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Trailer Wiring Harness Drawing Power from 2008 Mercury Mariner Hybrid with Vehicle Off  


I installed this wiring harness in a 2008 Mercury Mariner hybrid. The next day, my battery was dead. I used a voltage tester and determined that the part was drawing power even when the vehicle was off: When I removed the fuse, my vehicles battery voltage rebounded slightly. The battery maintains normal charge with the wiring harness fuse removed. What are the most likely causes of this problem?


Expert Reply:

Because trailer wiring harness # 118551 requires a direct connection to the vehicle battery, the power wire will be energized all the time because it is connected to the battery. However, with nothing drawing power on the 4-Way end that power has no where to go (nothing is drawing it). So just having the harness installed cannot drain the battery unless something is plugged into the 4-Way and is drawing power.

If you have the power wire connected anywhere other than the vehicle battery it could result in unwanted performance.

However, a short could cause power draw. You will need to make sure that the converter white wire is grounded to a clean metal surface. No paint or undercoating so you may need to sand away to get to bare metal. Also ground it by itself instead of with a common ground location.

Make sure the cover is on the 4-Way end so that nothing can get in and corrode the pins and cause a short.

Inspect the connections on the harness and the vehicle to make sure there are no bent or broken pins. And on the power wire you will need to check for any places it might be pinched or where copper strands are touching bare metal as this could also cause a short.

If everything checks out fine and you cannot find any problems with the installation, connections, or ground then you could have a bad converter and the harness will need to be replaced.

expert reply by:
Michael H

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