Why do "they" always say to disconnect the Negative cable??? - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

 
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post #1 of 7 (permalink) Old Jun 4th, 03, 09:59 AM Thread Starter
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Jim
 
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I'm a senior year mechanical engineering student and I'm trying to figure out why it is always advised to remove the Negative cable from the battary as apposed to the Positive when working on a car.
I know that when jumpstarting a car it's better to make the last connection the Negative on the good car at ground, not the battary, but that's to eliminate the possibility of a spark getting near the bad battary and igniting the fumes...
Does the negative removal have to do with the flow of electrons always in the Neg to Pos direction?
Any ideas are welcome [img]graemlins/thumbsup.gif[/img]

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post #2 of 7 (permalink) Old Jun 4th, 03, 10:29 AM
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I don't know....how's this for a guess?:

Some electical components might hold a small amount of current when disconnected, and if you disconnect the positive, and accidentally ground the positive cable any time in the process of working on the car, the component that is holding the charge might short out and fry?

Sounds good to me
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post #3 of 7 (permalink) Old Jun 4th, 03, 10:42 AM
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Unhooking the ground first doesn't allow you to arc your wrench by mistake when disconnecting the positive. Easier on electronics that usually work off a ground. Easier on FRED.

jamie
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post #4 of 7 (permalink) Old Jun 4th, 03, 12:09 PM
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Right on Jamie...it's to prevent a wrench meltdown. There isn't any reason other than that.

-dnult

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post #5 of 7 (permalink) Old Jun 5th, 03, 02:54 PM
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Matt Jones
 
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Actually, melting your wrench to the frame is only PART of the idea...if anything.

I asked this very same question to the professor of the Electrical Engineering department here at my school. I'm also a ME student.

His answer was so long and complicated I cannot begin to repeat it, but it made sense. If you really want to know the answer, I'd suggest you to do the same; ask somebody at your school who's been doing this stuff for years and has a doctorate to prove it.

Matt Jones
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post #6 of 7 (permalink) Old Jun 6th, 03, 12:06 AM
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Sean
 
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i'm with dnult. the first thing i learned was neg cable first, to avoid going to ground with the wrench. they taught us this in auto shop, and i was told the same thing when i was strarting out as a mechanic, out of high school. d.c. power couldn't care less which side is disconnected first, as related to a car, imo. i would be interested in hearing what your teachers explanation was however.

Sean

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post #7 of 7 (permalink) Old Jun 6th, 03, 06:18 AM
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No offence to egineers, you may know the math and all the fancy calculations, but when it comes to knowledge, the tried and fried is the only way to learn things, You may get away once or twice unhooking the positive cable, but for me it was probaly about the hundreth time when I arced the wrench, BOOM, the battery blew up in my hands, Shop foreman comes over and damit the evidence was right there, wrench on the positive battery cable. its something you never forget.
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