Why do we disconnect the negative terminal?

A little off topic, but I grew up in a small town of 3K. Saturday nights in high school we took turns with my friends driving around town late into the night. We usually parked the "extra" cars in the downtown area in front of the bank. One late night after our ride, I go to start up my '73 Monte Carlo and the battery explodes. It trips the bank alarm, alerts the police and the sleeping bank manager. It was a tense situation at first until everyone realized I did nothing wrong. I parked in a public parking space and my battery exploded at start-up. This was the one and only battery explosion I have experienced and I'll never forget how loud that sound was.
 
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gonefishin said:
A little off topic, but I grew up in a small town of 3K. Saturday nights in high school we took turns with my friends driving around town late into the night. We usually parked the "extra" cars in the downtown area in front of the bank. One late night after our ride, I go to start up my '73 Monte Carlo and the battery explodes. It trips the bank alarm, alerts the police and the sleeping bank manager. It was a tense situation at first until everyone realized I did nothing wrong. I parked in a public parking space and my battery exploded at start-up. This was the one and only battery explosion I have experienced and I'll never forget how loud that sound was.
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When I worked at a Sears Auto we had one battery explode on the slow charger that was in our battery rack room. Then we had another one let go in one of the fast chargers next to our battery service lane. At least the one in the battery rack room was muffled and the room already smelled like battery acid. The one in the toaster was loud.
 
bobdoo said:
The chassis of the car (or whatever) is all negative/ground.

Your wrench cannot short circuit from the negative terminal to anything nearby.

Conversely, taking off the positive terminal first, then hitting the chassis of the boat, WILL fry the electronics mounted on your Merc 25HP motor...
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This is the best answer, except I have no idea why a Merc 25 hp would do that.

At any rate, if you DON'T disconnect the negative first, and start working on the positive terminal with that wrench, YOU NEED TO BE VERY CAREFUL NOT TO TOUCH ANY NEARBY METAL WITH THAT WRENCH! If I could write that bigger, I would. If you do, you cause an arc which can cause the battery to blow up in your face.

NOW, once the negative terminal is disconnected, it's much safer. Nothing that wrench touches while working on the positive terminal, except for the NEGATIVE terminal itself will cause an arc. You can control that much better. During the disconnecting of the negative terminal, most vehicles being NEGATIVE ground, touching a wrench to any nearby metal parts will not cause a short circuit, or an arc.

Same goes for spark plug wires. Don't ever lay them on top of a battery and crank the engine. If they arc to the battery, the hydrogen inside will explode and it will blow up!
 
zzyzzx said:
I really hate the way every service seemingly begins with this, even for stuff not electrical... I pretty much only do it if removing the battery or alternator.
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It's still good practice to disconnect the battery. I've seen some weird stuff happen because careless mechanics that would have been avoided if the negative cable had been disconnected.
 
Remove the negative cable from the battery . Take your wrench and short the positive to any other metal part . What happens ? Nothing . THAT'S why you remove it .
 
zzyzzx said:
Such as?
I'm inclined to think that it's mostly stupid people that have stuff like this happen.
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You never dropped a tool or a part while working under the hood ? So , if you replaced a radio or other electrical component , you " worked it hot " ? You are just too cool .
 
nomas said:
You never dropped a tool or a part while working under the hood ? So , if you replaced a radio or other electrical component , you " worked it hot " ? You are just too cool .
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Like so many other things, yes, you CAN do it YOUR way and it will work just great 'cuz you're perfect. Rest of us idiots, just take the ground off 1st and put it on last :cool:
 
For battery work I use my little composite 1/4 drive harbor freight rachet after learning the hard way. Also helps prevent over torque.
 
nomas said:
You never dropped a tool or a part while working under the hood ? So , if you replaced a radio or other electrical component , you " worked it hot " ? You are just too cool .
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I used to be an Electrical Engineer, and I know what I am doing. Yes replaced a radio and other electrical parts and NEVER disconnect battery unless it's in the way, or changing the battery or alternator. Otherwise totally unnecessary. Only exception is if removing an air bag I pull the fuse for it.
 
zzyzzx said:
I used to be an Electrical Engineer, and I know what I am doing. Yes replaced a radio and other electrical parts and NEVER disconnect battery unless it's in the way, or changing the battery or alternator. Otherwise totally unnecessary. Only exception is if removing an air bag I pull the fuse for it.
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That's fine , but the " stupid " thing was a little over the top . Whatever .
 
zzyzzx said:
I used to be an Electrical Engineer, and I know what I am doing. Yes replaced a radio and other electrical parts and NEVER disconnect battery unless it's in the way, or changing the battery or alternator. Otherwise totally unnecessary. Only exception is if removing an air bag I pull the fuse for it.
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PHBBB, Pal, I used to rack and stack -48 muxes. Lotsa times we worked hot. Until one Sat morning I took down an entire head end with a slipped screwdriver because the power distribution was engineered incorrectly. :cool:
 
I can’t recall which, but dangit I learned the hard way that Volvos are actually sensitive to which post you disconnect. The airbag system is rather pervasively wired in these, for instance, disconnecting the HVAC controls without removing the battery will set an airbag error code, as well as a bunch of other things in the car. I also learned, through experience, that its sensitive to which terminal on the battery is removed first. Never seen that in any other vehicle.

once I sold the last Volvo, the details left my head too, use it or lose it…
 
zzyzzx said:
Such as?
I'm inclined to think that it's mostly stupid people that have stuff like this happen.
Click to expand...

Setting the vehicle on fire & blowing holes in aluminum A/C lines are 2 I've seen most. I've also seen Throttle Cables & Dipsticks get burned in two.

Unexpected things happen, Disconnecting the battery removes one of those variables.
 
zzyzzx said:
I really hate the way every service seemingly begins with this, even for stuff not electrical... I pretty much only do it if removing the battery or alternator.
Click to expand...
Imagine changing the fuel filter in your Quadrajet, and dropping the wrench on your alternator output. Now your garage and house are burning down because you couldn't imagine why you should disconnect the battery.
 
zzyzzx said:
I used to be an Electrical Engineer, and I know what I am doing. Yes replaced a radio and other electrical parts and NEVER disconnect battery unless it's in the way, or changing the battery or alternator. Otherwise totally unnecessary. Only exception is if removing an air bag I pull the fuse for it.
Click to expand...
Pulling the fuse will not render the air bag safe. They retain charge for a while, Mr Engineer
 
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