7-Way Trailer Plug Wiring Instructions & Diagram

This guide will help you wire a seven pin trailer plug connection.

Vehicles equipped with a tow package generally have wired connectors installed in the rear of the vehicle with wires extending to the front of the vehicle in the cab under the dashboard just above the brake pedal. Such may be just a bundle of wire ends or wired to a plug termination which may vary with the make of the vehicle.

Trailers with electric brakes need brake controllers installed in the tow vehicle to tow heavy duty trailers. 7 pin connectors are required in this case. These connectors also provide pins to supply auxiliary 12 volt power to the trailer and to power backup lights.

When referencing the 7-Way RV Plug diagram, make sure you are looking at the plug from the correct vantage point. Wire colors may vary. Matching the connection functions is the objective.

Make sure the ground wire [typically white] is securely attached to the frame on both the tow vehicle and trailer side. The ground wire should be large enough to handle the entire power load. Grounding through the trailer hitch ball is an unreliable ground connection.

Test to see if the brake lights work and the trailer marker lights work. If nothing works when turning on both at the same time, the trailer ground is the problem. Another test is to step on the brakes and and see if the trailer lights go out. If they do, the trailer ground is the problem.

Lastly, grease the trailer plug to help prevent copper oxide from building up which forms when oxygen from the air or water comes in contact with copper.

A "Trailer Test Unit" was built to test the tow vehicle and trailer wiring using the "Auxilary 12 volt+" charging circuit. The test unit can be used to test the tow vhicle without power from the "Auxilary 12 volt+" charging circuit. The trailer can be tested without the tow vehicle provided that the trailer has a battery connected to the "Auxilary 12 volt+" charging circuit. Otherwise, the trailer must be connected to the tow vehicle with the test unit inserted between the tow vehicle and the trailer. If changes to the test unit are required, its internal wiring is shown below.