Mod #1 Cooper Bussman power distribution box with 5 fused 35amp relays and 5 fused non-relay
Universal Relay Fuse Auxiliary Distribution BOX Block Jeep Cooper Bussmann | eBay
Disclaimer – This is what I did to the best of my memory to fabricate and mount a wiring harness and power distribution box. No warranties will be given or implied. Actual results may vary. I have seen several rather good write ups on installing a Power Distribution Box so I thought I’d add my own distinctiveness to the collective… Also, there is a lot of work involved. Enjoy…
A power distribution system is really just an additional fuse and relay box used to power your auxiliary devices. Reasons to have one: additional centralized power not patched into existing fuses boxes; organization of relays so you don’t have them and their wires all over the engine bay, and once it’s set up you can add auxiliary devices fairly easy.
This power distribution box has 5 fused 35 amp relays and 5 15 amp fused power sources for a total of 10 separate power sources. Wiring looms that come from the accessory may be cannibalized or may not even be necessary. In the end, the power box built will be connected with switches so when I add an accessory, I mount it and run the wire to the accessory line, ground, add a fuse to the power box where needed and the switch/accessory works.
The first step is to lay out a design of what you want to do. In my case, I am building the power distribution box to handle an air compressor, rear lights, spot lights, driving lights and an open relay for future use (Front Locker). I will also be using some of the 5 always on fused locations. Here is a Visio of my plan.
Visio diagram plan
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Next step is to get what you need to put together:
Switches from
Air On Board
Wire – 16g for power box to switches with selected colors to match the wires from the power block
Bullet connectors in 16-22 (you can never have too many)
Butt splices 12-14
Crimping tool! Yes, save yourself the hassle right now and just get one
Heat shrink tubing and mini torch
Soldering iron, electrical solder
Electrical tape, black and white (to write on)
Zip ties, lots of them in several sizes. Small sizes help build the wiring loom
Diodes – 1n504 3amp for Headlight and aux power sources (may already be in a wiring loom)
T-Taps or Scotch wire connectors
Split wire covering for wires the engine bay
High Temp black paint
Bussmann BP/HHH ATM Add-A-Fuse
Then, plan where you want to mount the new Relay box while laying out what you want in a wiring harness. This will determine the wire lengths, ground locations, etc. I chose a location next to the other fuse boxes because they are close to the battery. The location selected uses existing bolt holes on the driver’s side.
Fabricate a mounting Bracket: This took a bit of time trying to pick the right spot and fabricate the bracket. I used 1.5” support metal purchased at Home Depot cut, cleaned up, bolted and painted. I used existing M8 1.25 thread holes on the driver’s side engine compartment to mount fabricated L brackets with another strip bolted together and mounted close to the battery. I also used another piece of metal bolted to the underside of the bracket bolted to an existing M8 hole on the fender, for extra support.
Power Box Mounting Bracket, sanded – Before paint
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Power Box Mounting Bracket, sanded – After paint:
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Painted and attached
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Build the loom to match your switches and power requirements per your diagram. Mine looks like the Medusa’s spawn with wires for 5 switches. All switches from AOB have four wires: Power from relay, Power from a 12v source where the power can be power from a switched 12V (for accessories like a compressor) or 12v source (from headlight high beams) depending on the switches use, power for dash lights and Ground. I have a diode on the power from the aux switched and headlight sources to protect the switch and prevent power flowing back. Watch the way you mount these. The Gray bar on the diode is on the switch side.
Measure the lengths of wire needed from the switch locations to the fire wall then to the switch block. Measure and measure again then give yourself another six inches.
I built the wiring loom so I could have all the connections to the switches inside the cab and run just one end to the power distribution box through the fire wall. I did this to route the wire loom through the firewall easier. I also labeled all the wires by the number on the fuse block and the intended use. I used white electrical tape and a sharpie. The wire color purchased also matched the wire color of the wires from the switch block. Color coordinated and fashionable!
Once the wire harness was completed, I also used a multi meter on an ohm setting to test to make sure all wires are routed correctly. I then wrapped the loom in black electrical tape and taped a folded piece of spare wire through to a point and the end of the loom. This end will actually contains a wire for each switch that connects to the relay, the ground and the power wire for the headlight activation. I’ll use the length of wire taped at the end to drag the wire loom through the firewall. Note: the thin spare folded and taped wire’s sole purpose is to pull the wire harness through the firewall grommet.
