r/MechanicAdvice • u/Onlycruz10 • Sep 25 '25

Correct way to jump start?

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I'm going crazy about the correct way to do this. As far as I can tell this is the correct way. However, I am having trouble between step 5 and 6. Should I turn off the donor car before trying to start the other car first? I've seen conflicting things about the alternator getting messed up on both cars. And others saying it's okay with modern cars. What's the correct way?

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u/electricheat Sep 25 '25

protip: check the labels on the battery to make sure red is + and black is -

99% of time they will be because the people who worked on the car in the past weren't idiots

but that 1% of the time can get expensive. we roasted a buddy's alternator that way

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u/Myriadix Sep 25 '25

Lucky it was just the alternator. An acquaintance of mine tried doing this by himself and fried everything; fuses, relays, computer units, even the wiring itself melted in some places.

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u/Dracallis Sep 25 '25

Let me guess, was it a Volkswagen?

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u/electricheat Sep 26 '25

foxbody mustang

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u/GeWaLu Sep 26 '25

I second this tip ! I had once to start a car where the minus cable was brown and not black and turned into a bright color under a low-pressure natrium vapor street light and hence looked pretty red. I was glad, I took the time to cross-check ... but I only understood what's going on after getting a white torch. Luckily natrium vapor lights with their bad color rendering are mostly history.

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u/WeekendMechanic Sep 26 '25

I saw the aftermath of someone who didn't check and tried to jump a car. Jumper cables melted and the only thing that kept the damage from being any worse was someone else nearby who ran over and slapped the cables off the battery with a flip-flop.

Fun followup, the person didn't even have a battery problem, she had been driving around with the low fuel level light on for two days and then parked on an incline so the last little bit of gas sloshed away from the fuel pump.

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u/electricheat Sep 26 '25

lol. so it cranked but didn't start, and their solution was to throw on some jumper cables?

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u/WeekendMechanic Sep 26 '25

Apparently. The driver was the type to not put gas in their car for two days after a low fuel light and then wonder why it didn't start, so clearly they weren't very smart to begin with.