Troubleshooting a Flat Tow Wiring Harness Not Working When Changing RVs
Updated 05/12/2025 | Published 05/03/2025
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Question:
We have towed our 2015 Mini Cooper with a 2021 Airstream Atlas for a few years without problem - we are using the etrailer plug/harness/wiring kit 4 diode. We tried to tow the same car with the same 7 - 6 pin adaptor, with our new 2025 LTV Unity, but we can’t get any of the Mini indicators to work. As soon as we move the coiled adapter back to the Atlas, all indicators again work on the Mini. Seems to confirm that the Mini wiring and the coiled adaptor are working fine. Possible causes? I’ve seen some posts that mention issues with LED lights 2025 Unity to incandescent lights on the Mini. Different adaptors needed although I am fairly sure the 2021 Atlas had LED lights on the rear.
asked by: Les S
Expert Reply:
Thanks for all the information Les and congrats on your new motorhome. You've definitely narrowed the problem down. Have you checked the 7-Way connector on your new 2025 LTV Unity? From what you're describing it sounds like it is not functioning correctly. It could be missing a fuse/s, a ground, or it might not even be getting power.
I recommend checking your RV's 7-way connector with a circuit tester like # PTW2993. You'll need to have a friend sit in the RV and perform each of the connectors functions while testing the corresponding wires for each circuit on the back of the 7-way connector both before and after the plug. If you have power to all wires before the connector and not after, the problem is in the trailer connector. If you do not have power running to a wire before the 7-way then you will have to trace the wire from the 7-way back to its corresponding function while checking for breaks or frays in the line. You may also want to check your fuse box for blown or missing fuses.
Another issue you may want to check is if the 7-pin is wired correctly. I have linked a diagram below that will show you how it's supposed to be set up. It'll also show you how the functions line up on the connector.
(click to enlarge)
Product Page this Question was Asked From
Roadmaster 4-Diode Universal Wiring Kit for Towed Vehicles - 7-Way to 6-Way Straight Adapter Cord
- Flat Tow Wiring Harness
- Splices into Vehicle Wiring
- Diode Kit
- Universal Fit
- 7 Blade to 6 Round
- Roadmaster
more information >
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on Flat Tow Wiring Harness
Les S.
5/5/2025
Jerred, RV 7 blade connector checks out fine - used a test plug. Fuses are therefore ok. RV plug signals are also working fine when using a multimeter directly to the pins. But no signals detected when coiled wiring harness is inserted into RV plug and measurements taken on the other end at the 6 pin connector. Yet the wiring harness shows point to point continuity when checked on its own. Conclusion the 7pin plug on the end of the harness is not making good contact with the RV 7 pin plug. Going to replace that plug and hopefully that will resolve the problem.