Troubleshooting Trailer Running Lights Not Functioning After Installing a Brake Controller

Question:

After installing the electronic brake everything works except the running tail lights on the trailer. The brake lights, turn signals, and hazard lights all work on the trailer and the Aspen, but the regular tail lights do not come on on the trailer. Can you help?

asked by: Ray

Expert Reply:

The first thing I would do is check the pin in the 11 o'clock position on the front of the vehicle side 7-way connector with no trailer connected. use a circuit tester like # PTW2993. With the vehicle lights on, the tester should light up if it is receiving a signal. If it does, then there is something wrong on the trailer side. If it does not show a signal, then something is wrong with the vehicle side wiring.

There could be a fuse or relay related to the trailer wiring that you will need to check. Consult the owners manual for location of any fuses/relays and check to see if they are absent or need to be replaced. There could also be a broken wire somewhere and you will need to trace the wire back and check for damage. I also recommend cleaning the 7-Way connectors on the vehicle and trailer side if there is any dirt or corrosion.

If the 7-Way functions are working on the vehicle side with no trailer connected, plug in the trailer and probe the wires behind the trailer side 7-Way. If there is no signal for the running lights, something is wrong with the trailer side 7-Way and it will need to be replaced with a new one like # PK12706. Follow the wiring diagram for function linked below, to wire the 7-Way correctly. I have also included a video showing the replacement of a 7-Way.

If the signal is coming through behind the trailer side plug, then the problem is further back. You will need to trace the wire for the running lights back to check for damage or a loose connection. Finally, check the trailer ground at the 7-Way and the grounds for each light on the trailer. Taillights usually ground through the mounting studs and require a clean, corrosion-free bare metal connection to function properly.

One last possible issue that could be causing the problem is if the trailer has numerous lights like marker and/or clearance lights that come on with the running lights, the power draw could be too much for the system thus not allowing the lights to function properly.

expert reply by: Michael H