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I have a 2017 Subaru Legacy. A month ago U-Haul installed a trailer hitch and electrical kit. I used my 5 by 8 trailer (bought at Tractor Supply) happily for three weeks. Then one day the car did not start when the trailer was connected. We tried disconnecting the trailer's electrical ribbon and voilà, the car started. We moved the ribbon over to where it would get pinched less from the trunk closing on top of it, and then all seemed well.

However, the next day, the problem returned, worse than before. The car would not start with the trailer connected or without. We tried a jump with our other car but that didn't help. We had the car towed to the AAA approved shop in our area, which delayed looking at the car for three days, and then referred us out to the dealer.

The car was then towed to the dealer, arriving mid-afternoon on Friday. The dealer said the arrival was too late in the day for it to be looked at before Monday.

Now they have looked at it, and say that there was a short with the U-haul wiring kit installation, and the battery is fried, because it was fully discharged for too long, and should be replaced.

I read that fully discharging a battery can indeed cause irreversible damage. (1) At what point did the irreversible damage probably occur? (2) If we ever have this problem again, what, if anything, should I do differently?

(3) Should I now install a Subaru wiring harness instead of having U-Haul try to improve things? The Subaru dealer service manager says that after-market parts are not always completely compatible.

If the two types are equivalent, I'll go with U-Haul because I need to be using the trailer. The dealer said the harness needs to be ordered and would likely arrive on Wednesday (in two days).

The most important question in the short term is (3).

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  • A piece of wire is just a piece of wire. Subaru don't sell special wire that goes through a secret process in their factory! From what you said there are two unanswered questions: (1) why did the harness short out at all, and (2, more important that 1), why wasn't there a fuse in the wiring so the only effect of the short what that your trailer electrics stopped working? In a sense, you got lucky. You might have had your car burned out when the harness caught fire.
    – alephzero
    Aug 12, 2019 at 20:33
  • …. I would guess the root cause of the problem was the U-haul installed the harness incorrectly, so it was rubbing on something and the insulation was worn through in 3 weeks, or else it was touching something hot which burned through it. The problem wasn't the quality of the wire, but the guy who put it in the wrong place.
    – alephzero
    Aug 12, 2019 at 20:35
  • @alephzero - Maybe I shouldn't have said "fried." I meant ruined. There was no fire.
    – aparente001
    Aug 12, 2019 at 20:39
  • My point was that you were lucky there wasn't a fire. Apart from the electrical heating, shorting a battery is likely to produce an explosive mixture of hydrogen and oxygen gas (and I mean explosive, not just "burns pretty fast").
    – alephzero
    Aug 13, 2019 at 0:38
  • @alephzero - Ah. I will ask whether it was the battery that was shorted, or something else. Thank you.
    – aparente001
    Aug 13, 2019 at 4:18

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It is difficult to say if the factory harness is better than the aftermarket unit from U-Haul or other specialty supplier. The quality can be as good as Subarus' or a universal unit designed to attach to almost anything with the use of adapters. The fact that the wiring was pinched in the trunk lid during normal use leads me to believe that it is a universal design. It is likely that the primary design criteria was ease of installation not durability. A factory installed harness will typically involve a "T" type water resistant electrical connector that extends the rear harness outside the vehicle near the trailer hitch. You want to avoid tap slicers also referred to as "vampire taps". These connectors are pinched over the wires' insulation with pliers to splice in an additional wire. They tend to fail over time resulting in inconsistent connections.

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  • Thank you. Are you saying that it would be best not to have to close the trunk lid on the wire ribbon? I can ask the Subaru dealer if their harness would function that way. Hopefully yes. If not, where could I get that kind of installation done?
    – aparente001
    Aug 13, 2019 at 4:20

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