On a standard 4-pin flat trailer connector: white is ground, brown is tail and running lights, yellow is left turn/brake, green is right turn/brake.
Those four carry over to 5-way and 6-way connectors, and to most 7-way blade harnesses. Then there's the 7-way blade, where the industry never agreed. It's wired to two different color conventions, and on the older one, green means taillights and brown means right turn — backwards from every 4-pin diagram you'll find. That single reversal is why so many trailer wiring jobs get done twice.
This article covers every connector, both 7-way conventions side by side, and the one habit that makes the whole problem disappear.
Every connector at a glance
| Function | 4-Way Flat | 5-Way Flat | 6-Way Round | 7-Way Blade (SAE) | 7-Way Blade (RV) |
|---|---|---|---|---|---|
| Ground | White | White | White | White | White |
| Tail / running | Brown | Brown | Brown | Brown | Green |
| Left turn + brake | Yellow | Yellow | Yellow | Yellow | Red |
| Right turn + brake | Green | Green | Green | Green | Brown |
| Electric brakes | — | — | Blue | Blue | Blue |
| 12V auxiliary | — | — | Black or Red | Red | Black |
| Reverse / backup | — | Blue* | — | Black | Yellow |
*On a 5-way flat, blue is reverse-activated power. It may feed backup lamps, a surge-brake lockout, or both, so confirm what's on the other end before you connect it.
Which one do you have? 4-way flat is standard on utility, cargo, and small boat trailers. 6-way round shows up on horse trailers and older campers. 7-way blade is on most modern trucks and travel trailers, and it's the only one that needs electric brakes and 12V power.
The 7-way problem, explained once
Two color conventions are in active use on the same physical connector. Two of the biggest names in towing equipment publish different ones.
Convention A: the SAE-style layout
Keeps the 4-pin colors and adds three.
| Wire | Function |
|---|---|
| White | Ground |
| Brown | Tail / running |
| Yellow | Left turn + brake |
| Green | Right turn + brake |
| Blue | Electric brakes |
| Red | 12V auxiliary |
| Black | Reverse |
Convention B: the "RV standard"
Also called the traditional 7-way. Still on plenty of trailers.
| Wire | Function |
|---|---|
| White | Ground |
| Green | Tail / running |
| Red | Left turn + brake |
| Brown | Right turn + brake |
| Blue | Electric brakes |
| Black | 12V auxiliary |
| Yellow | Reverse |
What flips between them
Only white and blue mean the same thing in both. The other five change jobs:
| Wire | Convention A | Convention B |
|---|---|---|
| Brown | Tail / running | Right turn + brake |
| Green | Right turn + brake | Tail / running |
| Yellow | Left turn + brake | Reverse |
| Red | 12V auxiliary | Left turn + brake |
| Black | Reverse | 12V auxiliary |
When it actually bites you
Good news first: the terminal positions are identical. Two factory-terminated plugs work together no matter what colors are inside them. If your truck and trailer both have original plugs, none of this affects you.
It only matters when you're matching loose wires by color (terminating a new plug end, repairing a harness, or splicing in a replacement lead).
Now, get it wrong and your running lights come on with your right signal, and your turn signal lights the taillights.
The costly version: you can also feed 12V auxiliary power into a lighting circuit, or wire a trailer battery to the truck's reverse output. That backfeeds and damages things. Verify each terminal, then test everything before you tow.
If a previous owner has already been in there, replacing the plug end and running fresh wire beats reverse-engineering their work.
Shop trailer plugs, harnesses, and junction boxes →
Commercial trailers use a different standard entirely
Semi tractors and commercial trailers run SAE J560 7-pin round. Don't apply light-duty color codes to it. The big structural difference: stop lamps get their own circuit (red) instead of sharing with the turn signals.
| Pin | Wire | Function |
|---|---|---|
| 1 | White | Ground |
| 2 | Black | Clearance, side marker, ID lamps |
| 3 | Yellow | Left turn + hazard |
| 4 | Red | Stop lamps and ABS |
| 5 | Green | Right turn + hazard |
| 6 | Brown | Tail, license plate, clearance |
| 7 | Blue | ABS primary power / auxiliary |
Two federal rules govern the rest: 49 CFR 393.11 sets which lamps are required by vehicle type and width, and 49 CFR 393.28 covers how the wiring has to be installed. Our LED trailer light retrofit guide walks through what that means for a fleet.
