Seven prong       Four prong

Trailer Wiring

Caution: the diagrams below are for general reference, however you must check the wiring on both the vehicle and the trailer to verify the the wiring scheme is coordinated, as there seems to be a few different school of thought on which way is proper. We have decided to show only the most common methods and hope that someday all trailer wiring will be universal so you can just hook up and go. 

Caution: always install the appropriate fuses or auto-resetting breakers.

Seven prong for towing a trailer with electric brakes with a truck that has a trailer brakes assembly.

This seems to be the most common method to wire  the 7 way connector and most trailers requiring electric brakes come from the manufacturers wired in this manner.

NOTE the connectors shown here need to be removed from their housing to access the wire lugs.

Wiring Code 7-way Vehicle End # Color Gage Circuit Function *** Viewed from interior of connector.*** Loosen screws on sides an push it out 1 white 10 common ground 2 blue 12 electric brake 3 green 14 tail/running lights 4 black 10 battery charge (+) 5 red 14 left turn/stop 6 brown 14 right turn/stop 7 yellow 14 auxiliary/back up NOTE: #7 is the center terminal and currently is most commonly used for backup lights or as a source of power to operate accessories inside the trailer such as fans and lighting. Be sure to install the appropriate inline auto-reset breaker.

Wiring Code 7-way Trailer End # Color Gage Circuit Function *** Viewed from interior of connector.*** 1 white 10 common ground 2 blue 12 electric brake 3 green 14 tail/running lights 4 black 10 battery charge (+) 5 red 14 left turn/stop 6 brown 14 right turn/stop 7 yellow 14 auxiliary/back up NOTE: #7 is the center terminal and currently is most commonly used for backup lights.

Four prong wire connectors whether they be flat or round are typically wired as shown below: Most any automotive supply store sell these connectors and the standard 4 strand trailer wire. The colors are universal.
The vehicle end of the connectors have shielded prongs because they have a 12Volt current induced into them whenever the function (eg. brake lights) is activated and must be shielded to prevent them from accidentally shorting out..

4-Way Flat Trailer Connector	4-Way Flat Vehicle Connector	4-Way Round Trailer Connector	4-Way Round Vehicle Connector

Caution: Some vehicles have a diagnostic indicator that tells you when a lamp is burned out or not functioning, If your vehicle has this feature you may need special wiring adapters and testers. Not using these adapters may cause damage to the vehicle wiring or computer.

Even though 5 and 6 prong connectors (usually on non-American vehicles) do exist they are not commonly used and will not be covered on this page,

The single most common cause of trailer lighting problems is a poor ground on the tow vehicle or trailer. It is highly recommended that trailers with electric brakes use a #10 ground running all the way back to the negative post of the battery.
 This is not a bad idea for the lighter trailers that are not required to have brakes as well.  In lieu of this, be sure to rigidly attach the end of the ground wire  from the connector to the frame of the vehicle using a screw or a machine screw and a locking nut. Do not hope for a proper connection through the trailer hitch.. Poor ground can cause an the lights to not function when required or create an intermittent signal which may be mistaken as an indication of intent to turn.

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