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Trailer Wiring Issues with Running Lights Blinking


Perplexing trailer wiring problem. Right turn signal and clearance light its mounted on the brake light case blink together when right turn signal is on. The left is normal clearance light is on, but doesnt blink when the left turn signal blinks. Left signal has brown wire for running lights and yellow for turn signal. Right has green for turn signal and brown/yellow or yellow/brownits hard to tell with the fading for running light. If I turn the parking lights on and no key in the ignition, the lights on both sides come on, but are dim both clearance and turn. They blink on both sides also, although faintly. I tested the harness at the truck and the brown wire shows constant power while it blinks if I test the green and yellow wires. The trailer has a five wire plugwhiteyellowgreenbrownbrown/yellow or yellow/brown. Is this a short somewhere? All Rights Reserved, R


Expert Reply:

Step one is to isolate the problem to either your tow vehicle, your wiring harness or your trailer wiring. To begin, disconnect the trailer wiring harness from the towing vehicle.

First focus on troubleshooting the trailer. Check the white ground wire from the trailer plug to ensure that you have a clean secure ground connection to the trailer frame. Internal trailer wiring can over time develop wear spots in the insulation that can create a dead short. Such a condition will quickly damage a module, so it is important to check that the trailer wiring is not faulty. You can also try applying 12V power one by one to each circuit on the trailer plug. If the all of the light circuits behave normally when powered directly from a 12V battery you can move on to testing your harness. If the lights do not behave properly then you can troubleshoot the trailer wiring.

Next use a circuit tester on the wires leading INTO the wiring harness. These are the wires that carry the vehicle lighting signals to the harness and on to the trailer. Test the yellow wire with the left turn engaged; you should detect an on-and-off signal. The green wire should behave the same way for the right turn signal. The brown wire should carry a steady voltage whenever the vehicle lights are on. If the correct signals are going into the harness but not coming out of it, then the harness may be bad.

Carefully check the white ground wire to your harness to confirm the contact surface is clean, free from corrosion and that it is very tightly secured to a clean unpainted part of your vehicle, or better still to an actual grounding point. A bad ground could cause your problem. It is a good idea to also check for secure ground wire connections at each light. (The lights operating but being dim suggest that a bad ground could be the cause of the problem). If this ground wire check fails to remedy the issue, then we will move on to check the output side of your module.

Use a circuit tester such as # 40376 to test the module output for the right side running light (yellow/brown wire) when the tow vehicle lights are on. The wire to the right side running light should carry a constant voltage since this is intended to be on at all times that the vehicle lights are on. If the output wire carries a correct continuous signal then there is a problem in your trailer wiring. Again we will test the brown right side running light wire going to the module input. If this carries the correct constant voltage when the vehicle lights are on, then the module is bad. If this input wire to the module is showing an intermittent signal, then there is a problem with your vehicle wiring.

Your description of the wiring colors suggests that you may have a wishbone-type harness, which has both the brown and yellow-brown running light wires exiting the harness from the same pin location. If this is the case you will want to verify that the harness is in fact wired correctly. I have attached articles concerning trailer wiring which may be helpful as you troubleshoot.

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