Troubleshooting Trailer Lights - A Simple Guide
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How To Test & Troubleshoot Trailer Wiring

Your trailer's lights are essential to safe operation. Oftentimes, these lights prove problematic in the warmer months when the trailer is used at a high frequency after sitting idle for the winter. Address the problem now so you don't have to fuss around with your trailer once the cold weather arrives. Let's take a look at how to test trailer lights.

Troubleshooting Trailer Lights Strategy

The best way to solve your trailer lights wiring issue is to eliminate possible problems one-by-one until you have pinpointed the root cause of the malfunction. It is possible that one of the bulbs on your trailer burned out. So check the lights. If the bulbs are not the problem, disconnect the trailer's wiring system from the tow vehicle. Test all of the lights, from the backup lights to the turn signals and brake lights to ensure the problem is not the tow vehicle itself.

trailer with led lights

If the vehicle's lighting system functions as designed, grab a trailer wiring tester to check the tow vehicle socket. However, the tester must fit the tow vehicle socket. It might be necessary to use an attachment to properly test the system. Plug in the tester to the socket. Check the indicator(s) to determine if there is an electric current. There should be an indicator for each individual function. If the tester responds to each individual system, the trailer wiring system is probably the cause of the problem. If one or several test results do not provide a response from the tester, the tow vehicle itself might be the problem. If the tester illuminates when it should not (example: a phantom turn signal illuminating), it is a sign there might be a short in the tow vehicle wiring. Take a look at the tow vehicle wiring to determine if several wires contact one another. It also might be possible that several wires are connected to improper connection points. If this is the case, use wire cutters, a crimper and a wire stripper to perform a swap. Be sure to check out our trailer wiring diagrams for wiring identification and how to wire a trailer for additional information.


► Testing Trailer Lights: Check the Grounds

Ground problems are the top cause of issues with trailer wiring. Begin by cleaning the ground connections. Each component should have its own ground to the frame of the trailer. If the trailer lights flicker on and off, there is a good chance the trailer is grounding by way of the hitch instead of through the trailer's wiring connector. Check to see if the vehicle ground wire is stacked with additional wires on a screw. If so, consider moving it to a screw of its own or at least move it to the stack bottom. If the wiring is corroded beneath the insulation, replacing the entire wire is prudent. At a bare minimum, the last several feet of corroded wire should be replaced.

► Inspect the Terminals

Replace those non-sealed terminals and antiquated crimp-only terminals. Put Crimp and Solder Seal Terminals in their place. The problem with crimp-only terminals is they tend to have a weak connection even though they might feel quite tight. Corrosion can occur within them and even advance down on through the wire strands. A sealed connection formed by Crimp and Solder Seal terminals is more formidable than the wires. It does not allow for a voltage drop through the connection. This sealed connection will also prevent a breach by contaminants.

► Check the Fuses

Check the fuse on the powered converter to ensure it is still functional. Take a look at the tow vehicle lighting fuses. Examine the wiring on the trailer side and vehicle side for in-line fuses. If the blown fuses continue, it might be the result of the circuit being overloaded. One solution is to replace the trailer lights with LEDs.

► Determine if the Issue is With the Tow Vehicle Wiring or Trailer Wiring

Disconnect the trailer from the vehicle and run a test to determine if the trailer lights function properly. Wire the vehicle side wiring harness to a standalone battery. All you have to do is link up the white wire to the negative and the colored wires to the positive. If the trailer lights function with such a setup, it is clear the issue is with the tow vehicle.

► Re-wiring

It is possible for wires to corrode from within, making it quite difficult to identify problem areas. Insulation gradually cracks across the aging process and can spur shorts. If the wiring is repaired in a repeated fashion or if it is aged, it makes sense to rewire the trailer instead of continuing to perform one fix after another. Trailer wiring kits are surprisingly affordable. Installation won't take more than a few hours to complete.

► When in Doubt, Always Mind the Grounds

It is worth repeating: the wiring should have secure and clean grounds at all times. If the grounds are compromised, perform a thorough cleaning and it just might do the trick.

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