Learn Me 7 Pin Trailer Wire Connectors.

So I realize that this probably belongs in the trailer section but this section gets more traffic.

So I have it mostly done but a couple of wires are perplexing me.

The diagrams I see say the black wore on the vehicle side is for 12v DC. What I am not wrapping my mind around is why do I even need 12v DC? Is this simply for the running lights and charging the battery for the brake away switch? 

Also where do I pull the 12v DC from? Battery? Lights etc?

The 4 pin that connects up two the 7 pin controls the lights blue wire is for the trailer controller/trailer brakes, the white wire is ground. 

After that I have a pale yellow wire that I think is for the back up lights.

(Excuse the scrap wiring on the existing 4 pin as it wasnot my doing)

Running lights have their own pin.

The +12 is for anything that you want powered whenever the truck is running.  On most trailers that's just to charge the breakaway battery, but you could also use it for stuff like backup cameras or trailer TPMS radio relays.  You probably want it wired to something that's switched with the ignition, to avoid the trailer draining your truck's battery if it's hitched up overnight.  If you can't find one with sufficient current capacity left, add a relay to the battery?

Colors and pinouts are not always standard.  etrailer.com has a good writeup on the whole thing:

https://www.etrailer.com/faq-wiring.aspx

What codrus said.

Here's the standard wiring diagram...

I have converted a vehicle from a 4-pin to a 7-in before.  I installed a brake controller which I powered from a direct connection to the battery with an in-line fuse right after the battery-side terminal.  The +12V came from a second fused direct battery connection.  Everything I have experience with has a constant +12V at the 7-pin, ignition on or not* from the factory, so I just followed that.  The only thing that the +12 did for the open trailer is to keep the breakaway battery topped off.  On dad's enclosed trailer, it also powered the inside lights and I didn't want to have to turn anything in the truck on to be able to see at night.  I also discovered that there are apparently two standards for 7-pin connectors: RV and cargo trailer.  I can't recall the difference exactly, but I seem to recall that it basically swaps two wire positions or something like that.

* The Mercedes is a little different in that before it will send power to the trailer, it needs to detect the presence of the trailer first, so if you have a bad ground there or something, it won't energize anything.  But once it knows the trailer is there, it keeps the 12 volts flowing down the red wire.

wae said:

* The Mercedes is a little different in that before it will send power to the trailer, it needs to detect the presence of the trailer first, so if you have a bad ground there or something, it won't energize anything.  But once it knows the trailer is there, it keeps the 12 volts flowing down the red wire.

Newer Ford's are same. I had to buy a little 7 pin to 7 pin adapter that basically puts resistors on the pins for the lights to trick the truck. Then I can use it to power my enclosed trailer 12v tongue jack.

So at the moment I am just trying to make sure I have tail lights and brakes.

I do have an unused connecter in the cab that is keyed hot or I can pull it from the coach battery which is much closer.

Many states require trailer brakes with a breakaway/battery to engage the brakes if the trailer comes loose.  It's also handy for charging an RV battery while driving.  I would definitely hook it up to a keyed source, so in case your trailer battery is toast, it doesn't drain down the tow vehicle battery while it's off.  If you ever plan on towing an RV, it's wise to put that circuit on a 10-ga wire with a relay so it gets adequate space to flow enough electrons.

The rest is just like the wiring diagram suggests, although I disagree with the picture Rodan posted.  That color scheme won't be found on any 7-pole connector anymore.  That follows the old 4-pin coloring for stop, turn, running.  The electrons don't care what color the insulation is, but human brains can be confused.  This is the more common color code you'll find.

The center yellow pin is technically for reverse lights, but unless you live in Europe, trailers don't have backup lights, and you'll usually find that the trailer pigtail doesn't even have a conductor for it to save pennies.  I have used that pin before for other things, like a redundant ground, or a higher-amp 12v+, but it could confuse a future owner, and it is almost always ignored.

If you're adding a brake controller, you might have a factory pigtail already under the dash.  It likely has red, white, black, and blue.  Red piggybacks off the brake light switch, white is ground, black is 12v+, and blue goes to the trailer connector.

What vehicle are you working on?

Spearfishin said:

wae said:

* The Mercedes is a little different in that before it will send power to the trailer, it needs to detect the presence of the trailer first, so if you have a bad ground there or something, it won't energize anything.  But once it knows the trailer is there, it keeps the 12 volts flowing down the red wire.

Newer Ford's are same. I had to buy a little 7 pin to 7 pin adapter that basically puts resistors on the pins for the lights to trick the truck. Then I can use it to power my enclosed trailer 12v tongue jack.

Do you have a link for that device?  What it takes to operate the 12V output for the boarding lights on the wife's horse trailer with her new F150 is way more complicated than required. 

It's something like engine running, keys in vehicle, driver's door closed, etc.

In reply to Curtis73 (Forum Supporter) :

My van is an RV so I won't ever be towing one.

I will go with the keyed source.

Karacticus said:

Spearfishin said:

wae said:

* The Mercedes is a little different in that before it will send power to the trailer, it needs to detect the presence of the trailer first, so if you have a bad ground there or something, it won't energize anything.  But once it knows the trailer is there, it keeps the 12 volts flowing down the red wire.

Newer Ford's are same. I had to buy a little 7 pin to 7 pin adapter that basically puts resistors on the pins for the lights to trick the truck. Then I can use it to power my enclosed trailer 12v tongue jack.

Do you have a link for that device?  What it takes to operate the 12V output for the boarding lights on the wife's horse trailer with her new F150 is way more complicated than required. 

It's something like engine running, keys in vehicle, driver's door closed, etc.

TOW READY 20142 7-Way Flat Pin to 7-Flat Plug to Plug Adapter, Regular https://a.co/d/0RiAb2W

And yes that's exactly what this avoids. I couldn't consistently get that sequence to produce power to the trailer plug.

Also works when the trailer has a light out or a funky ground and the truck is constantly dinging at you "trailer disconnected!" every time you hit a bump. 

In reply to Spearfishin :

Thanks!  I'll give it a try.