Troubleshooting Malfunctioning Trailer Lights

Question:

Trailer lights work except for right side turn signal Only. Right side bulb was changed and I even swapped lights to see if it was the light itself. Only one filament lights up on right side. Is there a fuse on the truck that controls just that side. My truck turn signals work fine

asked by: Thomas F

Expert Reply:

This will depend on the year, make, and model of your truck, but there is typically a number of fuses for towing a trailer. You will want to check your owner's manual for the specific fuse locations for these functions. If the fuses all look good, however, it may be something else.

Using a tester, like the # PTW2993, engage the right blinker and check the output on the 3 o'clock position of a 7 Way plug, or the green wire port on a 4 Flat plug. These will both handle the passenger (right) side blinker function. If you are not getting pulsing power, you may have a break in the wire. Especially if the harness was not a factory component. This will require you to trace that wire and either repair it or replace it. If you are getting power, however, it is going to be something on the trailer.

With the blinker still on, you will need to connect the trailer plug to the vehicle. Again using a tester, check the 3 o'clock position on a 7 Way or the green wire on a 4 Flat, but this time directly after the trailer plug. If you are not getting power directly after the plug, you will need to replace it with # PK12706 for a 7 Way, or # HM38138 for a 4 Flat. If you are getting power, you will need to check you ground connections.

A bad ground can cause all sorts of weird things to happen. Though not completely necessary with incandescent lights, it never hurts to have a separate ground for each component. All grounds should also have a clean, solid connection to bare metal that is not aluminum.

If this all checks out and all your grounds are good, odds are the light will just need to be replaced.