Possible Causes for Dim Trailer Lights

Updated 07/09/2025 | Published 09/12/2017

Products Featured in This Question

Dielectric Grease for Electrical Connectors, 2 oz.

(1460)

$10.96

Digital Multimeter with 42" Long Test Leads - LED Display

(2)

$21.68

Question:

My trailer lights work but they are weak...what could be the problem?

asked by: Werner

Expert Reply:

Most often the root cause for issues with trailer lighting is a weak ground. Strange as it may sound, a loose or rusty/corroded ground can cause a vehicle's trailer wiring harness to badly misbehave and shut down some functions. It may also just limit the amount of current that can flow to the lights and this will result in dim illumination.

The easiest way to check the trailer ground is to disconnect it from the vehicle and apply 12V power (from a well-charged 12V battery) directly to the trailer connector. You can use your vehicle battery or a spare. Apply the negative terminal to the connector ground and then one by one apply the positive terminal to the pins for the various light circuits.

If you find the lights are still dim even when directly powered from the battery then this tells you to focus on the trailer's wiring. Find the main ground wire for the trailer's wiring harness, usually a white wire, and check its connection to the trailer frame. The ground wire's ring terminal should be tightly secured to a clean bare metal surface. You can sand off that spot if corroded and/or clean it with a solvent to remove any grease, oil, dirt or other contaminants. Sanding off the primer/paint from that grounding point also helps. When you make the new ground connection apply dielectric grease # 11755 to protect that new clean connection.

On the vehicle if you have a multi-meter like # PT89ZR you can test the voltage on the trailer connector. If you read much less than 12 volts then check the ground for the vehicle wiring harness in the same way.

If you still would like brighter light output even after you address any grounding issues you may be able to substitute super-high-efficiency LED type bulb replacements for the standard incandescent bulbs. LEDs use about one-tenth the power required by typical bulbs and so they provide generally provide much brighter light. The linked page shows you all LED equivalents to the most popular bulbs. Just match up the your bulb number, such as a 194, which is quite popular. For this bulb we offer many LED alternatives.

expert reply by: Adam R

Wjw

1/26/2025

Thank you. I cleaned the grounds and everything works now. You saved me a lot of time and busted knuckles. :-)

Robin H.

1/26/2025

@Wjw we're always happy to help!

Frank

3/27/2025

I have 13v connect to light its dim. i even used jumper wires from juntion box on tonge to lights dim. Connect light directly to a different12v battery they work fine. Everything is brand new wiring and light.

Jameson C.

3/27/2025

@Frank How are they grounded? To the trailer frame? If so is the frame aluminum? I'd be willing to bet getting the grounds a little better will allow them to work better but you may have to run ground wires back from each light to the main trailer wiring harness.

Bob

7/3/2025

Ground is good. All the marker lights work but the running lights are very dim. Turn signals and brake lights work OK,

Kate F.

7/3/2025

@Bob It sounds like you're on the right track already since your ground is solid and your marker, brake, and turn signal lights are working. The fact that the running lights are coming on but look dim is usually a sign that they’re not getting enough power, even if they seem “on.” One common cause of this is a voltage drop somewhere in the power line that feeds your running lights (usually that’s the brown wire). Even if the ground is good, if the power wire has corrosion, a loose connection, or a break inside the insulation, the voltage can drop just enough to make the lights glow weakly. A good way to check is with a multimeter, see what voltage you’re getting at the lights themselves when they’re turned on. If it’s well below 12 volts, you’ve found the issue. It’s also a great idea to take a look at the trailer plug, both on the trailer and on the tow vehicle. Corrosion or even just grime on the pins, especially the one for the running lights, can create resistance and cause that dim glow. Cleaning them up with a wire brush and some contact cleaner can work wonders. Adding a little dielectric grease afterward helps keep them protected, too. Another sneaky culprit can be the light fixtures themselves, especially if you’re using the trailer’s frame for grounding at each light. Even if the main ground wire is perfect, sometimes each individual light might not be grounding well, rust, paint, or even just an iffy screw can stop the ground path from working properly. Try running a temporary ground wire from one of the dim lights straight to the trailer frame and see if it brightens up. Lastly, if you’ve recently added more lights or accessories to the running light circuit, it’s possible the circuit is a little overloaded. That can also cause dimming. In that case, you might want to rewire some of the lights to a separate fused power line. Do you have the headlights in the tow vehicle set to on? If not try them with the headlights in the on position. Please let me know what you find out Bob =)

Robert S.

7/8/2025

@KateF Thanks very much for the help. I went to the junction box on the trailer to clean up the wires, The wire for the running lights broke as soon as I touched it. It must have been hanging on by a thread. The lights work well by a simple fix. Thanks again.

Kate F.

7/9/2025

@RobertS Anytime Robert! I am glad you were able to find the issue to get it fixed. If you ever have any questions in the future, you know where to find us =)