Adapters: when the truck and trailer don't match
An adapter beats splicing almost every time. Two things it can't do:
It can't create circuits that don't exist. A 4-pin source has no brake, 12V, or reverse wire for an adapter to pass through. Need those? You need a harness upgrade.
It can't convert between signal types. Light-duty wiring combines stop and turn; J560 separates them. That conversion takes electronics, not a shape adapter.
The three that cover most mismatches:
4-way flat to 7-way RV blade for a 4-pin trailer behind a 7-way truck
7-pin to 4-pin for the reverse
7-way junction box to terminate a harness cleanly at the tongue
Test, don't trust the colors
Every honest source on trailer wiring ends up here, so we'll say it plainly:
Wire colors are a strong assumption, not a fact. Manufacturers deviate. Previous owners improvise. And the two 7-way conventions guarantee some trailers won't match your diagram.
Find the ground first. It must contact bare metal on the frame, not paint or rust.
Probe every circuit. Have someone work the headlights, both signals, and the brake pedal while you label each wire by what it actually does. Reverse and 12V need separate checks. Brake-controller output often reads nothing while parked, so use the controller's manual override.
Cut power before you cut wire. Unplug from the truck and disconnect any trailer battery or solar feed. Auxiliary circuits stay live with the ignition off.
Match by function, not color. Then seal every connection with dielectric grease and weatherproof splices.
Five minutes with a test light saves the afternoon you'd spend chasing a fault you created.
What lights does your trailer actually need?
| Your trailer | What to run | Shop |
|---|---|---|
| Utility, landscape, cargo (under 80" wide) | Stop/tail/turn lamps, side markers, license lamp | Tail lights · Markers |
| Boat trailer | Fully sealed submersible lamps — standard LEDs won't survive the ramp | Boat trailer lights |
| Enclosed / long trailer | Same as above plus clearance lamps along the run | Complete kits |
| 80" wide or over | Adds clearance and identification lamps under federal rules | Complete kits |
| Any trailer, any age | Sealed IP-rated LEDs draw less current and outlast filaments | All trailer lights |
Lower current draw is the quiet advantage of LED here: less voltage drop over a long trailer, which means the lamps at the back stay as bright as the ones at the front.
Common questions
What's the standard color code for trailer wiring?
White ground, brown tail/running, yellow left turn/brake, green right turn/brake. That holds for 4-way flat, 5-way flat, 6-way round, and SAE-style 7-way blade. RV-standard 7-way is the exception.
Which wire is ground?
White, on every common connector including SAE J560. It must land on clean bare metal.
Are 4-pin and 7-pin colors the same?
On SAE-style 7-way, yes. On RV-standard 7-way, no: green carries running lights and brown carries right turn.
What does the blue wire do?
On 6-way round and 7-way blade, electric brakes. On 5-way flat, reverse-activated power. On SAE J560, auxiliary and ABS power. Test before connecting.
My lights work, but the wrong ones come on. Why?
Usually crossed wiring from the wrong 7-way convention. But a poor ground backfeeding through other lamps does the same thing, so check the ground under load first.
We'll help you get it right
Wiring questions are the ones we get most, and we'd rather talk it through than have you buy the wrong thing twice.
Once your circuits are verified, the lamps are easy: LED trailer lights, tail lights, and complete kits built sealed and waterproof for road salt and boat ramps.
Already wired and still misbehaving? Our trailer light troubleshooting guide covers that — most of those faults turn out to be grounds, not diagrams.
Not sure what fits your trailer? Call 1-888-562-5125 or email info@UltraBrightLightz.com. We do this all day.
