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2017 Ram 3500 Trailer Brake/Turn Lights Not Working

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Mark S. · rep 0 · May 25, 2:31 AM · 1 replies · solved · Question

Vehicle: 2017 Ram 3500 Longhorn

I've got a frustrating problem with my 2017 Ram 3500 Longhorn's trailer lights. When I hook up my utility trailer, the brake lights and turn signals don't work at all, but the running lights and marker lights are fine. The truck's own brake lights and turn signals work perfectly, so I know it's not the truck's bulbs.

I've already checked the trailer's wiring and bulbs, and they're all good when tested directly. This makes me suspect it's an issue on the truck's side. Could it be a fuse, a relay, or something more involved with the trailer wiring harness or module?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:31 AM

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ClearTheCode admin answer:

  1. Check Truck-Side Connector: The most common issue for trailer light problems, especially when the truck's lights are fine, is corrosion or damage in the truck's trailer wiring connector (7-pin or 4-pin). Inspect the pins inside the connector on your 2017 Ram 3500 Longhorn. Look for bent, corroded, or pushed-in pins. Clean them thoroughly with electrical contact cleaner and a small brush. Ensure the cap is always on when not in use.
  2. Test with a Multimeter: Use a multimeter or a dedicated trailer light tester to check for voltage at each pin of the truck's connector. With the truck running and the appropriate signals (brakes, left turn, right turn) activated, verify that power is present on the correct pins. This will confirm if the truck is sending the signal correctly.
  3. Check Fuses and Relays: Ram trucks have dedicated fuses and sometimes relays for trailer lighting circuits. Consult your owner's manual for the fuse box locations (usually under the hood and/or inside the cabin) and identify the fuses for trailer brake, left turn, and right turn lights. A blown fuse will prevent power from reaching those specific circuits.
  4. Trailer Brake Control Module (TBCM): Your Ram 3500 likely has an integrated trailer brake control module. A fault in this module or its wiring could disrupt signals to the trailer, particularly for brake lights and turn signals. While less common than a fuse or connector issue, it's a possibility if other checks yield no results.
  5. Ground Connection: Ensure the trailer has a good ground connection to the truck. A poor ground can cause various intermittent electrical issues on the trailer, though it usually affects more than just specific lights.
  6. When to See a Shop: If you've cleaned the connector, checked all relevant fuses, and confirmed power with a multimeter, but the issue persists, it's time to visit a professional. A dealership or a specialized trailer hitch/wiring shop can perform more advanced diagnostics, including checking the trailer brake control module and its associated wiring, to accurately pinpoint the fault in your Ram 3500.
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