Troubleshooting Tekonsha T-One Vehicle Wiring Harness Quit After First Use

Updated 01/12/2026 | Published 08/30/2024

Question:

I have a T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector Part Number: 118747 by Tekonsha, installed by a U haul dealer. The vehicle is a 2021 Mercedes GLC SUV. After the first use it quit working. The fuse was not blown, so it is likely the module blown. U haul told me that I should disconnect the 4 flat connector from the boat trailer when launching the boat. When the trailer lights are submerged it could short out the system. OK they are probably right, but why would the module blow and not the fuse? I would think it would be designed to blow the fuse if there was a short between any of the powered wires and ground. The fuse is supposed to protect the module. I bought my own replacement kit. Should a lower amp fuse be used or possibly a fast blow fuse? I think it comes with a 15 amp fuse.

asked by: Peter Z

Expert Reply:

Hi Peter; if the fuse on the # 118747 didn't blow I would agree that it was the converter. However, I don't think swapping out fuses would prevent the issue. The converter likely had a different problem to begin with. Tekonsha does a great job with their wiring so if it was enough to damage the converter the fuse would have definitely blown unless things weren't installed correctly.

I wouldn't expect any other problems as long as you're disconnecting when you launch. Even though you already bought a replacement it would be worth checking to see if you can get the old one replaced under warranty assuming it was within the first year.

expert reply by: Robin H

Robin H.

1/12/2026

Hey Peter, did the replacement Tekonsha wiring harness work better after you started disconnecting it before launching the boat? Knowing how the new one held up could help others dealing with the same issue.