fog light wiring

[xbmxxx]

Accidentaly posted this in the wrong section before. Mods please delete that one, I can't figure out how to from my droid.

I picked up all the supplies on my lunch break to build and mount 4 fogs on the front on my 05 STi. I copied a few members on here ideas and decided to mount 2 to the front plate mount and then 2 in the factory fog locations.

Has anyone gotten away with connecting them to the factory fog light harness? Id really like to not have to run all the wiring through the firewall and mount another switch. Im afraid they'll draw far too much for that fuse though.

[xbmxxx]

I finally got them to work, but their really dim so I need to do something different. Basically, the factory fog harness has a 12v and a Ground out of it. If i hook the light straight to that it works, but barely. I think I need to run a 12v straight from the battery, to a relay, then out the relay to the foglamp. then the factory hot wire will go to the relay to switch it, and then theres another wire off the relay that should go to ground. Does that sound right? I had it that way, but it kept popping the 10a fuse for the fogs in the dash. I dont think the relay should take more current to flip then powering 4 fogs should, and its doing that without it popping.

[Usher]

What size is the fuse? Your fog lights are probably drawing around 20 amps, but I think the stock fuse is a 15 amp fuse. Try replacing it with a 25 or 30 amp fuse.

[xbmxxx]

added 2 relays, got them to work.

[ShadowImg]

What size is the fuse? Your fog lights are probably drawing around 20 amps, but I think the stock fuse is a 15 amp fuse. Try replacing it with a 25 or 30 amp fuse.

Not at all OK.

You need to replace the wiring of the entire circuit in question if you do something like this, or it's quite dangerous.

OP, you did it the right way. Always add separate wiring that's heavy enough for the load, and use relays appropriately. Overloading wiring is bad juju.

[Usher]

Not at all OK.

You need to replace the wiring of the entire circuit in question if you do something like this, or it's quite dangerous.

I'm fairly sure that the fog light wiring is oversized for the two stock lights and will be fine with the additional load.

Disclaimer: I'm going from memory here and I'm currently 2,000 miles from my car so I can't just go out to my car and measure the wire size.

[wolfman79]

The stock fog wiring, along with most of the wiring in the car is going to be 16/18 gauge. It's not enough for just stock lights let alone anything above and beyond. No your car probably will not burst into flames but why pay good money for better lights if you're not giving them enough juice. Run some relays and use the stock wiring and switch to trigger them.

[rollo]

Truth, relays are definitely the way to go. Cramming a ton of current into components that are not sized appropriately will make them get hot. Many stories of switches melting in dashboards, wiring harnesses failing due to overheating, etc etc.

[Usher]

Well I was bored on the subway this morning, so I pulled up the factory Subaru wiring diagram. The factory fog lights are wired through a relay, so there is no current going through the switch. The fog light wiring is 16 gauge, and the factory manual specifies 21 amps allowable for 16 gauge wiring.

So to answer the original question:
Has anyone gotten away with connecting them to the factory fog light harness? Id really like to not have to run all the wiring through the firewall and mount another switch. Im afraid they'll draw far too much for that fuse though.
Yes, you can run them off the factory fog light wiring if you don't want to mess with running your own. There is no safety issue, you just need to increase the fuse to a 20 amp fuse. Of course this is moot now, since xbmxxx has already gone ahead and changed his wiring.

As for increasing the output of the lights:
Assuming you have 2 meters of 16 gauge wiring, you should have 0.05 ohms of resistance in the wiring and about 0.3 ohms at the relay, so a total of 0.35 ohms for factory wiring. Assuming a constant 2.65 ohms for each bulb (lights actually increase resistance at higher temperatures), the factory wiring will give you 7.85V at the bulbs, so 23W each. Wiring up a relay for each pair will give you 9.49V at the bulbs, so 34W each. Wiring up a relay per bulb will give you 10.6V at the bulbs, so 42W. These numbers are kind of deceptive, because the resistance of the light bulb drops when they are cold, so the actual wattage will be much closer than I have estimated here. Also, I'm assuming 12V, but you'll probably be closer to 14 when running your car, which would give you 32W, 46W, and 58W.

In my opinion, if you're going to be changing the wiring to increase the light intensity, you should just go all out and put a relay on each bulb, otherwise just save yourself the trouble and run it off the factory wiring. Personally, I just run my auxillary driving lights off of one relay, since I was feeling lazy when I wired them and I doubt I would notice a <3 dB difference in power.

[wolfman79]

Great info. My only question is were your figures based on using 16ga wire like stock? Most people, at least that I know of, usually upgrade to 14ga when running relays. Just curious what the numbers show with heavier wire.

[Usher]

I just used 16 gauge resistivity for everything, since it doesn't make a significant difference in the power estimates. For the estimates, I assumed 16 gauge wires and 2 meters of wiring = 0.05 ohms. Shortening the wiring by mounting the relays at the lights would probably get you down to about 1 meter = 0.025 ohms. From there, if you increase to 14 gauge wiring you are only going to drop down to 0.016 ohms. Since all these numbers are so much smaller than the contact resistance in a typical relay, it won't have a significant effect on power delivered to the light. (about 1W gain by halving the wiring length and 0.25W from going to 14AWG wire)

From these calculations, it's clear that the most important thing is to make sure you have excellent connections at the ends of your wiring, keep the wiring runs as short as possible, and if you want the most power delivered to your lights, wire up a separate relay for each one.

[ShadowImg]

Now that's the kind of information you want people to see when they search for this next time. :thumbsup:

Someone should toss a wiring diagram for a standard relay / aux fog setup too if it isn't already posted around somewhere.

[rollo]

The factory fog lights are wired through a relay, so there is no current going through the switch.

Yeah.. to clarify, I wasn't suggesting that the Subaru lighting goes through the switchgear, more as an illustration of the kind of bad stuff that heat caused by resistance can do.

I've always done relays simply for the sake of overbuilding, but it sounds like you're on it as far as the calculations.

[xbmxxx]

I ended up putting 2 relays at the end of the harness, where the factory fog bulbs used to be. used the factory 12v source to trigger the relay, and the harness ground to ground the relay. I grounded the bulbs directly to the chassis, then ran a 14ga wire straight from the battery, through the relay, to the bulbs. I then just piggybacked the other 2 lights off those. Much brighter then when they were just plugged in to the factory wiring.

[andrew's bug eye]

awesome, i had this same question also. i have a custom light bar with hopefully 2 hella 500's and 2 hella 500 fogs.

[spiwrx]

just wanted to add: from the diagram in the earlier post, the power to the switch should be fused also or tap from the '30' terminal of the relay.

The diagram does show the power to the switch coming from the fuse box, just make sure it's after a fuse....

(& the fuse should be appropriate size for the wire being used).

I know the intention was probably there but just wanted to clarify.

Personally I like to switch the ground and add (+) jumper form 30 to 85, as a short to the switch would then just turn the lights on. However this diagram includes an indicator light which complicates that. (also much of the factory wiring switches to ground)

[xbmxxx]

I am re doing mine (with solder and heat shrink this time, not twist and tape) and i think I have this right. heres my awesome drawing.

http://i53.tinypic.com/r2vpfq.jpg

(the two squares in the lower corners are the factory harness fog plugs)

does this look right?

[spiwrx]

Looks good to me, don't forget to fuse near the battery.