Troubleshooting Trailer Lights That Do Not Work After Outside Winter Storage

Updated 04/22/2019 | Published 04/20/2019

Products Featured in This Question

Hook Probe Circuit Tester

(66)

$14.36

Dielectric Grease for Electrical Connectors, 2 oz.

(1493)

$10.94

LCD Digital Circuit Tester

(28)

$28.06

Question:

I just bought a new trailer and the lights were working fine it sat outside during the winter I went and plugged up the trailer and no lights on the trailer will work. When I open my door then the lights will come on and that’s all as soon as the interior lights go out so will the trailer lights

asked by: Charles M

Expert Reply:

Most of the time when trailer lights stop working correctly the problem can be traced to a bad ground, either on the vehicle, on the trailer, or both. The other common cause is corrosion on the connectors, which can be corrected with an aerosol electrical contact cleaner like Caig De-Oxit and with fine-grit (500) sandpaper.

The best way to narrow down the cause is to test the trailer on its own using the truck's battery or a spare if you happen to have one. Testing separately will save you time in the long run.

You'll want to apply the direct battery power to your trailer connector, one circuit at a time, to see if the various functions work. If the trailer lights operate properly with this direct power then you'll know to focus on the truck to locate the cause for the problem. A set of jumper cables and two short lengths of wire will make this easier.

Connect the jumper negative clamp to the battery negative terminal and the other end to the wire, which you'll connect to the trailer connector's ground contact. Connect the positive clamp to the battery + terminal and the other end to the second wire. Now test each pin on the trailer plug one at a time to see if the lights work. If everything works in this test you know to check the main ground on the truck's wiring harness. Disconnect it and sand off that spot to expose clean shiny bare metal. Then apply dielectric grease # 11755 and re-attach the ground securely.

If the battery test does NOT allow the trailer lights to work then focus on the trailer's main ground wire in the same way. Disconnect it from the trailer frame, sand off that spot and re-attach it using the protective grease at the ground point. The included article will show you which function is carried on which wire for both 4-pole and 7-way setups.

If you happen to have a circuit tester like # PTW2992 you can apply this to the truck's connector contacts. This lets you see if the vehicle signals are present on the truck. Again, refer to the article to see which pin carries each function.