Left Side Trailer Lights Not Working With New LED Trailer Lights Installed

Updated 05/25/2021 | Published 01/19/2013

Question:

I have recently bought LED lights for my boat trailer and I get every function on the RH side but I get nothing on the left, I have checked the wiring on the car and matched the wires on the trailer plug with the socket but still nothing,I checked with a circuit tester and everything seems to be okay at The plug , because I bought the lights already wired up this should be a standard change and the issue is doing my head in is there anything that Im missing here Asimov out of answers Cheers

asked by: Dwayne

Expert Reply:

The first thing that I would recommend checking are the ground connections for the trailer side connector and the lights. Poor ground connections are the most common culprit of trailer wiring issues. The lights will have either a ground wire or they will ground through the mounting hardware. Each of the ground connections should be secured to a clean, bare metal surface on the trailer frame or to the negative terminal of a trailer battery if you have one.

If the ground connections check out fine, then I would start at the trailer side connector. With the trailer connected to the tow vehicle, use your circuit tester to test the left side stop/turn and running light circuits. Probe the wires, past the trailer connector, at different points working your way toward the trailer lights. This will tell you if there is a signal on those circuits and, if so, where the signal terminates.

If you do not have any signal on the left stop/turn or running light circuit, then the issue could be in the connector itself. The pins could be bent or damaged. If you wired the connector yourself, remember to wire the connector to the socket by function, not color. Aftermarket manufacturer wiring and even OEM wiring can use different colors for corresponding functions.

I have attached a trailer wiring FAQ for you to check out that will help when troubleshooting the wiring on your trailer. If you are unable to find the problem or a solution after following these steps, let me know what you come up with and we can do further troubleshooting.

expert reply by: John H

Ken

5/5/2020

Lights work on driver side but not pass on trailer pass side. Have 7 wire, but it reduced to a 4 wire.

Chris R.

5/7/2020

Have you tested the tow vehicle's connector to see if you're even getting power on the passenger's side circuits? You mentioned reducing a 7 wire to a 4 wire - do you mean that the trailer has a 4-way connector that's being hooked up to a 7-way on the vehicle via an adapter? Let me know and we'll go from there.

Bob L.

5/25/2020

I just purchased and installed an LED trailer kit, and I also cannot get the left side to work. Since my trailer is old and bolted together, I must have a ground wire run all the way to the rear member that the lights are mounted on. To do this, I presumed that I could take one of the COMMON running light strands (in this case the Yel/Brn) and devote it to a ground (attach to white at connector) and use the Grn/Brn for both left and right running lights. Son of a gun if only the right side lights! Not a grounding issue, and good 12V at the left side... but the LEDs just barely glow. This is mind numbing, so after three hours of pulling y hair out, I'm here looking for answers, too. I suspect the left and right must remain distinct and the same length and that means I'll need to replace the hacked cable. Would love an explanation.

Chris R.

5/28/2020

I would definitely try to keep the running lights circuit wires separate along the trailer like it was originally wired. It sounds like your trailer uses a wishbone harness (5 wires going to a 4-Way connector at the front). The connector itself is probably having trouble with combining the circuits like this. Use the separate running light wires on each side for each of the assemblies and simply ground each to a nearby bare metal surface. This should hopefully resolve the issue.

Kevin

6/15/2020

I also have an old rusty trailer with a 4-wire connector. Only one side worked until I got the ground issue straightened out. My problem was that the housing on the side not lighting up was not properly attached to the frame. The frame itself is part of the circuit. If the light housing does not have a separate white ground wire, then it likely grounds through the mounting posts. I had a plastic license plate frame interfering with the ground - just took the right combination of washers and where the plastic license plate frame was positioned. I also ended up scraping a ton of rust off and tightening up several places under the trailer where there were slight "ground gaps." For example - the pin that holds the entire bed to the center tube was a little wiggly. That affected the light that wasn't working even after I had the housing properly attached. So, make sure the housing is attached to the frame for a good ground, then make sure the frame doesn't have gaps - wiggle the pins and other "jump points" and you'll find the issue.

Chris R.

6/18/2020

This is fantastic info. Thanks so much for sharing.

Shaun

8/1/2020

I have the same issue with just the left side. I hook my trailer up to another vehicle and it works fine. But when I hook it up to my vehicle the left side doesn’t work. What should I do?

Jon G.

8/11/2020

With that being the case you're going to need to troubleshoot the wiring on your tow vehicle. Check the connector for any corrosion or debris (and clean out) then move on to any grounds or fuses that would affect the left side of your trailer wiring circuit and replace/reinstall accordingly.

Karl

5/23/2021

@JonG is it possible that the wiring module can fail? I have the same issue with the left side brake light and turn signal not working when my trailer is hooked up to my 01 BMW X5, but the trailer lights works perfectly when I hook them up to my buddies Tacoma. I re-wired all of the connections on the X5 and all lights on the X5 work perfectly. However, the left side trailer brake light and turn signal remain inoperable. All of the the trailer running lights, right side brake, right side turn signal work as intended. I have installed a jump wire from the pin harnesses all the way back to the left turn/left brake (yellow wire) output on the wiring module and still no left brake/turn light. I’m assuming the module has failed after many years of use; correct?

Jon G.

5/25/2021

@Karl It is possible for the module to fail. Before replacing the entire wiring harness though I recommend using a circuit tester to test the wire functions on the input side of the module If there is signal going into the module but not coming out then the module is definitely the problem. If you don't have a signal on the input side then the issue is further up the wire, possibly at a fuse.