use this type of switch make sure to insulate the extra pin.

    Step 4) From the second pin on the switch run two wires, one will go to pin 86 on the Relay and the second wire will go to the power side of the Indicator Light.
    Note: Some Switches come with indicator Lights inside of them and all that is usually needed is an additional wire to ground but everyone is a little different so double check what connection on the switch do what and hook up accordingly.

    Step 5) Hook up the Ground of the Indicator Light – Most DC lights have two wires, one power coming from the switch and the second is the ground and the ground needs to be connected to a good metal grounded point on the body of the vehicle.

    Step 6) Connect the Wire from the Switch (Step 4) to pin 86 on the Relay.

    Step 7) Run a 16-ga wire from pin 85 to ground, use some external tooth star washers on the connection to make sure the connection makes contact with new metal.
    Note: I like to use a Conductive Contact Paste on the ground connection to keep things cleaner longer.

    Test Point…

    Step 8) At this point you should be able to flip the Dash Switch and hear the Relay click, if you don’t hear the clicking re-check all of the connections or if you hooked up power to the switch as a Key–On Power only, then turn on the key. Once you hear the Relay clicking then you are ready to hook up the power to run the lights.

    Hooking up Battery Power to run the lights:

    Step 9) Run a heavy gauge wire from the battery positive post to Pin 30 on the Relay, make sure to have the higher amperage fuse in-line with this wire.
    Note: Mount the fuse close to the battery, this is a safety link and should blow out as first sign of rising heat in the line (a shorted circuit that is).

    Step 10) Run a heavy gauge wire from the Relay, pin 87, to the first light and over to the second light. Allowing for routing the wire and connecting the wire to the second light.

    Step 11) Make the connection to the first light - To make a connection, strip some of the sheathing (outside covering) off of the power wire, make sure NOT to nick the wire. Wrap the Light Wire around the Power Wire (as flat as possible – do not tin before soldering) and solder this connection. Clean it and slip a piece of Heat Shrink with adhesive inside, on to the power wire but do not shrink it at this time just slide it over the joint. (Note: When attaching the Light Wire to the Power Wire, allow for heat Shrink Application either before or after soldering, watch the direction of the two wires)

    Step 13) Make the connection to the second light - Run the wire from the first light to the second, making sure to slide a second piece of Heat Shrink on the wire. Cut the wire to length, solder the connections and clean them, again do not shrink the Heat Shrink just slide it over the joint.

    Step 14) Grounding the Lights – You can ground them to the closest metal that will allow you to secure or you can do it the way I do. Run an additional wire, the same gauge as the power wire, from the Lights to the negative side of the battery or a grounding point near the battery that will be connected to the battery. Attach by preparing the wire and soldering the same as the Power Wire and don’t forget the heat shrink. Note: By running a separate Ground Wire back to the Battery you will reduce the main reason for Lighting Problems – POOR GROUNDING and in turn allow the lights to burn at their brightest and burn longer!

    Test Point…
    Flip the switch and the lights should come on. If they don’t you will need to re-check the Power Wire only from the battery to the Relay and then from the Relay to the lights; that is if the Relay is still clicking from step 8.


    a) There are too many if, an, nor, for’s to state every possible reason for the lights not working but usually if the wiring is OK then you need to check the Fuse or the Light Bulbs. If those are OK, re-check the wiring just to be sure, double-check all of the connections.
    b) There are different type of heat shrink and most will work OK but the most important thing to remember is that you want to keep as much dirt and moisture out of the connections as possible. What is the best way; use heat shrink with adhesive inside of it and when you shrink it, the adhesive will melt and seal the joint.
    c) The wire gauge that you use will depend on the total current (amperage) draw of the lights, usually based on the wattage of the lights. Using a larger gauge is better than a smaller gauge wire because of safety factors and the larger gauge wire will allow the light to run at their brightest and last the longest.
    IF you have smaller gauge wire than what should be used, i.e. 16-ga instead of 12-ga, consider wiring the lights with two pieces of wire, each light would have its own power wire from the relay BUT both wires would go back to pin 87 on the Relay.
    d) Always fuse the power wires especially the battery wire.
    e) Always use a Relay to control the battery power.

    f) Here is a quick wire gauge reference:
    16-ga = 10 amps
    14-ga = 15 amps
    12-ga = 20 amps
    10-ga = 30 amps

    Keep to these wire gauge sizes and you will not have a problem with under-rated wiring and the lights will burn their brightest and I think that is why we put lights on in the first place to be bright and to let people know we can do it, I did.

    I hope this isn’t too confusing; I can get carried away very easily.

    Any questions e-mail me @


    1) If you want to use this schematic to wire Fog Lights and have them come on with both the high and lows beams, automatically, replace the switch with a wire that attaches to the Parking Light Wire.
    An indicator light from pin 87, will let you know the relay has switched and the Fog Lights should be on.
    Last edited: Feb 6, 2013
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  3. good wiring info!
  4. OP

    Scrambler82 Old Guy User

    Re-edited and posted.
  5. 2004xlmiller

    2004xlmiller Member

    great writeup

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