Seven-way round trailer plugs enable a tow vehicle to provide more than just trailer lighting. Three additional poles provide trailer battery charging, electric brake control and auxiliary power to the trailer. There are two types of seven-way trailer plugs: round pin and flat blades. The round trailer pins slide into round cavities on the tow vehicle instead of connecting with blades. Both types fulfil the same function. The trailer plug wiring is mirrored by the tow vehicle wiring to ensure the wires are mated properly when connected.
Grasp the wiring harness on the front of the travel trailer and pull approximately 2 feet of the wire beyond the travel trailer tongue. Label the wires according to the associated function.
Remove the travel trailer plug's service cover to access the wiring poles. Most plugs are held together with screws. Loosen the screws and pull the casing apart to expose the wiring poles. Turn the plug so the key slot---the jutted out piece used to align the connections---faces directly upward.
Mark or note the position of the first pole on the back of the plug. Installation of the remaining wires will be clockwise from the first pole when viewed from the back of the plug.
Connect the heavy ground wire, usually white or exposed, to the first pole.The plug key is the raised portion of the round-pin plug. It allows the plug to be inserted only one way.
Attach the left turn and brake lights to the second pole clockwise from the first. Connect the tail and running lights to the third pole. Attach the battery charging wire to the fourth pole. Check the installation before continuing. The fourth pole is directly opposite the ground wire on the first pole.
Connect the right turn and brake lights to the fifth pole counterclockwise from the first. Attach the electric brake wire to the sixth pole on the plug. The centre plug---the last remaining plug---is the auxiliary wire pole.
Reattach the plug casing with the screws. Connect the plug to the tow vehicle. Turn the ignition key to the start position and test the trailer connections.
Many travel trailer wires are colour coded according to function. Check the trailer's owner's manual for colour coding labels. Pole connections are either clamp, screw or tension, depending on the manufacturer. Before closing the trailer plug, tug on each of the wires to ensure they are securely connected to the poles.
Colour coded wires might not be correctly installed. Confirm the labelling of the wiring is accurate before installing the plug.