Informational - Relays, fuses, switches, aux lighting, fog lights, etc!

Discussion in 'GM Electrical Tech' started by Springthing, Aug 8, 2010.

  1. Springthing

    Springthing Epic Member 5+ Years 1000 Posts

    Well, it comes up once in a while so I figured we'd get this down to making a thread about it to refer people to.

    Questions like "How do I hook up fog lights?", "How do I power this", "How do I have these turn on when this is? etc etc...normally can be answered relatively easily. It's generally an easy mod to do and once you know the basics you'll be able to do it no problem.

    For these few posts I'll concentrate on two questions and show a solution for both. With a few word swaps I'm sure these two examples can be broadened to cover quite a number of other questions/scenarios.

    1.) I want to install driving/fog lights and have them come on when my high beams are on. How do I wire this up?
    2.) I want to power my new spot lights. How do I wire them to a switch on my dash?

    Before we get answering the questions directly, let's look at a few of the components you'll need.

    ---------- Post added at 06:37 PM ---------- Previous post was at 06:10 PM ----------


    Think of a relay, essentially, as a switch. The switch works like any other - it turns things on and off. The only (major) difference is that to flick the switch you apply power to it or disconnect power to it.


    Well, relays can handle a lot more amperage than your usual automotive/12v switches. If you were to wire a regular switch directly to power your 100W spot lights you would melt your switch. Period. There would just be too much power going through the switch and most are not able to handle full amperage that is drawn from accessories.

    Relays, then, are used as a secondary switch that CAN handle all the amperage. All your dashboard switch is powering up, then, is the relay, which uses hardly any amperage - there's no load there to drawn any amps.


    Relays come in a few different configurations but the most common is the five terminal.

    The above picture is a four terminal.

    Check the packaging/configuration before buying. All look basically identical.

    For most applications what you are looking for is a relay that is Normally Open. This means that power needs to be supplied to the relay in order to toggle it on.

    Confusing, right? Let's have a look at it visually, maybe it will make a little more sense. :)


    In the above image is a typical five terminal relay. I dare say this is the one you will find 9 times out of 10.

    Numbers 85 and 86 are the coil power. These are the two terminals that need to be powered in order for the 'switch' to go on.

    85 is simply wired to ground, aka your vehicle's body/frame/engine.
    86 is wired to a source of power. This is the one you'd wire to the switch on your dash.

    Both of these will be relatively low in power draw. They do not require a large diameter wire.

    30 is the terminal that needs to receive high power rated wire gauge. This terminal will be supplying power, in the end, to the accessory you are hooking up. We'll look at amperage draw later on in the thread.

    87 and 87A are what will be wired to your accessory. In this five terminal relay 87A is Normally ON, and 87 is Normally OFF. As it sits, with nothing powering the 'trigger' power WILL go from terminal 30 to 87A. At rest this relay feeds power straight through. Powering this relay (switching it 'on', triggering it, etc) will CUT the power to 87A.

    Terminal 87 will NOT be receiving power until the relay is triggered. At that time power will flow from 30 to 87, powering your accessory.

    I hope this makes a little sense!

    Just remember...a relay is a switch that is powered on by electicity.

    More on purchasing a relay later in this thread!

    ---------- Post added at 06:47 PM ---------- Previous post was at 06:37 PM ----------



    Various switches can be purchased, and the sky is the limit on how they operate!

    Most people think of switches as ON/OFF but look closely when you buy!

    On the top in the picture you have regular automotive toggle switches.

    On the left you have an ON/OFF/ON switch. Meaning you can choose to power either one accessory or another, but not both at the same time. The toggle is in the middle when OFF then flipped in one direction of the other, depending on the accessory you want powered.

    To the right of that you have a lighted rocker switch. This is your simple ON/OFF switch but when it is power on it lights up to let you know.

    Both others, to the right of that, are mini switches - one toggle, the other rocker type. Both of these, with their three terminals, show that they can be wired either normally ON or normally OFF. Complicated but that means with one switch you can have an accessory powered and one the flick of the switch power diverts from one accessory to the other. Neither of the two accessories can be OFF at the same time.

    More on buying switches later in this thread!

