Troubleshooting Trailer Lighting Where Only One Running Light Works

Published 12/19/2014

Products Featured in This Question

4-Pole Trailer End Connector

(39)

$7.47

Replacement Side Marker Light Bulb # 194

(204)

$5.19

Double Bullseye Trailer Clearance, Side Marker Light, Two-Bulb - Red

(134)

$4.46

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

(4181)

$15.37

Question:

One running light does not work, but otherside does?...brake and turn signal works on both sides

asked by: Paul S

Expert Reply:

Since all of your trailer lights operate properly with the exception of one of the running lights, this tells us that the vehicle wiring harness is probably working properly.

I would first carefully check the bad light's ground and power wires to ensure that both are secured. If the light uses a separate ground wire, usually a white wire like on # MCL36AB, that ground wire needs to be affixed to the trailer frame, to a clean rust-free bare metal surface. If the light is the type that uses its threaded mounting studs to make the ground connection, such as # MC42RB, then make sure there is clean bare metal at the mounting point to enable a secure ground. You can use sandpaper to remove paint and rust at the grounding point.

If you find no issue with the light's wiring then I would check to see if you have a bad bulb, by replacing it with a new one.

The link at right will take you to our main page for replacement bulbs. A common bulb for marker light applications is a 194-type, part # A194B. Your existing bulb should have a reference number on it to help you select a replacement. If you have LED-type lights, which do not use replaceable bulbs, then it could be that the light itself is damaged. You can also try applying battery power directly to the terminals/wires of the one malfunctioning light; this will tell you if the light itself is bad or if just the connections are bad.

Next you can check your main trailer ground at your 4-pole harness. It can happen that if the main ground is not 100-percent solid, some but not all lights will function. This happens when the ground is not solid enough to pass the full required current for all lights, but only enough for some. Find the point where the trailer connector's white ground wire attaches to the trailer frame. Again make sure the grounding point is a clean bare metal surface free of grease and dirt. You can always try relocating that main ground wire if there is a better spot for it. Make sure it is very tightly secured to the trailer frame.

I have also linked for you two articles with helpful troubleshooting tips.