Tracking Down Possible Trailer Ground Problem to Repair Running Lights

Updated 12/18/2012 | Published 12/17/2012

Products Featured in This Question

Optronics LED Clearance or Side Marker Light w/ Reflex Reflector - 6 Diodes - Square - Amber Lens

(4189)

$15.37

Hook Probe Circuit Tester

(60)

$12.41

Question:

I have installed all of the wires and lights, I check the brake lights and turn signal and they all work except the running light. Now the side marker light only had one wire at the back of it which is brown, Do I need to add a ground wire to it in order for it to work, I will check fuses also and also purchase a new battery. on the trailer on left I have yellow and brown wire, on right I have green and brown wire, you assistance will be highly appreciated

asked by: Terry

Expert Reply:

As a first step, you will want to determine if the wiring problem is on the tow vehicle or the trailer.

You can begin by using a circuit tester like part # PTW2993 to test the trailer connector on the tow vehicle while a helper activates each function. If you get no signal on the pin that feeds the running lamp circuit, you know the problem lies with the tow vehicle. If this is the case, you will need to check fuses or the wiring connections of the trailer connector. Also, check the connector for damage or excessive corrosion and clean or repair as necessary.

Make the same check of the connector on the trailer. Follow the wiring harness from the trailer connector, and make sure the white ground wire connects securely to the trailer frame, and that this ground connection is not corroded. Trace the running light circuit, which is typically a brown wire from the connector toward the rear of the trailer. Look for pinched wires or loose connections. Make sure that all the wire insulation is in good condition, with no cracks or worn spots that would allow the copper conductor to short out.

Each individual side marker and taillight will either ground via a separate ground wire, or through its mounting stud being in direct contact with the trailer frame. If a marker light that grounds through its stud (like the one you mentioned) is attached to plastic or wood or a metal place that is not in good solid contact with the trailer frame, you will need to run a separate ground jumper to the trailer frame to maintain a continuous ground circuit. Make sure that the grounding point for each lamp (whether it be the mounting stud or separate ground wire) is securely fastened to the trailer frame, and that the connection is not corroded.

What I mentioned above are the most common culprits for the problem you describe. If you make the checks I recommended, and still do not locate the problem, please get back with me.

I have linked you to or Trailer Wring FAQ article. The article contains wiring diagrams and troubleshooting strategies that you might find helpful. I have also linked you to a video showing how to use a circuit tester to check the function of a trailer connector.

expert reply by: Mike L