    ---------- Post added at 07:02 PM ---------- Previous post was at 06:47 PM ----------


    Fuses are a big deal.

    Just know that., right?

    Fuses should be used in any circuit that is powered(+). If you have power from the positive(+) side of your battery it should have a fuse. WHEN (not if..but when) you get a short at some point, or if an accessory fails you will keep from having a major problem on your the very least melted wires and burned out worst a fire!

    Depending on how intricate your mod/installation is, you can either have simple single fuse holders:


    .....or a full buss with 4, 6, 10 fuses:


    Single fuse holders are easy to use, easy to wire in your circuit and are well protected.

    Most busses are NOT well protected but can eliminate a lot of wiring. Busses have one single power(+) wire going in and the different fused circuits going out. Soooo..with one power wire from the battery you can power, and have a fuse in line with, 6 circuits with the above buss.

    For any project you do on your vehicle, a spade-type fuse should be used.

    Lastly.....any circuit that is receiving power(+) SHOULD BE FUSED ON THE BATTERY SIDE, AND AS CLOSE TO IT AS YOU CAN. The longer you have a length of wire, the more chance you have of getting a short. TECHNICALLY there is no difference how far away from the battery the fuse is, as long as it's on the power(+) wire somewhere. But I always advise 'the closer the better'. If anything goes wrong, a fuse close to the battery will blow and ensure your entire circuit loses power, no matter where it is, no matter what it's touching, no matter what you have in your circuit.

    Make sense? No? Good...moving along then!

    ---------- Post added at 07:06 PM ---------- Previous post was at 07:02 PM ----------


    I don't know why but it seems a lot of people forget about the usefulness of plugs!


    They're cheap, they're very handy, and they're easy to install!

    Next time you're at a salvage/junk yard, go through a few cars and pull plugs! Get some two wire ones, three wires, 10 wires, one wire...whatever you can get! They'll come in handy at some point, I promise you. If've spent an extra few bucks on something to fill up your parts bin in your garage!

    When it's time to clean your fog lights you'll be happy to have used plugs for them so you can take them off without cutting any wires. If you need to quickly cut the circuit to any accessory, pull the plug!

    ---------- Post added at 07:21 PM ---------- Previous post was at 07:06 PM ----------

    Buying any of the above

    I just want to give you an idea of what to calculate before you go buying this stuff. How much amps are you going to draw? What size fuse?

    There's a simple equation you need to keep in mind, and this will serve you well in the future and will help you pick the right pieces to the puzzle:

    Watts \ Volts = Amp

    Keeping in mind that automotive electrical systems are generally in the range of 14V

    With lights that are 55w (EACH!!!), the example would be:

    110 \ 14 = 7.9

    So you're looking at a fuse one size up from 7.9A

    If you have lights that are 100w each:

    200 \ 14 = 15.7

    So a fuse greater than 15.7A

    The same goes for your relay. You have to make sure your relay can handle the power that will be going through it when your accessory is powered. Look on the packaging or look on the relay itself. It SHOULD tell you what the amp rating is on it, as on most electrical components:


    This relay should be fine to power your two 100W lights.
    Last edited: Aug 8, 2010
    1 person likes this.
  2. Springthing

    Springthing Epic Member 5+ Years 1000 Posts

    1.) I want to install driving/fog lights and have them come on when my high beams are on. How do I wire this up?

    Easy! Have a relay triggered by your high beams.

    Determine which wire in back of your headlight (either one) is the high beam power.

    Splice in a wire that goes from that one to the 85 terminal of the relay. Wire number 86 to ground, somewhere near the relay.

    Bring a power(+) wire, fused, to terminal 30 on the relay.

    Have terminal 87 go to the positive(+) side of your fog/driving lights.

    Wire the ground(-) wire of your lights to the frame somewhere either using an existing bolt or screw, or driving a hole and adding a screw right near the lights that you can wrap your ground(-) wire to.

    ---------- Post added at 07:52 PM ---------- Previous post was at 07:38 PM ----------

    2.) I want to power my new spot lights. How do I wire them to a switch on my dash?


    The correct way to go it is:

    Have a small gauge power(+) wire come from the battery, to a small fuse (2.5amp should do, I think), to the inside of the vehicle to the dash.

