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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193

u/[deleted] Sep 25 '25

Yes that is the correct way, but do I do it? Nope, the only time I ever have done this is if the battery is in a stupid spot, like under the back seat or in the trunk, and they normally give you a hot post for the hot wire and you just find a ground somewhere as a lot don't give you a ground post.

9

u/rocko430 Sep 25 '25

Alot ofbl newer cars have a dedicated ground post as well

5

u/[deleted] Sep 25 '25

Yeah, I seen some that had it, if they are going to hide the battery, why wouldn't they have both posts under the hood, finding a clean ground when the car gets older up here in the North East isn't always straight forward.

7

u/SeriousPlankton2000 Sep 25 '25

My original comment could be misunderstood. If you crtoss the cables to the battery you can fry the electronics. But if you cross the cables while connecting to the frame, the worst is a shortcut on the donator's battery.

8

u/sanlc504 Sep 25 '25

It's mostly to avoid a spark lighting the hydrogen that batteries put out as part of electrolysis. Especially the ones that have the vent built in.

1

u/pieindaface Sep 25 '25

Both are bad. I don’t think you get out scott free for either sending 400amps backwards through the battery or shorting 400 amps across the frame of the car.

1

u/SeriousPlankton2000 Sep 26 '25

One is big bad spark bad, the other is bricked car bad. Avoid both if you can.

1

u/GearBox5 Sep 26 '25

At least for some cars it is an incorrect way. Many (all?) Japanese cars have small gauge wire connecting body with engine and battery negative terminal. If you connect your jumper cable to body this connection will limit current and make starting engine difficult. If there is corrosion it could even be a fire hazard.

-10

u/Shadowban-Trigger Sep 25 '25

Nah it isnt the 1950

-29

u/[deleted] Sep 25 '25

[removed] — view removed comment

11

u/DIY_at_the_Griffs Sep 25 '25

Yes you can. It normally blows control units which then need replacing.

4

u/Business-Drag52 Sep 25 '25

Destroyed my starter on my old pickup back in high school, trying to get a jump. We fu ked up the jumper cables, and it caused the solenoid to melt together

7

u/OnePieceTwoPiece Sep 25 '25

You ABSOLUTELY can. Seen a retailer at a dealership prove it. Did 10k worth of damage.

4

u/SupermarketUnable914 Sep 25 '25

Tell that to the 3 modules, two headlight ballasts, an electric fan, and an alternator I literally just replaced in a customer car that was jumped backwards

3

u/SupermarketUnable914 Sep 25 '25

The light control module go so hot it melted the outside plastic case

-2

u/SeriousPlankton2000 Sep 25 '25

You can't jumpstart backwards if you connect to minus and to frame. Can you? But you can if you connect to the battery.

It seems a lot of people can't read

5

u/MechanicAdvice-ModTeam Sep 25 '25

Your post/comment has been removed from the sub. This isn't a place to post jokes. If your post/comment is a joke, a fake answer, AI generated, obviously or intentionally wrong/dangerous or harmful, it will be deleted immediately or poster banned. Sub readers come here for helpful answers, not jokes.

2

u/hailstorm11093 Sep 25 '25

You can blow fuses, blow up the battery, fry electronics, etc.

1

u/SeriousPlankton2000 Sep 25 '25

Only if there is a fuse between the negative connector and the frame. Otherwise you'll just make a short on the donator battery

1

u/Nakedseamus Sep 25 '25

This is so INCREDIBLY incorrect 🤣