BoredKender

I have searched this forum, as well as 4 others, and I cannot find why one needs a relay for fog lights instead of wiring the switch straight to the battery. Can anyone answer? I am working on installing my fogs this week.

Mark Doiron

If you use wire and a switch that are heavy enough to carry the current for the for lights, then you may not need a relay. However, by using a relay, you shorten the distance that you must pull the thick wires (especially through the firewall and under the dash), and you reduce the need for a larger switch.

However, having said that, if you plan to connect to any of the stock JK electrical wiring (such as wiring them to the stock switch), then you must pay careful attention to what current levels the JK is expecting on that wire else you cause unexpected failures. IOW: If the JK is expecting 100 milliamps (and I made up that number), don't try to wire two 100-watt bulbs directly on that circuit. You may blow a fuse, you may cause the computer to croak, you may burn up wiring, or any of a lot of other bad things.

BoredKender

Thank you very much. If my switch is rated for 25A, will that be sufficient to run straight off the battery? It came as a kit with the lights, so I would assume it would all be designed to work together without a relay, but for 20 bucks assuming doesn't always go right.

vgullotta

I have a pair of KC Highlights Daylighters (100W) and they came with a switch but no relay, so I have two questions:

1. Do you think the switch is fine without a relay in a normal setup?

2. If I added a second pair of Daylighters and tied them into the same switch, would that be too much for the switch?

I've never put in a relay before, but maybe it's just worth it to do so just for the efficiency of the deal, but I was curious if I could run all four lights without a relay, that would be the easiest solution as far as installation was concerned.

Thanks in advance for any advice =)

whpony96

For each set of lights you should use one relay. First it will make the lights brighter. No matter how thick the wire is it will have a amount of resistance to it. The longer the wire the more resistance it will have. The relay is like a remote switch and moves the connection between the lights and the battery closer together. The shorter the wire distance from the light to the battery the brighter the light will be. This is one of the reasons why relays are used. The second reason is safety. If you do not use a relay and you have to much draw from the lights being on the entire length of the wire could heat up and the insulation could melt and then it will be a fireworks show if the fuse doesn't blow.

So in short get a relay set up.....

vgullotta

going to buy them both now, thanks for the info =)

BoredKender

Would someone be kind enough to post a laymans diagram of wiring a relay for us non-schematic reading rookies?

ohboyX3

there may be a diagram on the forum, but here is the link to one i have referred to...

http://www.classictruckshop.com/clubs/earlyburbs/projects/bosch/foglites.htm

Mark Doiron

Two 100-watt bulbs will draw about 17 amps. So the switch will be fine. However, that's a lot of current draw, and if you don't use a relay you're running those thick wires for much longer--which means you need even larger wire to prevent power loss and overheating from voltage drops. Depending on the rating of the insulation, you're probably looking at 16, maybe even 14 gauge wire. It's not battery cable by any means, but it's stout, stiff stuff. Difficult to work with, especially when you're bringing it through the firewall and up under the dash and trying to get it to wherever that switch will be located. I wouldn't want to run it. A relay will make your install easier.

As for running an extra pair of bulbs on that switch: Too much current, which means the switch will overheat. And, the wire size is unbelievably difficult to work with. But, put in a relay with terminals rated at that current and you'll be good to go with even a light-duty switch (as long as it handles the relay's coil current--the current it takes to operate the electromagnet in the relay).

vgullotta

I bought the Pilot 12v 30A relay at Pep Boys and found this diagram for it if this helps.

I put it in-line between the fog and the switch.
85 goes to ground
86 goes to the switch
87 goes to the fog lights
30 goes to the battery