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