How to Test to Determine Trailer Wiring Functions on Dream Coach Horse Trailer  

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Expert Reply:

The wiring diagrams in our linked help article include the wire colors that are most commonly used for each particular trailer electrical function. For example, a blue wire on a trailer is most often used on the brake circuit to deliver DC power to the electric brakes.

However, common use is not the same as a rigid standard and there's no telling what wire color could have been installed on any given trailer to make a quick roadside repair or to complete a new unit in a production run. Wire color can indicate the function it provides but there is no guarantee.

The best way to figure out what any given wire on your trailer does is to apply power and see which function activates. You can use a well-charged spare 12V battery or the one in your tow vehicle. Testing is pretty straight-forward. Connect the trailer ground wire to the battery negative terminal; trace that wire back to the trailer frame to ensure it is in fact the ground wire. Ground wires are often white. Then connect the other wire (the red one) to the positive terminal and note which lighting (or other) function is activated. Test wires one at a time until you know what circuit each one handles.

On a vehicle you can determine the function carried on any given wire by having a help activate the various functions (brake lights, turn signals, etc) while you apply a circuit tester like # PTW2993 to the wire. Once the wire goes hot you will then know which vehicle function it handles.

Once the trailer and vehicle wiring functions are confirmed it is a simple matter of making the appropriate connections for the connector type used. The linked article will show you which plug and socket contact to use for each function. For example, in standard RV 7-way wiring the center pin provides reverse light signals.

expert reply by: Adam R

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