Trailer Truck Cable

What is Trailer Cable

A trailer cable is a multi-pole electrical connector between a towing vehicle such as a car or truck and a trailer. It is intended primarily to supply automotive lighting on the trailer, but also provide management and supply to other consumers.

It is also referred to as “trailer wire,” “trailer connector,” or “trailer connecting cable.” Another feature the connector offers is feedback from the trailer to the towing vehicle, from being as simple as a fault indication for ABS brakes to advanced command, monitor and control using CAN bus for EBS brakes.

Type of trailer connector

According to more than 25 different international standards, trailer connector is used all kinds of vehicles. These range from agricultural machinery to cars and commercial vehicles to defense. Depending on area difference, there are different standard, such as Euro trailer connector, north America Standard trailer, Australian standard and etc. According to the pin number, there can be 4-pin, 7-pin, 6-pin, 5-pin, 12-pin, 13-pin.

There are several standards for trailer wires, and if you search, you’ll find a different Trailer Wiring Diagram for each.  Each standard has it’s different purpose, so please don’t just make it up as you go.  Don’t make a weird wired trailer.  Follow these guidelines and make it right!

The approach for you depends on your electrical needs.  To start, every trailer needs lights — brake lights, turn signals, and tail lights.  Some also need side markers and running lights.  Some brakes need electricity too — to actuate electric brakes, or to disable hydraulic brakes when backing up.

The following trailer wiring diagram(s) and explanations are a cross between an electrical schematic and wiring on a trailer.  We recommend these standards because they are pretty universal.  That said, for specific situations, there are industrial standards with different connectors and wire arrangements.  It can get confusing, so if you don’t already have a specific standard in mind, follow these.

4-Pin Flat Connector

At a minimum, all trailers need at least 4 functions:  Tail lights, Brake lights, Left & Right signals.  4 wires will give these functions, so the simplest scheme is a 4-pin connector.

The most common 4 wire connector is the 4-Pin Flat Connector as shown here.  Trailers that use this are usually fairly light weight and don’t have brakes or other power accessories.  It’s the most common style for “consumer” type trailers.  Small utility trailers, light boat trailers, little campers, off-road trailers and many more use this traditional 4-Pin Flat connector.

Lighter Duty Trailer (No Brakes) = Use a 4-Pin Connector.

1.  White = Ground  (See White Wire Notes below.)
2.  Brown = Tail Lights, Side Markers and Running Lights  (See Brown Wire Notes below.)
3.  Yellow = Left Turn Signal & Left Brake Light
4.  Green = Right Turn Signal & Right Brake Light

Please see the Trailer Wiring Diagram and Connector Application Chart below.  The 4-Pin connector only has the first 4 items listed.  The rest you can ignore.

5-Pin Flat Connector

(Round style 5-Pin Connectors also exist.)

Trailers with capacity over 3000# Total Gross Trailer Weight should have brakes.  That’s not mandatory everywhere, but it’s a good idea.

If a trailer has brakes, then it needs a connector with at least 5 pins.  The 5th pin, a blue wire, gives power to operate (or disable) the trailer brakes.

Traditional Trailer + with Brakes = Use a 5-Pin Connector.

1-4  Wire the first 4 pins (White, Brown, Yellow, Green) just like the 4-pin connector above.
5.  Blue = Electric Brakes or Hydraulic Reverse Disable (See Blue Wire Notes below.)

In the Trailer Wiring Diagram and Connector Application Chart below, use the first 5 pins, and ignore the rest.

If your truck has a built-in 7-pin socket, but you only need 5 of the pins.  Use the 7-pin connector anyway (see below), and just leave out the last 2 wires.  It accomplishes the same thing for 5 wires, but with a connector that’s compatible with your truck.  The 5-Pin flat connector above is nice for easy handling, but if your vehicle already has a 7-pin, just use it.  It’s OK, within the standards, to leave out wires for your custom situation.

7-Pin Connector

For trailers that have a little more going on electrically, we recommend 7-pin connectors.  The 2 added pins are typically for Auxiliary Power and Back-up Lights.

Expanded Use Trailer + with Brakes, Aux Power & Back-up Lights = 7-Pin Connector.  (6-Pin Connectors also exist, but they are less common.)

1-4  Wire the first 4 pins just like above, and the 5th line goes to the brakes.
5.  Blue = Electric Brakes or Hydraulic Reverse Disable (See Blue Wire Notes below.)
6.  Red (or Black) = 12V Auxiliary Power (See Red Wire Notes below.)
7.  Purple = Back-up Lights (Sometimes another color.)

7-Pin Connectors like the one pictured are very common for RV’s and other bigger-ish trailers.  This is the style we recommend.  Other styles exist — though the pin-outs are often different.  Several industrial styles are similar and definitely use different pins.

It is OK to leave a pin or two blank (unused and unconnected).  For instance, looking at the trailer wiring diagram, if you want Auxiliary Power, but don’t have back-up lights, then just leave the purple wire out.  A blank spot (unconnected pin) doesn’t hurt anything.

The Trailer Wiring Diagram and Connector Application Chart

See below video about why tension and strength are important: