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Discussion Starter #1
I'm by no means a DC wiring expert, but I know many of you are. I bought a light bar off a forum member, and was going to wire it in on my roof. I was going to make my own plug so I can connect/disconnect the light bar at will. But when I search MLS (they originally sold the light bar), they offer the wire harness complete with a switch, a 30 amp fuse, and a 40 amp relay. Why do I need a relay if the power line is already fused? I simply don't understand this stuff. Thanx

SS
 

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The relay can handle a higher amp load. The switch will use a small amount of power,( keeping the wires in the cab smaller and cooler) to control the relay. The relay can then handle the higher amp load of the light bar.
 
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Discussion Starter #3
OK, that kind of makes sense (to this neophyte). I currently have 2 pod lights up there. I've grounded them to the roll bar (ROPS). I want to pull them off, and just wire the light bar to the existing power line I have up there, and then ground the light bar to the ROPS just as I did the pods. The power comes from my BUSS bar under the passenger seat. So I could just wire the relay into this power line under the seat, ground the light bar to the ROPS, and be good to go?
 

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Discussion Starter #4
Let me be a little more clear. Does the relay go between the BUSS and the light switch, or between the light switch and the light bar?
 

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the ground can go to the roll bar, but i really suggest a dedicated ground wire. In the picture above, substite the buss bar for the batter and this should help.
 

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part of that got cut off.  switch in cab should be fused from fuse box or another 12 volt positive source.  when power is applied from the switch to the relay, the contacts in the relay close and allow power to go from the battery (buss bar) to the light.
 

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Discussion Starter #7
OK, here I go again. The relay's I've used (for trailer brakes) Have 2 connections. Basically inline on the power wire. I want to use my existing wiring. So if this is what I use (because I currently have one in the shop) here's what I'm assuming. Power line is fused at the BUSS. The power goes to the light switch which I've already installed adjacent to the stock light switch. So if I use the 40amp relay I have, simply put it in after the BUSS, or after the light switch? Or does that even matter since it's gonna be after the fuse. Keep in mind....old dawg, new tricks!!!!
 

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Discussion Starter #8
Oooops, we overtyped each other.
 

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appropriate size fuses for your application should be used. also power wire to switch can come from fuse box or buss bar. relay takes very little power to control.
 

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To keep it simple a relay is used because most switches cannot handle the same current as a hi-output light bar(they can melt). Wiring size can be reduced for the battery-switch-relay wiring only as well(18 guage is fine), battery-30 and 87-lightbar must be larger wiring(14 guage minimum) though and fuses are needed for safety. Most light bar harnesses will work but may need wires lengthened to fit your machine.
 

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Discussion Starter #12
This stuff is all foreign to me. I already have a separate switch wired to the BUSS. So if I understand everyone............30 amp fuse in the BUSS. Hot wire to switch. 40 amp relay after the switch. Continue hot to light bar. Have I got it yet??
 

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This stuff is all foreign to me. I already have a separate switch wired to the BUSS. So if I understand everyone............30 amp fuse in the BUSS. Hot wire to switch. 40 amp relay after the switch. Continue hot to light bar. Have I got it yet??
Brian, follow Firebird's diagram in post # 9 above.

This is the right way to do it!

C-Ya!!
 

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substite battery with buss bar.
The only issue with this is the light bar could be switched on with the vehicle off, leaving you with a dead battery. I recommend getting the power for 85 from a switched source like the accessory plug behind the dash or under the seat. Then the light bar will only turn on when the switch is on (unless your buss bar is switched).

The only other thing is check your wire size. 14 awg is probably enough, unless you have one of the 300w light bars some people are running. Here's a chart I like to use. I usually bump the wire size up (down in awg size) by one just to be sure.
 

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Discussion Starter #15
I'm trying to avoid tearing things down so I can get into the wires in the tunnel. Apparently that doesn't seem to be an option. Maybe someone wants to buy a light bar. :puke:
 

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You can pull a wire through by just removing the battery cover. Connect your light bar positive there (battery) and rout it forward with a straightened clothes hanger or similar. Then crawl under the front and grab the wire and route it up.
You can take the switch power from one of the dash switches (incoming power side).
 

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This stuff is all foreign to me. I already have a separate switch wired to the BUSS. So if I understand everyone............30 amp fuse in the BUSS. Hot wire to switch. 40 amp relay after the switch. Continue hot to light bar. Have I got it yet??
If you use a 30 amp fuse and your light bar only pulls 15 and your wire is rated to less than 30 amp there's no point in using a fuse. You could burn up your light bar and possibly melt your wiring and/ or start a fire. You need to match the fuse to the light bar and wire capacity to prevent issues.
 
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SS-

Run it on the outside of your cage, wired like shown in post 9 and don't look back. There are no good shortcuts in good wiring...
 

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Discussion Starter #19
OK, so I bought the wiring harness from MLS. (Hoo boy, this might be a stupid question!). Since I already had power under the hood, I just changed the fuse in the BUSS to a 30 amp. Now I have double fuse and a relay. The install was pretty straightforward, but,,,,,,,,,,,,,,,,,, since I'm not wiring direct to the battery, I have a LOT of extra wire on the harness. Maybe 5-6 feet. I want to cut out this excess so I don't add to the spaghetti under the hood. The question is, are these wiring harness designed with ohm/impedance calculated for the length, or can I cut and paste a lot of this wire. I was gonna cut the section out, but my wife says,,,,,you better ask first. Soooooo, I'm asking. Happy wife, etc!
 

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Cut away Brian, you will be fine.

C-Ya!
 
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