Matt's Guide to wiring accessory lights, relays, & switches
 

****NOTE 2022: Picture links are dead and were uploaded using a third party hosting site, therefore they are gone for good. The member who created this guide no longer logs in, so there is no retreiving this info or these photos. ****


I made this to serve as reference for one of the most frequently asked questions. HOW DO IT WIRE MY FOG LIGHTS (or driving lights or off-road lights). The following three schematics in plain english should illustrate how to properly and safely wire accessory lights to your vehicle.

Schematic #1: Two lights with a relay controlled by a switch in the cab. Most of the kits you can buy come with all the equipment to do exactly this.

http://www.nmvfc.org/matt/fog-light-relay.gif

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Schematic #2: Rather than using a switch, this will synchronize your accessory lights with one of your truck's stock lighting systems. I personally have my off-road lights setup like this so they come on/off with my high beams. You can use anything you want.

http://www.nmvfc.org/matt/fog-light-relay2.gif

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Schematic #3: This is a combination of both. You have a three-way switch in the cab. The switch's positions are basically ON-OFF-AUTO. With the switch on, you get lights on no matter what. With the switch off, the lights are off no matter what. With the switch in auto, they will work like #2 and follow the existing high or low beams or whatever you choose.

http://www.nmvfc.org/matt/fog-light-relay3.gif

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All wire-to-wire connections should be made with automotive butt connectors. It is good to put some silicone RTV in the ends to seal them up from the weather. Do not use household wire nuts!
http://www.westgateparts.com/picture...connectors.jpg

The relays can be connected using insulated female spade connectors. Do NOT use uninsulated connectors and put some silicone RTV on them to seal them up.
http://thumbs.ebaystatic.com/pict/130115600689_1.jpg


Make sure the fuse holder for the 10-12 gauge wire is also rated for that current. You can't put a whimpy fuse holder on a big circuit.

Nice write-up, good graphics. I typically do pretty much the same thing for most the stuff I wire up in my vehicles, with one exception: instead of putting the dash-mounted switch on the positive voltage to the relay coil, I run the ground side through the firewall to the switch, and then from the switch to ground.

This way, if the grommet falls out (or rots, or whatever) and the wire frays against the bare metal of the firewall, it won't short out the circuit and blow the fuse (always, always, always, always, always use a fuse!! They're cheaper than a new wiring harness or a new vehicle!) Wired as I described, if a short in that circuit were to happen, the fog lights (or whatever) would come on without interaction of the switch, and you'd know it was shorting to ground somewhere between the relay and the switch.

...that, and I don't like to unnecessarily run power wires through the firewall if I can help it. Signal wires (such as temperature sensors, etc.) and ground wires are fine, but I avoid running hot wires through the firewall.

nice write up Matt.....i had thought about doing this because of soooo many questions......glad someone else got to it besides me:006:

Good job on the right up. Is it going to be posed in the "how to" section?

screwed up about fog lights
 

Ok, yes I screwed up about not getting fog lights and living in central valley of California. I'm pretty sure the wiring is there I think and haven't been able to find a wiring diagram for a 2008 XLT Super cab. Any help out there? I have found the fog lights but not any thing else I need like the relay and the correct light switch.
Thanks in advance.

good idea I have never thought of running it like that.

Images aren't popping up for me, and I am definatly interested in the On/Off/Auto setup.. anyone happen to have the link they can send over to me via PM perhaps?

Sorry guys, I accidentally deleted the entire contents of my web server while uploading skiing pictures on vacation last month. Apparently someone needs to come up with a integrated breathalyser for MS Front Page so this doesn't happen in the future. I'm slowly putting stuff back online as I notice it missing. I'll add this to my list for this evening.

...yet another reason to always use a text editor (preferably vi) when editing text, instead of relying on some corporate bloatware to "do the right thing" when it creates text for you from pretty pictures... ;)

Actually its more like "another reason not to keep hitting OK without reading the message box first". Specifically where it said "Do you want to delete all these files?" and I apparently clicked YES.

exactly. that's why I recommend `vi` as the editor... you won't accidentally do stuff like that. ...at least, not more than once. ;)

Tada. All fixed. I'll try not web edit under the influence anymore.

web edit under the influence? lol. so now i know who edits my posts for me so they make no sence!?

bringing it back but,


would a 30amp relay and 20amp fuse be enough for 2 100watt lights? or do i need the 40amp relay?

EDIT: also 85 get grounded right?

They'll draw about 15 amps so the 20 amp fuse should work. Either relay will work since there both sufficiently rated for the load. Terminals 85 & 86 are interchangeable unless the relay has a built-in flyback diode.

interchangeable? either one can be + or - or are they both +? i know fuses but relays are a whole new thing for me

radioshack only had a 30amp and 40amp relay

Terminal pin out for a typical SPST (4 prong) relay:
Term 30 power in
Term 87 power out
Term 86 control (power from switch)
Term 85 ground

However, either one (term 85 or 86) can be used for the pos or neg so long as one is connected to your switch and the other to ground unless the relay has a built-in flyback (spiking) diode.

so the relay does have to be 20amp along with the fuse, i havent found a 20amp relay yet but plenty of 30's

You can use any SPST relay so long as it meets the minimum circuit load. In your case thats 15 amps so anything rated above 20 amps will work.

sweet thanks

is there any install pics...?

Pics of what? This is just a generic wiring diagram, not something specific to an install.

lol.





Hey Matt....just to make sure........

87a = secondary switch?

Using a 5 prong relay......Like say i'm doing backup lamps, and i switch the relay via stock reverse lamp on the 86 spade. And a switch in the cab switching the relay via 87a. That way when i put the truck in reverse, lights will come on. AND I can manually switch them on. Am i right?

87A is the Normally Open (NO) side of the relay if so equipped. When the relay is off, 87A is connected to 30. When the relay is on, 87 is connected to 30.

I believe that 87A is NC to 30, 87 is NO to 30. The rest of the statement is correct.