Troubleshooting Blown Running Light Fuse That Occurs with Two Different Trailers  

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If both of your trailers cause your tow vehicle's running light fuse to blow this tends to suggest that the problem is on the vehicle.

If you have previously towed these same two trailers without a problem using the same vehicle the most likely cause is a short on the running light circuit. Running lights on a 4-pole wiring harness are carried on the brown wire, which is next to the ground pin. On a 7-way socket like # 37185 the running lights are carried on the contact at the 11-o-clock position as you look at the trailer socket from the rear of the vehicle. You can refer to the linked article on wiring to see these circuit configurations.

For either setup you want to check the condition of the connectors to see if there is damage or corrosion on the contacts. In a 7-way, the running light circuit is right next to the 12V trailer power circuit (which is at the 1-o-clock position). Corrosive build up inside the socket or a piece of foreign material stuck between the two contacts could cause these circuits to become bridged together. If this happened, then 12V power from the battery would be shorted to the running light circuit and this would instantly blow the fuse.

Corrosion can occur on the outside of a 7-way where it is fairly easy to spot but it can also happen on the inside, where some trapped moisture can over time cause corrosion that stretches between the two adjacent contacts, causing a short. Be sure to check the back of the 7-way too, including the connector that joins to the back of the socket assembly. There are spray-type electrical contact cleaners like Caig De-Oxit and others that can dissolve corrosion and one of these may work for you.

The other thing to check is the condition of the running light circuit's wire, all the way from the connector at the back of the vehicle forward along the frame. Worn or cracked insulation on this wire could cause contact that would short the circuit and blow the fuse.

Naturally you will also want to ensure that the correct rating of fuse is installed in the power distribution box.

expert reply by: Adam R

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