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Here's a strange situation I encountered last night that I'm trying to wrap my head around.

My mother in law drives a 2011 Subaru Legacy. Last night, her battery (since replaced) died -- not totally dead, but no longer had enough power to fire the starter.

I brought my wife's car, a 2018 Chevy Traverse, to provide a jump. I made the negative connection on the provided ground post rather than to the battery. When I made the final connection, the entire electrical system on the Legacy went dark -- lights, accessories, everything. As soon as I disconnected the jumper cables, everything came back. During this time, the battery voltage reading on the Traverse was reading around 14 volts.

Unable to successfully jump, I swapped cars and brought my car, a 2006 Kia Spectra, and repeated the process. Everything worked as it should.

My question is this: what was going on that the jumper cables from the Traverse made the Legacy go completely dead? And what was different that it didn't happen when I used the Spectra? The polarity was correct, both cars work properly, no fuses were blown. I'm at a loss to figure out what happened.

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Feb 20 at 21:31
  • Are you certain that the Traverse is body negative and that the jumper cables were the right way round? It sounds like what happens when a body negative car is connected to a body positive car, body to body. Or that battery A's positive was connected to battery B's negative. If the jumping operation was being done in low light it with cables that are hard to tell apart (some people use cables of the same colour) it increases the likelihood of this occurring – Caius Jard Feb 21 at 2:55
  • Is anything made these days positive ground? I've never seen one. – Tristan Feb 21 at 15:04
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One of two things happened:

  1. While you were attaching the jumpers to the Legacy, you dislodged the connection and caused it to go black.
  2. You attached one side or the other backwards on the initial attempt, causing a short, rather than a jumper circuit.

I'd suggest your issue was actually #2. Since there was a lot less juice in the Legacy battery than what the Traverse was putting out, the Traverse just overrode the Legacy causing the blackout. When you brought the second vehicle in to jump, it was then connected correctly and things worked as they should have.

  • If it was #2, that would have created a dead battery-to-battery short within the cables, wouldn't it? There was no heating in the cables, and by all accounts I've read, reversing the polarity when jumping a Legacy blows all kinds of fuses and often requires replacing the ECU -- the car is driving perfectly normally now. On #1, the weird thing is that when I disconnected the negative from the Traverse, the lights came back on in the Legacy. – Tristan Feb 21 at 15:00
  • @Tristan - If the battery was dead enough, it may not have had enough juice to heat the cables. And yes, it can blow all kinds of things. It doesn't mean it will blow all kinds of things. Mind you, me saying #2 was just my gut check. These are the only two reasons I can think of where you lost all power in the car when it was attached to the jumpers. Might be something completely different. – Pᴀᴜʟsᴛᴇʀ2 Feb 21 at 15:16
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Your mother in law quite logically turned the lights off SOS all available power would go towards recharging her battery.

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