Quick answer
A 4-way flat connector runs a trailer's basic lights: running lights, left and right turn/brake, and ground. A 7-way connector (usually the round RV blade style) adds a circuit for electric trailer brakes, a 12V power line and usually a reverse circuit. Light utility, jet ski and small boat trailers usually use a 4-way. Campers, car haulers and most trailers with electric brakes need a 7-way plus a brake controller in the truck.
Side-by-side
| 4-way flat | 7-way RV blade | |
|---|---|---|
| Circuits | Ground, running lights, left turn/brake, right turn/brake | The same four, plus electric brakes, 12V power and an extra circuit often used for reverse lights |
| Electric trailer brakes | No | Yes, with a brake controller in the vehicle |
| Typical trailers | Small utility, boat, jet ski and motorcycle trailers | Travel trailers, larger car haulers, equipment and enclosed trailers with electric brakes |
| Shape | Flat plug with four wires | Round plug with flat blade terminals |
The 4-way flat connector
The 4-way flat is the most common plug on small trailers. The common color code is:
- White: ground
- Brown: tail and running lights
- Yellow: left turn and brake
- Green: right turn and brake
If your truck has no trailer plug, a vehicle-specific "T-connector" harness plugs in between the tail lights and the factory wiring, so you get a 4-way without cutting wires. Some vehicles use separate bulbs for turn signals and brake lights (often amber turn signals). Those need a powered converter that combines the two signals into the format a trailer uses. The product listing will say if a converter is included.
The 7-way connector
The 7-way RV blade connector carries the four light circuits plus:
- Electric brake output from the brake controller to the trailer's brakes.
- 12V auxiliary power, which can charge a camper battery or power a breakaway system's charger.
- An extra circuit, most often used for reverse (backup) lights or, on surge-brake trailers, to lock out the brakes when backing.
Many newer full-size and heavy-duty trucks come from the factory with a 7-way plug, and some also have a built-in brake controller. If yours has the plug but no controller, a vehicle-specific brake controller harness lets an aftermarket controller plug into the factory wiring under the dash.
Do I need electric trailer brakes?
It depends on the trailer's weight and your state. Many states require brakes on trailers above a set weight, and the limit varies from state to state. Larger campers and equipment trailers almost always have electric brakes, and those need a 7-way connection and a brake controller to work. Check your trailer's manual and your state's rules.
Mixing trailers: adapters
If your truck has a 7-way and you also pull a small trailer with a 4-way flat, a 7-way to 4-way adapter lets you plug it in. Adapters also go the other way, but a 4-way plug can't power electric brakes on its own; you'd need added wiring and a brake controller.
What we carry
Shop trailer parts and wiring for vehicle-specific T-connectors, 4-way and 7-way connectors, adapters, converters and brake controller harnesses from brands like CURT and Husky Towing. Each vehicle-specific harness lists the exact vehicles it fits. Finish the setup with a receiver hitch and the right ball mount and hitch ball.
Related guides: trailer hitch classes explained and do I need a weight distribution hitch?
Have a question about fitment? Contact us. More guides: Truck Buying Guides.
Frequently asked questions
Can I use a 4-way for a trailer with electric brakes?
Not by itself. A 4-way flat only carries lighting circuits. Electric brakes need a brake output wire from a brake controller, which a 7-way connector provides.
Does a 7-way plug work without a brake controller?
The lights will work, but the trailer's electric brakes won't. The brake circuit needs a brake controller in the vehicle, either factory-installed or aftermarket.
What is the difference between a 7-way RV blade and a 7-way round pin?
They are different plugs that don't connect to each other. The RV blade style is the common one on pickups and RV trailers in the US. Round pin 7-way plugs are mostly found on some commercial and agricultural trailers.
Why do my trailer lights work on one side only?
The most common cause is a poor ground. Check the white ground wire connection on the truck and trailer, then check for corrosion in the plug and blown bulbs.