    Hook up the power(+) wire to one side of your switch.

    Another wire to the other side of the switch will go right back through the firewall to the engine compartment.

    The wiring, then, is the same as the above.

    The power(+) wire coming from the switch is put to terminal 85.

    86 goes to ground(-)

    30 is larger gauge power(+) wire from the battery which is...?...anyone..?... fused. Right... The guy in the back got it...

    87 is wired to the positive(+) wire of your light

    Ground(-) the black wire of your lights.

    To add to the confusion a's a little extra information on how to simplify the switch wiring, but I don't recommend it..!!..

    To power your relay, nothing says you have to have your switch on the power(+) terminal. Really, if you wanted, you could have constant power(+) to the relay, but have the ground on a switch to complete the circuit.

    In other can go from the battery, to a fuse, right to terminal 85.
    Inside the cab...ground a wire to a screw, have the wire go to the switch. The other side of the switch the wire comes out, goes through the firewall and to terminal 86. So to trigger the relay you're still completing the circuit. The only difference is you are completing the circuit on the negative(-) side of the relay as opposed to the positive(+) side.

    I do NOT like this set up, even though it cuts down on running a power(+) wire to and from a switch inside the vehicle, because if there is a rub anywhere on the negative(-) wire your relay will trigger ON because the wire will ground itself. Not too bad with lights'll just realize it at night or when you go out to your car one day and the battery will be totally dead. But other accessories you might rather want a fuse to blow than for a circuit to be completed. Nothing comes to mind right now but there's got to be a few examples. :)

    ---------- Post added at 07:54 PM ---------- Previous post was at 07:52 PM ----------

    That's pretty much it. This should get you started on your next wiring project using these components.

    I do not claim to be an expert. Simply sharing my knowledge. If you follow any of these instructions and your truck catches fire well...I'm sorry. It wasn't supposed to. My culpability, however, stops at " probably shouldn't have followed any of the instructions or listened to anything I said."

    ---------- Post added at 07:59 PM ---------- Previous post was at 07:54 PM ----------

    Here are a few links to posts with examples and with a little more information on the above information: relay relay relay relay
  3. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    You are awesome Steven! This is some very helpful information. For me, this stuff is second nature, but there are definitely a lot of people that don't neccisarily know how this stuff works. I'll try to make a couple add-ons to this with some schematics I've made and some write-ups.

    Let's keep the helpful information flowing, however, lets try to keep things neat, and accurate. Depending on how this thread ends up, I might trim some posts to keep the good info to the top of the thread.

    EDIT: Of course, if I had bothered to click the links, I would have noticed your first link was to one of my threads....Excellent choice Steve! :great:
    Last edited: Aug 8, 2010
  4. theindnman

    theindnman Member


    I appreciate the information. Very usefull.
  5. CDN_BigBlazer

    CDN_BigBlazer New Member

    Awesome write up! Glad there's a lot to learn from the people on this site

  6. Enkeiavalanche

    Enkeiavalanche Loving the Outdoors Staff Member 5+ Years ROTM Winner 5000 Posts

    Very nice write up... Well done......
  7. JonTiffany1

    JonTiffany1 New Member

    i have the full time amber drl's and the all on high mod done to my avy, sure lights up the road, however, i made it a little easier than the way you did it. I bought a 50 pack of adaquately rated diodes from radioshack, wrapped them in heat shrink, leaving only about a centimeter exposed on the ends, and wired them to and from the ground relay coil in the headlamp hi and the devices i wanted to stay on when putting the highbeams on, which were the lows, drls, and fogs. im currently trying to see what other cool mods can be done with the diodes in the under hood fuse panel :p
  8. Grizzly Guy

    Grizzly Guy Rockstar 100 Posts

    excellent write up in deed .helps me understand this wiring thingie better for sure.
  9. Jeremy09LTZCrew

    Jeremy09LTZCrew Epic Member 5+ Years 1000 Posts

    I really appreciate you writing this. I've read on these forums before when people wired their fogs to come on with their headlights and I've wanted to do the same thing. This, along with some of those other forums will give me some good information to start playing around with it myself.
  10. H82typ

    H82typ New Member

    Well done! I'm linking to this, if that's ok!

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