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Relay Basics

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    (mods move this if in wrong place)

    This is an informational post. This is to explain and teach the basics relay function and how to wire up a relay with either an off trigger or on trigger set up. Of course this information does not mean that you will be a genius when it comes to wiring up things with relays, but will give the basic knowledge of relays and their function.

    What is a relay?
    A relay is a simple electromagnetic switch that when the coil is energized it causes the switch to either open or close. In Automotive it is used to allow smaller wires at the trigger source, such as headlight switch, and higher amperage wire to carry the load of the work a shorter distance. Relays can be used in either on or off coil states and can power almost anything in a car from a horn to hydraulics.

    It is important to understand what you are dealing with and as such you need to learn the basics of automotive electricity and how it works. I will give a brief description of the basics below before I continue on the relay section this is going to be a good refresher for some and new stuff for others.

    The Basics:
    Battery: Big ugly thing under the hood that holds a charge and has a + and a – printed on it. The battery is the key to electricity to a car; when you do any electronic work on a car please disconnect the battery so you don’t blow fuses or burn wires.

    Positive: +, hot, positive are all terms that mean the same thing it is the power side of a circuit.
    Ground: -,Ground, cold are also terms that are the return side of the circuit (complicated don’t ask)

    Short circuit: When two opposing poles of electricity come in contact ie.. positive and negative touch. This can be bad as it will melt wires at the worst if it continues or blow fuses at the best.
    Trigger: It is something that causes a reaction. For example pushing the horn button is pushing a trigger. Triggers can be either + or -.

    Relay:
    Welcome back to the relay portion of the show. Here is a basic diagram of a 5 prong relay. I choose the five prong relay because it is a little less common but it can show you a different sort of relay option which is the off trigger relay.
    [IMG]http://i567.photobucket.c...oob1000/relaydiagram1.jpg[/IMG]
    One thing you should first notice is the numbers on the diagram. These numbers are the generic numbers used on almost every relay out there. It is a sort of generic coding if you will.
    #85 Trigger
    #86 Ground
    #30 Power wire
    #87 feeds the device
    #87a feeds the device

    For this example I made the trigger wire that feeds #85 positive, but it can be negative and if you make the trigger negative then the #86 must be a positive.
    Here is a basic wiring diagram to illustrate the use of a relay to light up a set of fog lights.
    [IMG]http://i567.photobucket.c...glight_wiring_diagram.gif[/IMG]

    You will see how the #30 comes straight from the battery (it has a fuse in between battery and relay) this will be the power that supplies the physical fog lights.
    #85 is to a ground source (remember car metal is ground).
    #87 connects the fog lights to the relay.
    #86 is fed by the switch, toggle, push button, light sensitive, etc.

    With this set up here is what should happen. Switch is clicked power goes to 86 the power of 86 and ground of 85 causes a coil inside to move the internal switch of the relay allowing power to travel from #30 to #87 and the fog lights come on.
    Now #87a works in reverse when the switch is flipped off then no power goes 86 hence the coil releases and moves the internal switch to pass power from #30 to 87a and power a device that only works when power is absent.

    I hope this helps you understand the basics of relays and the flow of power in a car.


    If you have questions please pm me or post here and I will answer plus add to this post to help others in this forum.

    I have a question. I have had an HID kit (6k) for about a year and outa the nowhere the driver side doesnt turn on anymore. Ive taken out the HID and added the stock bulb just to see if the problem is the HID ballast etc. but it doesnt turn on either. does this has to do with the relay or what? what can i do to fix it?
    Leo if you are not getting light output from the driver side with stock bulb then it may be the stock relay.

    Question now is are you plugging in the stock bulb to the ballast or to the stock wiring?
    If you are running a stock bulb through stock wiring then what it sounds like to me is your stock wiring is out and may need replacing, see if any power is going through it with a volt meter. Check the fuse box also (what exact fuse i do not know) see if any are blown.

    I know that this wiring issue can coast some big $'s
    dam man thanks for making me think str8 haha. it was a blown fuse. I jsut replaced it and it works perfectly now with the HID. Thank you =D Dumb me
    • Edited 1 times, last by xGhostx on .
    No prob glad to be of some help

    You should look into getting a relay harness though, it has a fuse built into it so this should not happen + it will save your stock wiring from going out as well.
    where can i find one? I am not very familiar with electrical stuff besides the basics
    well ty ima check it out. p.S srry for thread jacking rene
    Np, i don't think it is a thread jack being that it is about a relay harness? (now it is )
    Leo u remember wat fuse u change..... cuz I just got that same problem
    check the map for the right sided headlight fuse. it should say rhlb or somehting like that. I just changed the current one that it had (the blown yellow one) and changed it for the extra one the car brings
    i have hid's im looking in to the relay an was wondering if i need 2 one for each side or dose one kit cover both headlights?
    xXscionXxtc7 wrote:
    i have hid's im looking in to the relay an was wondering if i need 2 one for each side or dose one kit cover both headlights?


    you only need one and you`ll be gold
    Rene325ci wrote:
    Positive: +, hot, positive are all terms that mean the same thing it is the power side of a circuit.
    Ground: -,Ground, cold are also terms that are the return side of the circuit (complicated don’t ask)


    "hot" is only applied in AC systems not DC
    ground can be either the negative or positive though 99% of car use a negative ground
    I am aware of hot and being for AC, but it is commonly used in automotive correctly or not.


    Okay I am bumping because it is a good thread and recently was being asked about wiring.
    leo08scion wrote:
    stock wiring


    failllll ... i know its late.. but come on u gotta run a relay to be safe
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