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The Function of the Auxiliary 12-Volt Connection on the 7-Way Trailer Connection  


What is the function of the 12 volt battery in this type of trailer? Can the interior light be turned on with the trailer NOT connected to the tow vehicle?


Expert Reply:

The auxiliary 12-volt wire that runs from the 7-way connection to 7-way trailer plug will supply 12-volt power from the vehicle to the trailer. The 12 -volt power will allow interior or exterior lights to run that are not wired in to the trailer's tail and clearance lights. The light will operate separate from the trailer clearance and tail lights and will not need to have the truck light turned on to work. The auxiliary wire will also allow you to use other 12-volt trailer accessories, like an electric jack. This auxiliary 12-volt can also supply a maintenance charge to the trailer's battery and breakaway battery.

If you have a 4-way flat plug, then your interior trailer lights must work only when you have your tow vehicle's lights on and connected to the trailer.

Unless you have a separate 12-volt battery connected to your trailer, the interior trailer lights will not work if not connected to the tow vehicle.

Also, keep in mind that leaving the electrical connected between your vehicle and trailer over a period of time can cause your vehicle's battery to drain.

expert reply by:
Jared P



Where can you find the connection to the auxiliary lineunder the trailer

Etrailer Expert

Jesse M.


@Michael No way to give a specific location I'm afraid. Even if we knew exactly what trailer you have there is no way to look this up. Your best bet would be to simply trace that wire back from the trailer 7-way. Just start at the tongue and work your way back. If it were me, I would start by checking the driver side frame rail.

Sid 0.


When is the Aux power supply hot ? Only when the ignition is on ? -OR- Hot all the time like a cigarette lighter socket ? Thanks Sid.

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