Trailer Harness electrical

ISPKI

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Hey all,

So I got my 94 F-Superduty earlier this year and have been having a great time stocking up on firewood for this coming winter.

The truck has an extremely heavy duty bumper/tow hitch setup and I would like to setup a trailer harness and controller for the truck so I can start hauling trailers of wood.

What wire gauge size is recommended for the harness from the brake controller to the trailer connector? I have a 7+5 pin combination plug.

I read that 10AWG is sufficient. Now, is there a cable on the market that has the 7 conductors in a single jacket or would I be better off getting one 4 conductor and one 3 conductor for the run? Trying not to drop hundreds of $$$ on the wiring, but also dont want to hackjob it.
 

TNBrett

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According to Dexter(the axle manufacturer) 12ga is sufficient for two axles w/ brakes. And yes, 7 conductor trailer cable is available w/ the proper wire sizes and color coding.
 

ISPKI

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According to Dexter(the axle manufacturer) 12ga is sufficient for two axles w/ brakes. And yes, 7 conductor trailer cable is available w/ the proper wire sizes and color coding.

Got it. I am looking at a cable with 2 10ga 1 12ga and 4 14ga.

I think its laid out like this:
10ga is for brake power supply and ground
14 gauges are for lights (brakes, running, reverse, and ???)
12 gauge is some kind of auxiliary power for an RV

25ft looks like its around 90$. Seems like a good deal.
 

TNBrett

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I think typical 7wire trailer cable is as follows.

White 10ga ground
Black 10ga battery pos/charge
Blue 12ga trailer brakes
Yellow 14ga reverse
Green 14ga running/tail lights
Red 14ga left turn/stop
Brown 14ga right turn/stop
 

Big Bart

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Ispki,

I would recommend setting up a trailer wiring junction box on your frame or maybe you have a good spot on the underside of your flat bed. Not sure if 94 had any factory wiring you can use in the cab For your controller. As I recall my 95 F150 had some extra wiring in the cab for trailer brakes and a factory fuse spot. So you do not have to bring wires through the firewall. My 88 IDI to the best of my knowledge does not. So I had to do all the wiring.

The advantage to the junction box on your rig and trailer is you can easily change the 7way plug or pigtail on the vehicle or the trailer. You also can easily test at the junction box to tell if it’s the pigtail or the vehicle or trailer side wiring. Trailer wiring never seems to last more than a couple years. Wires break, connectors corrode, truck side receptacle gets smashed and cracks, pig tail gets dragged down the road, etc., etc., etc.

So you likely can just buy 10g wire for the trailer power to the brake controller and from the controller to the junction box. Then buy a pre built pigtail for the 7 way on vehicle and a pigtail for the 7 way on the trailer. (You also will want a breaker on the wire going from the battery to the controller.)

I highly recommend the Tekonsha Prodigy 3 controller. Feature rich, compact, can remove when not needed, can move between trucks, multi axle, and does electric and electric over hydraulic brakes

Trailer side wiring kit -

Vehicle side wiring parts - (You will cut off the male side of the extension cable And buy a separate junction box)




Factory trailer light kit. This might not be for your truck so look it up. (I think maybe it was you who had the challenges with wiring being butchered but maybe it was another member. If so you may not have factory connectors anymore.). Keeps you from having to puncture or cut your factory wiring.


Its a little more work the first time but then makes fixing down the road super easy! For me it also allowed for a quicker install of a second plug in the bed for a 5th wheel trailer.

Food for thought.
 

ISPKI

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Awesome, thanks for all that info. Yeah my truck had a utility bed on it and the end of my factory harness was chopped up and spliced into that. I have since reworked the rear of the harness by splicing in a 14ga trailer cable. The truck does have a pair of junction boxes, once of which I used to make the connections for the tail light harness. They are mounted inside the rear frame extensions so I will use one of those for the trailer harness connections.

