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Discussion Starter #1
2013 2.5 Premium - I added the aftermarket trailer wiring kit to give me the 4-flat connector.

No problems, everything is great.

A buddy has a trailer that I might borrow occasionally. It's well under any weight limits, but it has a 7-round plug. It has lights only, no brakes or any other options that would require the extra wires.

When I looked at e-trailer, they have 4-flat to 7-round adapters, but all have an extra ground wire. E-trailer says I have to use the extra ground wire. I'm assuming the ground on the 4-flat end isn't connected and that's why the other ground needs to be connected.

Why couldn't I just take the ground from the 7-round and splice it into the ground on the 4-flat? Why would I need to connect the extra ground pig-tail somewhere else?

It does look like the wire gauge is heavier on the 7-round, maybe the current draw on a typical trailer with brakes and other options needs the heavier ground wire?
 

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The extra ground is to handle the substantial extra current from the electric brakes, which that trailer does not have. While you could indeed just splice it to your existing connector, if you ever do hook up a brake controller, you will need to connect up a big ground separately.

I expect the lights will work just fine with that extra ground wire just dangling in space. Try it and see. If the lights don't work, connect the ground.

John Davies
Spokane WA USA
 

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Discussion Starter #3
I did some more checking - it seems like the ground wire gauge is really the issue. Some of the 4-to-7 convertors that I've seen don't even have a ground wire coming out of the 4-flat, the have to rely on the larger gauge ground. Others do have the ground coming out of the 4-flat, but it apparently isn't connected at the 7-round end.

Since this trailer only has lights, I could either get with my buddy and splice in a 4-flat at the trailer end so we could use either of them, or make a custom adapter that would take the 4-to-7 adapter extra ground wire and splice it in such a way that it goes into the ground on the 4-flat. Then this adapter would be used only for that trailer. What I didn't want to do was to have to connect the ground to a metal screw or other hard point, then have to deal with not being able to just disconnect it quickly and toss it into a storage bin when I wasn't using it. I didn't want to permanently mount it - I like the way the 4-flat cable stores in the spare tire well when I don't need it.
 

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I did some more checking - it seems like the ground wire gauge is really the issue. Some of the 4-to-7 convertors that I've seen don't even have a ground wire coming out of the 4-flat, the have to rely on the larger gauge ground. Others do have the ground coming out of the 4-flat, but it apparently isn't connected at the 7-round end.

Since this trailer only has lights, I could either get with my buddy and splice in a 4-flat at the trailer end so we could use either of them, or make a custom adapter that would take the 4-to-7 adapter extra ground wire and splice it in such a way that it goes into the ground on the 4-flat. Then this adapter would be used only for that trailer. What I didn't want to do was to have to connect the ground to a metal screw or other hard point, then have to deal with not being able to just disconnect it quickly and toss it into a storage bin when I wasn't using it. I didn't want to permanently mount it - I like the way the 4-flat cable stores in the spare tire well when I don't need it.
Pretty sure the flat 4 on the subaru end has a ground already. I know every vehicle I've had in the past 15yrs had the flat 4 grounded. You can easily just ground the trailer end to the trailer also not a big deal. Though trailer wiring that goes wonky is almost always caused by dirty grounds on the trailer end of the program. The smaller lighter weight the trailer is the harder it is to get the trailer grounded well. I had an old very light boat trailer that actually was so bad I ran a ground wire around the trailer to all the lights which usually ground to the trailer at the mounting point of the light fixture - then had the wire grounded to 3 different locations on the trailer. I went from having to rebuild the lighting system nearly every year to having far far brighter lights than I ever had and not having to mess with it for another 5yrs.

Even the very poor grounding issues on that trailer caused zero issues with the Subaru or any of the other cars I towed that trailer with. Only trailers I've blown fuses with and had them impact the tow car were Uhaul trailers with major electrical issues.
 

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Discussion Starter #5
Yeah - from the 4-flat to the subie it is grounded. But the adapters that convert from the subie 4-flat to the trailer 7-round don't carry that ground through, they rely on another, heavier, ground that hangs off the 7-round end that has to be attached, somehow, to the subie.

I think my best bet is to add a 4-flat to the trailer so he'd have both connectors on the trailer and then either one of us could use it.
 

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Yeah - from the 4-flat to the subie it is grounded. But the adapters that convert from the subie 4-flat to the trailer 7-round don't carry that ground through, they rely on another, heavier, ground that hangs off the 7-round end that has to be attached, somehow, to the subie.

I think my best bet is to add a 4-flat to the trailer so he'd have both connectors on the trailer and then either one of us could use it.
I've seen this you can just pig tail into the wires at the hitch with your 4pronger. Probably the easiest and cleanest way to go. Little tip do the small insulation strip without cutting or crimping the wires and drop some solder on it. Put some liquid rubber goop on it and wrap it with electrical tape. You'll never ever have any connection issues again related to the pig tails to the 4 pronger set up.
 
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