Troubleshooting Trailer Running/Parking Lights Not Working on Truck Wiring Harness

Updated 05/02/2019 | Published 05/01/2019

Products Featured in This Question

Pollak Metal 7-Pole, RV-Style Trailer Socket - Vehicle End

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16 Gauge Primary Wire - Black - Per Foot

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Curt 7-Way to RV Socket LED Circuit Tester - Vehicle End

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Deka Heat Shrink Butt Connector - 16-14 Gauge - Nylon Insulation - Blue - Qty 5

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Curt LED 4-Way Flat Connector (Vehicle End) Tester

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Digital Multimeter with 42" Long Test Leads - LED Display

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Question:

Running lights on trailer do not work. Truck issue? Hi I have been trying to trouble shoot why my running lights are not working on boat trailer, but turn and break lights are. I have an adaptor that has three little lights that tell me when it’s working. One blinks when blinker is on, the other blinks when other blinker is on. Both light up when breaking. The third one is for running lights. It does not light up and I am not reading any power there on multi meter. I have two of these adaptors so I doubt there both bad. Also checked all the fuses, couldn’t find any blown fuses. I looked at the wiring from the truck plug in to the front, couldn’t find anything wrong. I’m not really sure what to do next?

asked by: Nate

Expert Reply:

From the sounds of it, you have a truck issue. Since you have used a multimeter like # PT89ZR as well as a lighted tester like # I26 or # C73NR, it does sound like you aren't getting that running light power. This is often a separate circuit in trucks, so you will want to check your owners manual for fuses or relays that state trailer running lights or parking lights.

Fuses don't always look bad so you will want to use your meter to check for power on both sides of fuses and then you can determine if you have power from the fuse box. If so, I recommend checking your ground connection on your trailer wiring to make sure you have a good ground free of corrosion and debris. If that doesn't solve your issue, next check any connections for your wiring between the two points and if there is a factory harness check for bent pins and power on either side of the connection harness. Your final option is running a new wire # 16-1-1 from a location that you know has power to that pin on your trailer connector using # DW05744-5 to ensure your running lights will get power.

expert reply by: Jason S