car batterys

I am hooking up a harness for trailer wireing and am told to disconnect the ground from the battery …Why ? what happens if I do not ?

In case you accidentally touch a power wire to the body/ frame while working on it. Disconnecting the negative cable opens the circuit so if you accidentally short something, current has nowhere to go.

If you don’t disconnect the negative, at best you could blow a fuse. At worst the wire can overheat and melt or start a fire.

When making connections to the existing harness, you might ground a live wire. This will create arcing and excessive heat, not to mention scare the bejesus out of you. By disconnecting the battery ground, you will prevent that from happening.

excavating (for a mind)

if I disconnect the ground will I lose all my settings like radio…computer…seats …phone …etc ?

yes you might.

that is overall better than causing sparks, blown fuses.

Thanks johnpost…opened package that came with car…all the connections are plug ins…no wires to strip or to connect …just push the plug in …two on each side…sounds way too easy…but I think I can do it… still not sure about disconnecting the ground wire from battery…I don’t see anyway there could be a spark with the plug ins …but don’t know…

Before you disconnect the battery, make sure your car radio and/or any alarm system, remote starter, etc. all do not have any special requirements for removing the battery. On some systems, this triggers the security in the device which makes it think that it has been stolen and it won’t work again without a special reset from the dealer/factory.

On many vehicles this isn’t an issue, but on some it can be a real pain to recover from.

As others have said, connecting to live wires involves a risk of shorting things out, which can cause damage. Without seeing the plugs involved, I can’t say how easily you could end up shorting something. Many plugs can be accidentally inserted an enough of an angle to where things that shouldn’t contact can come in contact with each other.

Furthermore on a modern car you have to ensure that you follow proper procedures when disconnecting the battery or you could toast one of the many control units in the car.
This is considered not good in the trade.
Instructions for different makes of cars vary but in general you need to leave the key off and out of the ignition for a long enough period of time that the entire computer network has gone to sleep.
This could take upwards to 8 minutes or a bit longer.

even if I am a girl I do a lot of my own car work…Car in question ia a 2011 Chrysler Town &Country Van…how would I find out if there would be a problem disconnecting the ground? … Thanks again …all who have posted…you have given me a lot to think about…

the manual for the car will describe about disconnecting the battery.

if you don’t have one then go to

select year and model and you can download one.

it will speak about the model in general and its variations. you look for specifics for your particular version of the van.

anything added on later radio, alarm, remote starter would have its own instructions.

I wouldn’t think it’s necessary to disconnect the battery to plug a trailer brake controller into the existing socket, but if Rick say to do it, believe him and not me.

In this case, it probably is just an over-abundance of caution in the instructions.

Actually I didn’t say you had to disconnect it, what I said was you have to follow the proper procedure if you do.
Assuming that all the connections are plug ins (post #6) then disconnecting the battery could just be standard CYA put in there for no good reason.
[Story from my past] I was working as a line tech on Jaguars. At the time it seemed like every procedure in the Jag factory repair manual had 1, 2, or all of the following warnings:
[ul]
[li]Disconnect the battery[/li][li]Depressurize the fuel system[/li][li]Drain the cooling system[/li][/ul]
After reading these warnings (which usually were not necessary) for the 16,000th time us guys in the shop would get silly.
(tech pulls an oil change in to shop)
“Hey did you depressurize the fuel system?”
“No, but I drained the battery, and disconnected the cooling system, is that OK?”
“Close enough”
[/SFMP]
::: crickets:::
OK, you had to be there. Now get off my lawn.

Thanks Rick, appreciate your help. I am going to do this without disconnecting the ground…just being very careful…will let you know in a day or two how it comes out…

You made the right choice for this case, betty.

“Disconnect the battery” is a much overused instruction. It makes sense when working with a live positive wire that could fairly easily be grounded, such as when replacing a starter or alternator. It’s a wise precaution when working in a possibly touchy area with potentially severe consequences, such as an air bag system. But hooking up trailer wires? Ridiculous.

In the trailer circuits, there’s no live wire to ground, so long as the taillights, brake lights, and turn signals are off. It would make far more sense for the instructions to say “make sure those lights all work properly, and make sure they’re turned off” than to say “disconnect the battery.” There’s simply no advantage or benefit to disconnecting the battery, and with modern cars the disadvantage can range from annoying (reprogram radio memory) to disabling (reset onboard computer). I don’t know what the fools who write these instructions are thinking; in the real world it’s just silly.

I’m laughing on the inside. :smiley:

Disconnecting either battery terminal would allow safe electrical work. A complete circuit is only possible if both battery terminals are connected, but there is a very good reason the negative terminal is specifically called out.

You are instructed to disconnect the negative rather than the positive because there is a good chance the wrench you are using on the terminal will also touch the body or some other grounded point. If you are wrenching on the positive terminal while the negative is still connected, that is very bad. Depending on the wrench and battery, there will be, at a minimum, a big spark, and possibly the wrench could be heated glowing hot, resulting in burns or fire, and the battery might explode. Once the negative/ground connection is broken, it is then fairly safe to work on the positive terminal, as bridging a wrench to ground will not complete a circuit through the battery. You only have to take care not to bridge to the negative battery terminal, which is a much smaller target.

Thus, when removing/disconnecting a car battery, you should always disconnect the ground (negative) side first. When installing or reconnecting, you should connect the positive side first.

Finally, if you are working on a vehicle with two batteries (many diesel pickup trucks), you should disconnect both negatives before doing either positive.

You are usually told to disconnect the negative lead when doing electrical work, or when there is a chance of spilling fuel, which is easily ignited by an electrical spark. In many cases it is too easy to pinch a wire even when doing purely mechanical tasks, hence the general advice.

-Kevbo, who once lit a motorcycle on fire with a small electrical spark, and discovered that profanity is not an effective way to put out a fire, and ended up using a CO2 bottle instead.