reset for computer to relearn pos or neg?

[ccorbridge]

When disconnecting the battery should I undo the positive or negative terminal? I've searched the forum and found it both ways. Ill be cleaning my MAS and upgrading my cold air intake (K&N) and want to be sure the computer knows this.

 

[NickF]

The red terminal.

 

[trailmix]

i always pull the negative cable to reset check engine lights etc. if the negative cable is not attached the circuit wont be complete and nothing will get power

 

[javendan]

x2

negative cable.

joel

i always pull the negative cable to reset check engine lights etc. if the negative cable is not attached the circuit wont be complete and nothing will get power

 

[TheDurk]

As far as the ECM, goes, I am sure it makes no difference: power is cut either way. But it is ALWAYS good practice to remove the negative before you remove the positive on any (negative ground) car. If your wrench contacts ground while removing the negative cable, nothing happens. If you remove positive first and that happens, big sparks and possible damage to you, the car or the wrench. Similarly, always reconnect the ground (negative) last. Mechanics 101.

 

[NickF]

Removing the positive wire(red) allows you to remove power from things that are not grounded locally on the battery, so I make it a habit of disconnecting that. As long as nothing is turned on you won't get sparks large or hot enough to cause you or your tools any damage.

 

[ccorbridge]

I see there are advantages (and disadvantages) to both methods. Thanks for the info guys.

 

[snhrph]

Removing the positive wire(red) allows you to remove power from things that are not grounded locally on the battery, so I make it a habit of disconnecting that. As long as nothing is turned on you won't get sparks large or hot enough to cause you or your tools any damage.

Huh??? There are things that are not grounded on the battery??? Then what are they grounded to??? And where do they get the power from?

 

[NickF]

Huh??? There are things that are not grounded on the battery??? Thwn what are they grounded to??? And where do they get the power from?

They are grounded to the chassis, or other bare metal. They still get power from the battery/alternator, but they complete the circuit in a different location. Most aftermarket electronics are installed this way

 

[snhrph]

They are grounded to the chassis, or other bare metal. They still get power from the battery/alternator, but they complete the circuit in a different location. Most aftermarket electronics are installed this way

I think you are missing a very important point here - the "things" can be grounded to absolutely anything on your truck, even the exhaust pipe... But at the end of the wire(s), they all come together to a thick wire that connects to the negative terminal on your battery. If you disconnect the negative terminal, it breaks the closed loop in exactly same way as if you disconnected the positive terminal, minus the sparks... :biggrin2: So, to reiterate, any "thing" that draws power from the battery will have NO power if you remove EITHER terminal - positive or negative.

Unless, of course, the "thing" is powered by a different source...

 

[TheDurk]

Removing the positive wire(red) allows you to remove power from things that are not grounded locally on the battery, so I make it a habit of disconnecting that. As long as nothing is turned on you won't get sparks large or hot enough to cause you or your tools any damage.

Thanks to snhrph for responding to the first sentence. As to the second, if you think there is any difference in the amounts of sparks generated when a wrench on the positive cable accidentally contacts ground because 'nothing is turned on' you REALLY don't understand auto electrics. It's a direct short and load on the cable is irrelevant.

 

[Benski]

Thanks for clearing that up. I don't want fireworks while I'm working on my truck! The black/negative will always be first off and last on.
:thumb3:

 

[IanB]

I've always disconnected the neg cable, and if you're looking to reset the ecu you'll need to discharge the system even after you disconnect the battery, I do this by hitting the brake pedal (it tries to light up the tail lights, and uses up any residual charge in the system).

 

[Gerdo]

Removing either cable will totally disconnect the battery. But...

Thanks for clearing that up. I don't want fireworks while I'm working on my truck! The black/negative will always be first off and last on.
:thumb3:

Always the safest.
You still need to watch that your wrench doesn't touch the pos and neg at the same time. Removing the positive has the potential of a wrench touching the terminal and the body or engine, not good. Removing the negative removes that potential (touching a wrench from the negative terminal to the body of engine will do nothing- on our negative ground vehicles).

 

[Baja Runner]

Lol seriously guys ... you break the circuit and electricity cannot flow. Doesn't matter which one, but yes its safer to pull the negative first.

Electric field is the same as a gravitational field. Imagine your ground (negative) is ground level and your positive is 12 ft high. U put a ball on the ground it has no potential to move, drop or turn into kinetic energy. You raise it to 12 ft and it is going to want to fall to ground. The same thing happens with an electron in an electric field. (Voltage potential)

 

[TheDurk]

Removing either cable will totally disconnect the battery. But...



Always the safest.
You still need to watch that your wrench doesn't touch the pos and neg at the same time. Removing the positive has the potential of a wrench touching the terminal and the body or engine, not good. Removing the negative removes that potential (touching a wrench from the negative terminal to the body of engine will do nothing- on our negative ground vehicles).

Another point not made above, is that, if you are wrenching on the positive with the negative still connected, and any part of your body contacts bare metal grounded to the chassis, YOU, and not just the wrench, will be part of a direct short and you will not enjoy it. I grew up working on tractors and you have uninsulated metal all over the place.

 

[Nashvegas34]

The battery also gives off potentially explosive vapors. If you pull the positive cable first and cause a spark, there is the risk, although slight, of igniting the vapors and causing an explosion. Removing the negative first breaks the circuit and prevents that. After I lift the hood I usually let it air out for a few minutes to dissipate any vapors that *might* be hovering before doing anything. Better extra careful than extra crispy.

 

[TheDurk]

The battery also gives off potentially explosive vapors. If you pull the positive cable first and cause a spark, there is the risk, although slight, of igniting the vapors and causing an explosion. Removing the negative first breaks the circuit and prevents that. After I lift the hood I usually let it air out for a few minutes to dissipate any vapors that *might* be hovering before doing anything. Better extra careful than extra crispy.

I don't worry too much when dealing with fully charged modern maintenance-free (sealed) batteries. But a discharged battery is always a concern for H2 gas. When jumping, good practice is to hook up the positives first, then connect from the ground terminal on the charged battery to a chassis ground away from the battery (engine lift hook is good on ours) on the discharged car.