I have a hopkins 7 round + 5 flat connector with a 7 round connector and 30ft of trailer harness cable on the way for wiring it up. What i'll do is cut some feet off the 30ft cable, wire it into the junction box and then use the few leftover feet to rig up a pigtail to the plug at the back of the truck.
 

Booyah45828

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IMO, you don't need 25 foot, as only a couple of the wires will be making the full 25 foot run front to rear. The rest will connect to existing wires in the back.

Piggybacking off of TNBrett's list.

I'll run the pos/batt wire and brake controller wire from under the hood.
The ground will get connected to somewhere clean and rust free on the chassis.
And the lights will all be solder spliced into the rear wire harness that's usually located near the spare tire.

I've only ever had 2 issues wiring them like this.

One was a farmer with a livestock trailer that had 20+ amps of clearance lights. That one needed a separate wire and relay off the clearance circuit to operate.

The other was on a honda ridgeline, that had the brake/turn/clearance lights powered through mosfets in the BCM. On that one the OEM designed and required a separate trailer control module installed to power the trailer lights and keep the BCM from wigging out.

Something that you might find is that a lot of your heavier trucks will have a trailer wire bundle ran back and taped to the rear harness. And the fuse box already has fuse sockets in place for trailer connector. All you have to do is install fuses and wire the blunt cut wires to the correct spots on the 7 pin. I don't know if your 94 superduty is like this, but it'd be worth looking into.
 

ISPKI

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Good idea, I know it has a number of auxiliary fuses, relays, and electrical lines for generators, air compressors, welder generators, a bucket lift, etc, but I am not sure how its all organized. I have most of it disconnected currently from when I removed the utility bed.

I do have a large number of marker lights wired into my harness. My bed is extended to 14ft and has upwards of 20 small marker lights plus the LED round tail lights plus markers on the rear bumper. I really dont want to load anything else onto the main harness, even though it would probably be OK since everything is LED now but I am looking at wiring up some brighter lights on the overcab rack that would illuminate the work area at night.

SO. What I have to look into is wiring in a relay power distribution box for the tail and trailer lights.

I have a relay distribution box I had salvaged from an 01 explorer. has circuits for dozens of fuses and relays with a single input, looks like 4ga probably. Maybe that would be worth using, it would certainly make for a clean setup.
 

Booyah45828

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It's not a bad idea to do that. That explorer distribution box would be excessive, but if you have it, I'd use it. If it isn't water tight, you'll have to put it under the hood, and then run your 25 ft cable all the way to the back to the 7 pin connector. All of the wires you'll need for relay signal should be at the bulk head connector under the hood regardless. So it shouldn't be too hard to wire up.

Honestly, if you're wanting work lights or other accessories, that explorer box and it's extra relays/fuses might work out pretty slick.

In my list I actually forgot about a freightliner straight truck that had the same issue as the ridgeline. On both of them, I looked into installing the factory trailer light module, and both were over 1000 bucks. So a handful of water tight relays piggybacked off the truck lights and we had a functioning system, and it cost way less then the factory option.
 

ISPKI

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It's not a bad idea to do that. That explorer distribution box would be excessive, but if you have it, I'd use it. If it isn't water tight, you'll have to put it under the hood, and then run your 25 ft cable all the way to the back to the 7 pin connector. All of the wires you'll need for relay signal should be at the bulk head connector under the hood regardless. So it shouldn't be too hard to wire up.

Honestly, if you're wanting work lights or other accessories, that explorer box and it's extra relays/fuses might work out pretty slick.

In my list I actually forgot about a freightliner straight truck that had the same issue as the ridgeline. On both of them, I looked into installing the factory trailer light module, and both were over 1000 bucks. So a handful of water tight relays piggybacked off the truck lights and we had a functioning system, and it cost way less then the factory option.
Oh yeah, its way excessive but it was destined for the trash anyways and has all the fuses and relays in it with wire leads ready for splicing. It has some 10ga leads coming out of it too and I have the electrical schematic for it.
 
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