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disconnect + or - cable?

J

jCarroll

Well-known Member
Location
mid-Ohio
I grew up with disconnect the positive battery cable when doing electrical work which might involve a hazardous connection.

Now I'm reading instructions with parts that say to disconnect the negative cable .........

As a former secretary of state was quoted "What difference does it make?"

I'm working on a negative ground machine.
 
Okay here's the deal. The FRAME GROUNDED connection is the one I disconnect FIRST. That's because if your wrench comes in contact with anything metal nothing bad takes place HOWEVER if you first put a wrench on the hot ungrounded battery terminal and the wrench touches the frames metal OUCHHHHHHHHHHHHHHHHHHHHHHH could burn you badly and/or the battery blow up.

But yall feel free to do as you please

John T
 
So, you see, jcarroll, it depends on whether the machine that you are working on utilizes pos or neg ground.
The primary idea is to first wrench on whichever battery terminal (pos or neg) is tied to the chassis (ground), because if that wrench comes in contact with the chassis metal parts, there will be no spark. Then once that cable is disconnected, when wrenching on the other battery terminal, if that wrench contacts the chassis, there will also be no spark. Do it in reverse of this & you will make fire & sparks upon wrench contact!
So, in summary, it isn't whether to disconnect pos or neg first, but rather disconnect whichever is tied to chassis first.
 
Anyone involved with fire & rescue will tell you always the ground cable first you don't want any sparks at an accident scene.
 
EXACTLY, that's why I said "The FRAME GROUNDED connection" is the one to disconnect first however that may well pass by some people unfortunately grrrrrrrr

John T
 
Thanks, fellas, for answers which make sense.

I had not considered a wrench malfunction since I never make mistrakes.
 
Ha!
Along with unhooking the ground first, when going back together, always hook up the ground last. It's a good idea to gingerly/briefly touch the ground cable to ground before going right after it to fasten it, in case something is shorted somewhere.
 
As the others mention, disconnect the GROUNDED cable regardless of battery ground polarity.

A friend learned this the hard way. He lost his ring finger wrenching on the ungrounded battery clamp with the ground cable connected. Somehow his wedding ring got jammed between the wrench handle and a grounded frame part. The ring instantly became white hot and neatly removed his finger.

Safety first!
 
(quoted from post at 10:28:29 05/15/14) As the others mention, disconnect the GROUNDED cable regardless of battery ground polarity.

A friend learned this the hard way. He lost his ring finger wrenching on the ungrounded battery clamp with the ground cable connected. Somehow his wedding ring got jammed between the wrench handle and a grounded frame part. The ring instantly became white hot and neatly removed his finger.

Safety first!
Click to expand...

I was changing out the fuel pump on an IH Scout with the 345 engine. Had to sort of reach around and in behind the alternator. Suddenly I realized my watch band was getting rather warm, and then realized my watch band was in contact with HOT post on the alternator. No real damage done, but from then on, my watch always went into my pocket BEFORE I picked up any wrenches.
 
Haven't read all posts so may be repeating.

Basic rule of thumb. Unhook the GROUND first and hook up the GROUND last.

Reason being if your wrench touches the frame or some other part it won't short when working with the ground. Then having the ground unhooked while you work on the hot lead means there is nothing to complete the circuit if the tool touched the frame.

jt
 
Disconnect the ground first and hook it up last. doesn"t matter what the sign of the ground is. the chasis on a vehicle is grounded so disconnect the ground first. You won"t have a complete circuit when you are taking the other terminal off and touch a chasis part with the wrench.
 
As everyone has said chassis ground first.
Way back in high school welding class a classmate was showing off his brand new 20 foot tape measure, this was back when tape measures did not come from the land of the vertically challenged and Sal had paid good money for it.
The first time he pulled it out to use it he was going to build a new battery tray to replace the rusted out one on his old Malibu.
He pulled his car into the shop, opened the hood and pulled out his tape to measure the battery tray.
As he streched the tape out across the top post battery the sparks and smoke were followed by flick flick flick flick flick... as the spring in the tape retracted all but the last foot or so that had been melted off on the battery.
So depending on what you are doing sometimes it doesn't matter which or even if you disconnect first.
 
John T, and Bigjt, you forgot to add "it will make our wedding band red hot in an instant if you are holding the wrench in that hand and the ring touches the frame". Kind of makes you shake your hand a bit when there's a branding iron wrapped around one of the fingers. I still have an indentation around that finger forty years later. Jim
 
So many people think when they hear the word "Ground Wire" they automatically think "Neg Wire". That's not so! Ground means connected to vehicle frame. Looks like you have many good responses to your question.
One time years ago I was working on a 24-volt two-battery system. After removing the ground cable I accidently dropped my 1/2" wrench over the two posts of one of the 12 volt batteries. The wrench started to glow red in matter of seconds. there was close to 1,000 amps running thru the wrench. But any battery regardless of size will do the same thing. A person can never be too careful. I always tell people that "batteries and tires are bombs if you're not careful around them".
 
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