Testing Vehicle Wiring and Trailer Light Functions on 2017 F-350 Diesel and 5th Wheel Trailer

Updated 05/30/2018 | Published 05/26/2018

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Digital Multimeter with 42" Long Test Leads - LED Display

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Hook Probe Circuit Tester

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Question:

First time for 5th wheel attached to new F 350 truck and right turn signal and right stop light not working. Camping World says 5th wheel lights work for them so it must be truck. Ford dealership checked truck for 5th wheel cord and says all lights have output and work properly. I am not sure which brake setting I used on the console. If I used the electric over hydrolic setting 5th wheel has electric brakes in error would it cause this problem of right turn and break light not working. Left brake light and turn signal works but only one break light is on sometime when two lights should light up . Lights are Bargman 84 series triple light configuration. 5th wheel still in shop at Camping World so I cannot check at present but hoping correct setting will fix problem when it is ready to be picked up or we will be stuck with driving without correct brake and turn signals. Thanks for any insight to this problem

asked by: William S

Expert Reply:

In 7-way RV wiring the right stop light and right turn light signals are combined and carried on one circuit. If you refer to the linked photo of a 7-way socket you can see which pin within the 7-way handles these signals; it is the pin at the 3:00 position as you look at the socket on the truck.

Since you do not have the trailer handy for testing if you have a voltmeter like # PT89ZR or a circuit tester like # PTW2993 you can check the truck wiring. If you check out the linked video you will see how tester # PTW2993 should react to signals.

Once you again have access to the trailer you can test the light circuits easily enough just by applying 12V power from the truck battery directly to the trailer plug. This is done most easily with a set of jumper cables and two short lengths of wire. Connect battery negative to the ground pin on the trailer's 7-way plug (the pin at 7:00) and then connect battery positive to the other contacts on the plug one by one so you can check for light function.

Since the trailer brake circuit is totally separate from the lighting signals (the brake circuit is on the pin at the 5:00 position) the settings on the brake controller will have no bearing on the operation of the lights.

expert reply by: Adam R