How to Test to Determine Function Carried on Trailer Wiring

Updated 09/20/2017 | Published 09/19/2017

Products Featured in This Question

Wesbar 4-Pole Flat Connector w/ Jacketed Cable - Trailer End - 14' Long

(69)

$30.45

Hopkins 7-Way RV Style Connector with Molded Cable - Trailer End - 8' Long - RV Standard

(925)

$46.90

Question:

What if the trailer has 2 yellow wires????

asked by: Clayton B

Expert Reply:

Often trailers will use the commonly-accepted wire colors for specific functions, such as a blue wire for trailer brake power, but there is no guarantee that a wire's color correctly tells you what it does. The surest way to know what a trailer wire does is to directly apply 12V power to it and see what function on the trailer is activated. In typical 4-pole trailer wiring a yellow wire will carry left-side stop and turn light signals.

You can use your vehicle's battery (or a well-charged spare battery) to test each circuit on the trailer. Connect the battery NEG terminal to the trailer connector's ground pin. Then, one by one, apply the POS terminal to each of the circuits to see what light or other function is activated.

The linked article on trailer wiring shows all of the common types of trailer connectors and the circuits they usually carry. A 4-pole connector like # W787274 carries stop, turn and tail light signals. 7-way connectors like # H20046 carry these signals plus brake power, 12V power and reverse lights.

expert reply by: Adam R