Trailer plug types: 4-pin, 5-pin, 6-pin, 7-pin connectors
Identify trailer connector types by pin count, wiring color, and function so you can match your tow vehicle to your trailer without guesswork.
You need a 7-pin RV-blade connector for a trailer with electric brakes, a 4-pin flat for a small utility trailer or bike rack, and an adapter for anything in between. That covers about 95 percent of cases. The full breakdown of types, wire colors, and uses is below.
Quick match by trailer type
| Trailer | Connector you need |
|---|---|
| Small utility trailer, bike rack, light boat trailer | 4-pin flat |
| Larger boat trailer with surge brakes | 5-pin flat or 7-pin RV-blade |
| Single-axle camper without brakes | 4-pin flat |
| Travel trailer or camper with electric brakes | 7-pin RV-blade |
| Fifth-wheel or gooseneck RV | 7-pin RV-blade |
| Most factory tow packages on modern trucks | 7-pin RV-blade (often with 4-pin built in) |
How pin count maps to function
Every plug has one ground pin. The rest are signal/power pins, so a 4-pin has 3 functions, a 7-pin has 6.
| Pin count | Adds | Functions |
|---|---|---|
| 4 | — | Ground, running lights, left turn/brake, right turn/brake |
| 5 | +1 | Adds reverse light or surge brake disable |
| 6 | +1 | Adds 12V hot lead (battery charge) |
| 7 | +1 | Adds either reverse or electric brakes, depending on what’s missing |
4-pin flat
The smallest and most common connector. Four flat blades in a row. Used on anything that only needs trailer lights.
Wire colors:
- White: ground
- Brown: tail/running lights
- Yellow: left turn and brake
- Green: right turn and brake
If your trailer doesn’t have brakes, a battery, or reverse lights, a 4-pin flat is all it needs. Bike racks, kayak trailers, small open utility trailers.
5-pin flat
Same as a 4-pin with one extra pin. The fifth pin is either reverse lights or, on hydraulic surge brake trailers, a disable signal that cuts the brakes when you’re backing up. Common on boat trailers with surge brakes.
Wire colors add blue for the surge brake disable / reverse line.
6-pin round
Square or round shell, six pins in a circle plus a center pin. Adds a 12V hot lead (black wire) on top of the 5-pin functions. The hot lead charges a battery on a small camper.
This connector is uncommon in the US (Pollak / Bargman style). You’ll find it on some older campers and small RVs.
7-pin RV-blade
The standard for any modern truck and any trailer with electric brakes. Round shell, seven blade-shaped pins arranged in a circle around a center pin.
Wire colors (SAE J560 standard):
- White: ground
- Brown: tail/running lights
- Yellow: left turn and brake
- Green: right turn and brake
- Blue: electric brakes
- Black or red: 12V power (charge line)
- Purple or orange: backup lights / auxiliary
Color codes vary slightly by manufacturer, so always check the trailer’s wiring diagram before splicing. Bargman and Pollak both publish standards.
7-pin round (heavy-duty)
Less common but still around on commercial trucks. Same seven functions, larger pins, screw-in connectors. Adapters between RV-blade and 7-pin round are easy to find.
Adapters
Mismatched connectors are the most common towing pain point. The fixes:
- 4-pin trailer on a 7-pin truck: plug-in adapter, $10 to $20.
- 7-pin trailer on a 4-pin truck: doable but you lose brakes, charge line, and reverse. Add a brake controller and a powered adapter, $40 to $80, or upgrade the vehicle wiring properly.
- Heavy-duty 6-pin round to 7-pin RV-blade: specific adapter, watch the brake/charge pin mapping.
Wiring tips that save trouble later
Dielectric grease in every connector before you plug it in. Connectors corrode, especially boat trailers and anything stored outside. A pea-sized blob in the receptacle prevents 80 percent of failed-light issues.
Mount the truck-side receptacle high enough that it doesn’t drag. Most factory locations are fine; aftermarket installs sometimes drop it under the bumper where road spray and gravel chew it up.
For a trailer over 3,000 lb GVWR (1,500 lb in some states), you need electric brakes and an in-cab brake controller, which means a 7-pin connector. Don’t try to fake it with a 4-pin.
Picking yours
Look at the back of your truck. If there’s a round 7-blade receptacle (often with a small 4-pin tucked underneath), you have a full tow package. Match that to whatever your trailer expects, or run an adapter. If you only have a 4-pin, you can either add a 7-pin (a real wiring job, $200 to $500 installed) or stick to trailers under 3,000 lb without brakes.