HOW TO …..... Wire an electric Relay for cars



  • When you fit an electrical item usually aftermarket items they sometimes need a lot more power than your electrical harness fitted in the car can handle or you may want it to switch on and off with the ignition.

    The ignition cables are only low rated but that low amperage is enough to fire a series of relays to carry much higher amperage to your, wipers, starter motor, lights etc and even the instrument panel.
    Whenever you turn one of those on even just the ignition you can hear click and these are relays activating items through out your car.

    If you are fitting extra spotlights, in car audio anything that needs a serious amount of amps to run it you will need to fit a relay so the switch or cables do not get overloaded.

    Basically a relay is an electrically activated switch that can handle more power that the manual switch you use in the car to turn something on.

    Most Relays have their voltage and maximum amperage stamped on the top like the one below which is suitable for 12 or 24v up to 15 amps

    The other numbers on the label correspond to to the pins in the bottom, they tell you which pin does what, this one in the picture has 8 pins but isnt really suitable or used in cars

    Most cars use a 5 pin Relay like the diagram below

    This is an easy example and is suitable for most items in a car.

    Pin 30 and 86 are permanent live feed, usually wired direct from the battery positive with an inline fuse rated to the maximum of the relay

    Pin 85 is the earth from the relay but is still carrying 12v so this goes to the positive of your switch, the switch is then earthed and the relay is earthed when the switch completes the circuit

    Pin 87 goes to the positive of the item you want to power, the item must not exceed the amperage of the relay and then the negative of the item can then be earthed properly.

    Now when you turn the switch on, it will fire your relay and give you the proper voltage and amperage to your item without damaging your switch or your car loom


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