eddog

There ought to be a lot of opinions about this.

bangbgC6

Wouldn't a battery cutoff switch be easier?

rodj

As in when doing elec work on car?

zr1fred

It's a good idea if you aren't driving your car for extended periods of time and you don't have a battery tender or power to the car. If that is what you are asking.

Cliff Harris

If you disconnect the positive terminal and accidentally touch metal with your wrench then you will get a big spark at least and possibly worse. The battery can deliver enough current to weld a wrench to metal parts and it will then get REALLY HOT. I've heard of melted wedding bands and burned fingers.

If you disconnect the negative terminal, it's already connected to ground so there is no possibility of a short.

SQUIRMIN VERMIN 84

The chassis is no longer part of the circuit.

JrRifleCoach

Are you asking if the negative cable is pulled first?
Or the neg is pulled when storing the car?

Paul Workman

Aside from Cliff's excellent reference to safety, why the distinction?

P.

Frizlefrak

Be more specific with the question...but yes, when wrenching, it's a safety issue. As mentioned above, get your wedding band lodged between the positive terminal and the chassis and it can burn your finger completely off in a very short period of time. Gold is an excellent conductor.

markKlein

When working on or near any wiring it's an excellent idea you should always do (and which I am often a little too lazy to do).

Klondike

Electrons flow from neg to pos. I'm not exactally sure of what & why it happens in a computer chip, but it is safer for the chips when you lift the source of electron flow rather than the exit side (+). There is usually only one "ground" vs many possible positive paths through the computer boards and cables. If the batt. ground cable was still hooked up and the disconnected+ cable came in contact with ground, there would be a conection path for many charged capicitors in computer boards. Doesn't matter if the ground cable touches anything, it's already ground and it's away from the battery source.

Railroadman

No matter how many times we read this when starting out, I think every guy has - at SOME point - gotten a wrench or screwdriver across something that it shouldn't. Guarantee it will wake you up in a hurry and you'll be a whole lot more religious about preventing it from happening again.

3D87C4

I've done some arc welding that way.

mako41

Current flows to the negative not the other way around.
When you remove the negative battery cable on a vehicle with a negative ground system, you have removed the potential for current to flow. If you removed the positive battery terminal leaving the negative in tact you will still have the posibility of unwanted current flow, tools, jewerly ... anything that conducts electricity can short to the still connected ground.

To put it simply, all the electrical components on a vehicle are connected to the common chasis ground. They can be activated when the positive terminal on them is supplied with battery voltage. If you remove the negative ground no current can flow even if battery voltage is applied to the positive component terminal!

Calderone

I always do that, i never ever let my 87 parked with the neg cable on , i use a killswitch **** but that's in me , always do this and my battery is still good like new after 4 years.

eddog

Just quoting the mm beginning of every maint procedure,but wondering about frying computer box more than the common arc welding.

Midnight 85

While we are on the subject, it is also a real good idea to remove any watches or rings while you are working under the dash. I heard a story once where a guy shorted a hot wire with his watch and had a permanent scar around his wrist that looked just like a watch. This of course pertains to metal watch bands.

Sparkytfl

I did it doing something with a starter. "But it's only powered when you turn the key" sounds true, but in fact is wrong.

Paul Workman

NOT that it matters, but up until the the invention of the CRT (cathode ray tube - the display used in radars and TV picture tubes, etc) "conventional flow" (aka positive flowing to negative) was thought to be the case. Nobody could actually "see" current (i.e. electrons) flow, so the "convention" was current flowed from positive to negative. (To this day, when you look at a circuit schematic the "arrowheads" on diodes and depicted in transistors, etc, point in the direction of positive flowing (to negative potential).)

The CRT shoots a beam of current from the base, which is steered by parallel vertically and horizontally placed magnets to bend or steer the beam, to strike a fluorescent screen creating a visible spot of light. But, when applying a positive potential to the anode (at the base of the CRT) and negative potential on the screen, nothing happened until (what the hell?) someone swapped polarity, and walah!! It must have been quite a "shock" to the scientists standing around when they suddenly realized current flowed from negative to positive (giving way to "electron flow" i.e., negative to positive as opposed to "conventional flow" i.e., positive to negative). And, now I can imagine how the concept of current flowing opposite to what you thought...is causing you too to ponder the logic of what you have said.

My point? My point is the battery can be thought of as the "pump" in a closed circuit network - providing the force to move current around the components, and returning to the battery. So, regardless of whether you subscribe to "electron flow" (negative to positive) or "conventional flow" (positive to negative) if you remove either pos ro neg cable from the "pump", current stops...Well, almost.

Someone made the (excellent) point there are capacitors associated with various modules, e.g., the ECM, etc - think of them as the equivalent of mechanical springs. When voltage is applied, the capacitors are charged (the equiv of compressing springs). The battery (pump pressure) can be removed, but the capacitors remain charged - for a time - until current leakage around the capacitors depletes the charge until "the pump" is reapplied, a la connecting the cable(s) to the battery.

However, until the capacitors have a chance to bleed off the voltage, if the positive cable was disconnected and then came in contact with the frame, there IS a chance of the reverse flow from the capacitors to ground could do harm - especially in sensitive computer circuits.

For what it is worth, I was first taught "electron flow" in the US Navy, prior to fussing with missile and gun fire control radars and computers, etc. But, like you, I since converted to "conventional flow" theory because it made reading electronic schematic diagrams much easier: when they discovered the truth about "electron flow", they never changed the symbols (the "arrowheads") that indicated the flow direction of "conventional flow". The difference is when you look at a schematic, you can almost envision the current flowing and spreading out across the circuit, guided by all the "arrowheads". In "electron flow", current flows against the arrowheads - kinda like steering a car by looking in the rear view mirror. As long as I don't try to apply a positive bias to the anode of a CRT, it's all good.

Bottom line...

Disconnecting either cable will open the circuit - preventing current flow**.
Removing the negative cable is arguably safer because if the wrench comes in contact with the chassis in the process, there will be no danger.

Removing the positive cable** will open the circuit, but precautions need be used to avoid the tool used from shorting against the chassis, AND the open connection needs to be isolated from the chassis to avoid inadvertently discharging the capacitors (in the ECM, etc), at least until the "caps" potential has time to bleed off.

Your mileage may vary.

P.

PS: Correction: Changed the text to reflect it is the NEG cable that is easiest to get to on the C4. That, combined with the safety concern for removing the pos terminal makes removing the neg terminal - especially on a C4 - a no-brainer! (Memory is a terrible thing to loose!)

P.

Railroadman

Paul, thanks for putting it in those terms, makes a bit more sense. I had read something to that effect but could not remember enough to post a rational statement. I believe I also heard that lightning - not sure if it's ALWAYS or SOMETIMES - actually jumps UP from earth, not DOWN from the clouds. It happens so fast the eye does not catch the difference and since we're raised with the idea that it comes down, the brain "sees" it that way. At least that's what the article claimed IIRC - I read it quite a while ago.