Battery disconnection protocol?
Thread Starter
Senior Member
Joined: Apr 2018
Posts: 143
Likes: 31
From: New Mexico
Battery disconnection protocol?
Got some new headlights for my truck, and i'm wondering what necessary precautions you all take when doing electrical work on your trucks. I had some electrical issues in the past so I want to be safe.
Thread Starter
Senior Member
Joined: Apr 2018
Posts: 143
Likes: 31
From: New Mexico
I have to splice into the blinker wires for some of the features of the headlights, so I figure it's safest to go ahead and disconnect. Fair enough though, thanks for the advice!
Senior Member
Joined: Jun 2019
Posts: 643
Likes: 201
From: Twin Cities, MN
I agree probably not needed but as mentioned above always the negative first.
The reason why is if you have a wrench on the positive post and you are disconnecting that first, if your wrench touches something that is metal and is grounded to the negative post that is still hooked up, you will have a bad day. For instance (not saying this is physically possible based on component location) but if your wrench was on the positive connection first and your wrench touched the body of the alternator, it would complete the circuit to the negative post. That is just an example but it could be nearly anything in the engine compartment from the engine to the body. I wouldn't trust the paint on the body being thick enough to be much of an insulator.
If you disconnect the ground first you can touch from the positive to one of these conductors and there is no path back to the negative post because the wire isn't hooked up anymore. So there will be no fireworks.
I actually did something like this on a Hummer in the military. The battery box is located under the front passenger seat. The positive wires are run to a large post on the side wall of the battery box. I was half asleep from being out on missions and had to hit the maintenance shop for new batteries because we were having problems with the ones in it and had to keep jumping the truck. I had just dropped in the new batteries and connected the wires and went back to install the battery hold down bracket. I should have done it the other way around but hey, I was tired. So I was tightening down the bracket that holds down the batteries and not even on the wires thinking I am good. However, the open end of the wrench was on the battery bracket nut, and the box end of the wrench touched the post on the side of the battery box where the positive wires run come together. Massive amount of sparks were flying, the wrench got really hot. So hot that I melted the tips off the open end side of the wrench as well as a hole through the box end. I threw that sucker quick enough that I didn't burn my hand but it scared the crap out of me and woke me up. There are no fuses to blow since it is a direct connection to two really big batteries and the hold down bracket that must have had enough of a path back to the negative posts.
So now when changing a battery, I always secure it first, then positive followed by the negative wire. The negative wire is always the first to come off and the last to go back on.
The reason why is if you have a wrench on the positive post and you are disconnecting that first, if your wrench touches something that is metal and is grounded to the negative post that is still hooked up, you will have a bad day. For instance (not saying this is physically possible based on component location) but if your wrench was on the positive connection first and your wrench touched the body of the alternator, it would complete the circuit to the negative post. That is just an example but it could be nearly anything in the engine compartment from the engine to the body. I wouldn't trust the paint on the body being thick enough to be much of an insulator.
If you disconnect the ground first you can touch from the positive to one of these conductors and there is no path back to the negative post because the wire isn't hooked up anymore. So there will be no fireworks.
I actually did something like this on a Hummer in the military. The battery box is located under the front passenger seat. The positive wires are run to a large post on the side wall of the battery box. I was half asleep from being out on missions and had to hit the maintenance shop for new batteries because we were having problems with the ones in it and had to keep jumping the truck. I had just dropped in the new batteries and connected the wires and went back to install the battery hold down bracket. I should have done it the other way around but hey, I was tired. So I was tightening down the bracket that holds down the batteries and not even on the wires thinking I am good. However, the open end of the wrench was on the battery bracket nut, and the box end of the wrench touched the post on the side of the battery box where the positive wires run come together. Massive amount of sparks were flying, the wrench got really hot. So hot that I melted the tips off the open end side of the wrench as well as a hole through the box end. I threw that sucker quick enough that I didn't burn my hand but it scared the crap out of me and woke me up. There are no fuses to blow since it is a direct connection to two really big batteries and the hold down bracket that must have had enough of a path back to the negative posts.
So now when changing a battery, I always secure it first, then positive followed by the negative wire. The negative wire is always the first to come off and the last to go back on.