Home OF4WD Forum Technical & Trucks Technical Chit Chat What is a relay and how does it work?

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    (– by LugNutz #5331 –)

    Howdy all

      Well with the wheeling season fast approaching, many of us are scrambling to get those last minute items such as new lights, E-fans, radios and other electrical accessories installed and running. Over the years I've seen too many wiring nightmares and had to give a few Electronics 101 courses, particularly when it comes to the when, why and how to of using relays. I know a lot of you are scared by wiring, so thought I would take a little of the "bad voodoo" out of these things.

      In the simplest of terms, a relay is a remote control switch. It can be used to redirect power (either positive or ground) to a device, be used to isolate a device, trigger a factory device, or be utilized to control or deliver higher currents more efficiently to a device.

    The key to know is the numbers, usually found on the bottom of the relay.

    Pin 30- Generally used as the source input pin. ie a fused lead from the battery
    Pin 87a- This is the normally closed position. With the relay "off" or not in use, this will be a direct line to Pin 30
    Pin 87- This is the normally open position. With the relay "on" or energized, this will be a direct line to Pin 30
    Pins 85 & 86 These are your relay control terminals. Suppling a positive charge to one and a ground to the other will "turn on" or energize the relay

    Here's an example.

      Instead of running a heavy wire from your battery, through your firewall to a switch, then back out the firewall to a set of lights on your front bumper, you could mount a relay under the hood by the battery. Then you take a good gauge wire (WITH A FUSE) from your battery to terminal 30 on a standard Bosch style relay. Next you take another good gauge wire and run it from terminal 87 out to your lights. Now you have just shortened your power wire by about 10 feet or so, allowing a much shorter and direct route to the lights, making them brighter. But how do I control my lights now? Easy! There are a couple of ways to wire this now, all fairly simple. I will suggest the way I commonly wire them, which will give you the benefit of the factory headlight reminder. Find the parking light wire under the drivers side dash, it will be coming out of the headlight control switch (Make sure it is actually for the marker lights and not the dash lights or it will dim with them). Tap into that wire with a small gauge wire (18-20 Ga will be fine) and run it to the switch you wish to use. Now run the other side of that switch wire out through a rubber grommet in the firewall to the relay and plug it into terminal 85 on your relay. Finally, take another small gauge wire and run it from terminal 86 on the relay to a good ground. Now whenever you have your marker lights on and the switch flipped, you will have foglights, and when you turn your lights off, they will all go off.

      Now that was a pretty basic one that most guys may know, but did you know you can also use a relay to break a circuit such as an electric fan? Why would you want to do that? 2 words….Water crossings! By adding a control relay to your E fan circuit you will be able to cut power to the fans electric motor before it gets submerged which will help prolong it's life and stop the fan blades from splashing water all around the engine bay.

      If you have an aftermarket electric fan (or a Taurus fan) with a thermostat controller, you can simply use it's trigger wire option to shut it off, but if you've got a factory E fan or don't have the fancy thermostat controller then you'll want to add a control relay to the power wires running to the fan. Find a section of wire that leaves you enough room to work on it and cut the power wire. Put female spade connectors on both cut pieces and plug the supply side into terminal 87a, and the fan side to terminal 30. Some of you may think that is backwards, but it will work either way, and by putting the feed on 87a in this case will prevent you from having a live terminal (87) exposed on the relay when this circuit is turned off. Connect a ground wire to terminal 86. Now you just have to run a single small gauge wire from terminal 85 through your firewall to your switch and connect it to a feed, personally I would recommend an accessory feed. Now that this circuit is installed, your fan will work as it normally does, but when you activate the kill circuit, it will shut off power to the fan (recommended before water crossings).The nice thing about this is that the fans work with the circuit off, and it will allow them to run properly when the vehicle is turned off. I would recommend adding a small LED to the switched side of your dash switch, just to remind you that the fans are off. Also for those of you who always seem to find trouble, make up a short piece of heavy gauge wire (couple of inches) and put a male spade connector on each end. That way, if the relay ever fails, you can simply pull the fan wires and jump them.

      There's a lot more you can do with a relay and I will write up a few others in the future, in the meantime, here's a great interactive link http://www.bcae1.com/relays.htm for those still confused.

      Finally I'd just like to add a few safety reminders.

    I can't stress that enough. Any feed wire should utilize an appropriately sized fuse within 8 inches of it's attachment to a power source (Battery, Ignition feed). This is a very common mistake for guys running amplifiers assuming the 30 amp fuse built into the amp is protection enough. That internal fuse will only protect what's behind it (the amplifier in this case) but it will offer no protection to the wire before it. If the feed wire gets pinched, chaffs on the fire wall or gets a screw through it, it will provide a direct short to ground and the wire will heat up like a toaster element until something blows. Improper installation and a lack of proper fusing of aftermarket accessories is the major cause of automotive electrical fires.

    When you are installing a new wire run, pay attention to it's surroundings. Are there any sharp edges to rub through the insulation? Under the hood or even in some interior areas you should always protect your wires with split loom tubing and wire ties or electrical tape. When you have to pass a wire through metal such as a firewall, always find or add a rubber grommet to protect it. What about pinch points? Will the wire get trapped and crushed when you start bolting seats and panels back in, or if you dent in a panel? How about heat exposure? Are you near a manifold or is the catalytic converter right underneath that floorboard? And finally always use the appropriate gauge wire or larger for your intended current draw, overkill never hurts here.

    Whenever hooking up electrical items, I always solder all my connections and cover them in heat shrink tubing, dual wall heat shrink if it will be exposed to the elements. Now that's my personal preference, my wiring usually outlasts the vehicle, but quick crimp connectors are acceptable and easier for most when they are crimped properly…. vise grips crushing a crimp connector is never a good thing, use a proper crimping tool and seal your connections if they will be exposed. To add to that, never use wire taps or Scotch Loc connectors anywhere they will be exposed to the outside elements, they're not sealed, wear out quickly and expose existing factory wires to moisture and oxygen which are never good combinations for exposed copper. Use a good stretch tape or liquid brush on tape, and add a die electric grease to any quick connectors to ensure a long and reliable connection.

    Hope these little tricks help you to build and wheel a reliable trail rig.  🙂


    (– by 762X39 #2435 –)

    I just did a quick read through, your description and suggestions are spot on. Stressing the point about fusing circuits at the battery is more than good advice but could be a lifesaver in a mishap. Batteries can put a few thousand (I'm not kidding) amps into a direct short causing an explosion and/or fire.


    (– by 762X39 #2435 –)

    I thought I would post a couple of pictures of my Inverter install showing the 150 amp fuse bolted to the firewall and the connections to the inverter under the back seat. The fuse and holder are the forklift type and the wire is #4 welding cable which turns out to just meet spec for a 1kw inverter. The lugs are crimped, soldered, taped and protected with heat-shrink tubing.



    (– by whateverworks #5191 –)

    This diagram can be used for e-fans, lights, and whatever else would need a relay or high current usage… The amperage of the relay will depend on the need… (ex. e-fan will use 30 – 40 amp relay, lights will need 30amp)



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