Troubleshooting Right Brake Light on Trailer Not Working But Right Turn Signal is Working

Question:

My trailers running lights, side markers, turn signals, 4-way hazard lights are all work fine including left side brake light. The problem is that the right side brake light stopped working with head lights on or off. I initially thought that the LED tail light unit went bad so I replaced it with a new one but with the same result. Some more specific info: 1. Towing vehicle 1998 GMC Savana 2. Ground connections are all bolted no dedicated ground wires. 3. Both brake lights are working properly on my towing vehicle 1998 GMC Savana SLE just the right side brake light on my trailer is not. What could be the root cause of this symptom? Much appreciate your insight on this!!

asked by: RayS

Expert Reply:

Thank you for all of the details in your question. It looks like you have a faulty ground or some corrosion in one of your connectors.

On a trailer the functions for your turn signals are also tied into the brake lights so if your right turn signal is working on your trailer but the brake light isn't then there are a few things you can do. I recommend starting off by checking both the connector on your 1998 GMC Savana and the connector on your trailer. Check to make sure that they are clean and free of debris. Any white or green residue could mean that there is corrosion which would cause this issue as well.

If both of those look good then the next thing to do would be to isolate which end is causing the problem; your tow vehicle or your trailer. You can do this by testing the functions on your Savana with a Circuit Tester # PTW2992 and by using a separate 12V power source (different vehicle or battery) to test the functions on your trailer. If you have a 4-Way it will be the Green wire, if you have a 7-Way it will be the 3 o'clock pin on your tow vehicle which is the 9 o'clock pin on your trailer (see attached photo).

From here you will be able to tell which end the problem is on. You will then want to check all of the grounds making sure that they are secure and clean. Any corrosion or even paint could be causing this kind of problem. They need to be bare metal-to-metal connections.

If the grounds look good then move on to the wiring. Trace the appropriate wire and check for any places where the wire might be bent, exposed, corroded, or damaged in any way and replace it.

If you get through all of this troubleshooting and you are still experiencing this problem then you will need to take your setup to a qualified mechanic who can physically inspect it.

expert reply by: Jon G

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Jake C.

11/5/2021

Tow vehicle: 2013 Ford Edge limited. The edge has a 4 way connector wires are ran correctly good grounds from what I see got good power on vehicle side of connector but when plugged into my boat trailer or atv trailer the right tail light including all functions barely burn. You have to cover the light with your hands and look through small crack between my fingers to see the bulb barely burning. What is my problem?

Victoria B.

11/10/2021

@JakeC It sounds like you're having the same issue with 2 different trailers, your boat trailer and again on your ATV trailer. The issue you describe is something that typically makes me think the light has a weak ground, but if this is happening on 2 different trailers, while it could still be the ground on both trailer's right-side lights, I'm more inclined to think there is an issue at the vehicle connector. The trouble I'm having coming up with an obvious issue is that the light indicators for the right-side taillight come from 2 different wires, but you aren't having the same issue with the running lights on the left-side taillight. At this point, I would use a multi-meter to test the wiring in front of the vehicle connector as well as at the connector pins to make sure no power is being lost at the connector. If it all looks good, then I would plug one of the trailer in and test down the wiring to the light to see where the power is dropping off to try and narrow down the issue.