Compressor, Power Distribution Box, wire loom and assorted stuff… The wire harness has all black ground wires soldered together and only 1 ground passing to the fire wall. There are 5 switch accessory wires and a wire that powers the switch with 12v from high beams for the driving and spot lights…
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After building the internal loom, connect all bullet connectors to all the wire ends of the loom as well as the matching pair on the four wires to each switch. Do not put connectors on the side needing to pass through the firewall. (doh) This will be done later.
The 2015 4Runner Trail Premium has a spot in the firewall grommet that you can pass the wire loom through. It is a round nipple that can be snipped off leaving a rubber tunnel big enough for 8 16g wires to pass through. I pushed a coat hanger through first then the wire I taped to the wiring loom end and passed the wire passed right through. I then pulled that end of the wire through the firewall. My measurements were correct and the wires met at the relay box with enough to spare.
If you did it right, the color wires of the loom match the color of the wires to the switches on the relay box. I then added the bullet connectors and connected the switch wires to same color wires of the relay box. Add bullet connectors or butt splices to both ends, cover the wires with a wire covering for the engine compartment and zip tie it accordingly.
This note is in the compressor install… I only have the compressor installed right now so, I measured the accessory wire to the compressor, the pressure sensor wires, strung them from the mounting location to the relay box, wrapped in split loom wire cover, taped where appropriate and zip tied along the rear of the fire wall. Note: a pressure sensor is connected in-line to the compressor switch wire to turn it on and off at a range of 70-100 psi. This can be changed later too.
So double check connectors on the relay block will fit into the connectors on the wiring harness in the engine bay, check the connectors on the wiring harness that will connect to the switches from AOB. Then, check the connectors on the switches. Now’s the time to see if they are crimped well enough, trust me on this.
Since I labeled all the wires that match the switches and already added bullet connectors, the next step is rather easy. Connect them all up. The switch blanks in the dash are accessible by pulling down on the lower dash panel, next to the steering wheel. It pops out with a good pull. (Thanks Antman) The blanks have a spot in the top and bottom that can be pressed and they really just pop out. The blank next to the dimmer switch is a bit more challenging but a very small screw driver stuck in the bottom of the switch blank from the back pushes the mount clip and that too pops right out.
I have two aux devices that I want to run anytime; I used a Bussmann BP/HHH ATM Add-A-Fuse
that really just plugs into an open slot of the inside fuse box in the white box on the driver side box just under the left side of the dash, or replaces one of the fuses. Then, I connected bullet connectors to each side of the wire and plugged it in. I have this on un-switched power right now but will find a switched power fuse slot.
Pop out the old blanks, place the switch, with the connectors attached, through the hole… do not fully clip back in yet. Check wire length and see if your measurements worked. A second set of hands is really helpful in holding open the box, keeping a light on the work space and feeding you bites of slow roasted turkey sandwich and coffee.
Connect up the power… Measure the red and black wires from the power box to the battery. The red power wire is 8 gauge and the black ground is 10. Cut as needed and add a circle connector with a crimping tool. I Connected the power box ground, and the ground from the wire loom to the battery ground point on the side of the engine compartment next to the battery. Then I connected the heavy 8g wire to the battery. (no sparks… a good sign!)
Right now in my build, there are 3 fused relays and 5 fused power sources not used, but they will be. I pulled out the fuses then wrapped the aux wires in tape and split cover. When needed, I’ll pull out the appropriate “accessory” wire, add a fuse, connect the wire to the accessory and watch it work.
At this point I tested the switches and that I had power coming out of the accessory wire on the relay, with the multi meter. Everything works. Then I plugged everything back in, popped in the switches and buttoned all up.
What’s left? In my wiring harness, I have the blue wires connected as dimmer power so I just need to connect that wire to a wire coming from an existing dash illumination wire. I have read that wire is a green on any switch. I still need to connect the blue wire with a scotch or red T-Tap for the dash illumination. But, it’s tested with un-switched power. Next week, I will to tap into the high beams, for the driving lights but I decided to take Saker’s advice and use a 9005 extension that I’ll tap into. (just ordered from Amazon.
Dash opened up showing the backs of two AOB switches (green)
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Switches installed - AOB switches fit pretty dang good.
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Note my switch locations. The Air compressor is a long way away and I will not accidently press it. The others, well, I just filled in the empty blank holes. The stock switches are keyed and only fit in one location. I can grind off the keying and mount them anywhere I want but that will be saved for a later date.
Almost done....
