Which Parts Can Impact Your Trailer's Lights?

Supplying reliable electrical power to your trailer is an essential safety consideration. Without power, other drivers will be unable to see your running or brake lights, creating a potentially hazardous situation on the road. However, running power all the way from your truck to the back of your trailer isn't straightforward, and numerous parts may affect your trailer's electrical systems.

If you're experiencing issues with the lights on your trailer, it's usually best to start forward and work your way back when attempting to diagnose the problem. Make sure to check these four critical parts to determine why your trailer's lights aren't working.

1. Fuse and Relays

Start with the fuse box and relays in your truck. Circuit layouts will vary between truck manufacturers, so you'll need to consult your manual to determine the circuits that apply to your trailer. Check and replace the fuse if necessary, but be prepared to look for additional issues if the fuse continues to break. It's also a good idea to ensure that the trailer relay or relays are operating correctly.

2. Pigtail Connectors

The pigtail is the cable that runs between your truck and the trailer. This connection supplies power to the trailer lights and should have a 7-pin female connector on both ends. Additionally, you'll have 7-pin male bulkhead connectors that this cable plugs into on both your truck and trailer. You can test the connectors on the truck side for proper voltage output using a multimeter.

You can buy new bulkhead connectors if necessary, but start by cleaning the contacts and adding dielectric grease. If this procedure doesn't help, you'll most likely need to replace the connector.

3. Pigtail Power Cord

The pigtail or power cord between your truck and trailer can fail, leading to intermittent problems such as flickering lights. You can once again use your multimeter to test for proper voltage output by plugging the cable into your truck and then probing at the trailer-side female end. If you discover a problem, take the same approach as the connector: clean the contacts, add dielectric grease, and test again.

The 7-pin power connectors used on most semi-trucks are relatively dependable, but they aren't immune to physical damage. Carefully inspect the entire cord for signs of wear or other problems, and replace it if necessary.

4. Trailer Light

Finally, inspect the lamps themselves for potential issues. If you're sure the bulbs are good, then check the sockets and connectors for signs of corrosion. You can also use your multimeter to confirm that the brake lights, rear lights, or marker lights receive power at the sockets and the wiring harness. If everything checks out, you may need to replace the light itself.

Contact a company that sells truck equipment and parts to learn more.