why battery cut off goes on negative terminal instead of positive?

[andyt11]

Originally Posted by 80-911SC

it goes on the negative BECAUSE it does ....

If it works ... don't fix it ... don't ask why an orange is orange or would they taste different if it was called blue .... they are oranges in the end of they day ..

No one is trying to re-invent the wheel. Just trying to learn.

[cgarr]

I figured the fewer big hot 12v cable I had running around the trunk area that could get shorted out the better.

[andyt11]

^^

That was my thinking cgarr, but I guess it makes sense not to have a big lump of copper bolted to the pos teminal just waiting to make contact with the body in the event of a bump or shunt, (or even just a loose battery).

The end result is the same putting it on the neg. It still allows you to quickly disconnect the power, just must take on board the point of killing the ignition first.

[lhoudyshell]

Not to hijack, I appologize in advance, but could the kill switch as outlined on the negative post work on a 2004 Boxster? Lynn

[Hugh R]

The simplest reason I've heard is your swinging a wrench to disconnect and when undoing the ground your less likely to hit anything hot with the wrench. But if you undo the hot your likely to hit a body panel which is grounded. On old British trash like Aston Martins with + grounds you undid the + ground first.

[RoninLB]

FD cuts negative on a car job 1st

just because you disconnect positive 1st doesn't mean there isn't juce still available from some electricals somewhere + the tools to ground safety issues involved of going after the positive 1st

[a_bayuga]

On a different note, what is the philosophy behind putting the exterior kill switch here? Is this the most ideal location? Why not by the right side? Sorry for borrowing the pic.

Thanks

[80-911SC]

its close to you when you get out the door ????

on the other side you would have to crawl over the hood or run around ....

plus you can shut it off on your way to the front to open the hood ........

Steve

[aston@ultrasw.c]

Also, an emergency worker, given the choice, will approach the drivers side first. If they then need to disconnect power the switch is close by.

[hcoles]

Glad I found this thread.
I'm helping with a race car and they are using a neg. kill switch. The motor is new so we need to check that the alternator or other component doesn't keep something alive that you want kilt.

[winders]

Originally Posted by hcoles

I'm helping with a race car and they are using a neg. kill switch.

Change it so it breaks the positive line and removes power from the ignition system.

Scott

[Marc-André]

If I were to disconnect the positive battery lead (i.e. switch) while the engine is running, how could this damage the alternator? All the electrical equipment will be cut off at the same time: Ignition, fuel pump, alternator, lights, etc.

[patkeefe]

Electrons flow from negative to positive outside the battery. That is physics. Electrons flow to negative inside the battery due to the chemical reaction. That is chemistry. So, the path for the electrons is from negative to positive.

That being said, put the kill switch anywhere you want.

[Tippy]

Originally Posted by luftgekuhlten

The current flows from the negative terminal to the positive, so it follows that you should disconnect the outflowing terminal first to deenergize the system.

That's my understanding too. My EE coworker says that's what occurs from an electron physics standpoint.

It's baffling to most when you tell them this.

[patkeefe]

Originally Posted by Marc-André

If I were to disconnect the positive battery lead (i.e. switch) while the engine is running, how could this damage the alternator? All the electrical equipment will be cut off at the same time: Ignition, fuel pump, alternator, lights, etc.

Battery also acts as sort of an electrical "damper", and absorbs the transients and spikes the alternator can produce. So, you risk frying the diodes, and doing bad stuff to the electronics you may have. There is a lot of info on this in the internet.

[mreid]

Originally Posted by Marc-André

If I were to disconnect the positive battery lead (i.e. switch) while the engine is running, how could this damage the alternator? All the electrical equipment will be cut off at the same time: Ignition, fuel pump, alternator, lights, etc.

When properly set up, the battery acts like a large capacitor. It determines a reference voltage for the alternator through the regulator. When you suddenly remove the battery from the circuit, the regulator responds by trying to compensate and without the battery to set a reference voltage, a large current spike will flow through every active circuit. The higher the rpm of the alternator, the more massive the current spike. Race car cutoff switches usually have at least two poles (battery and alternator) and should be installed on the positive side disconnecting and isolating both the battery and alternator simultaneously. For us hobbiests, we use a cutoff switch more as an electrical system disabler allowing us to disconnect the battery when the engine is not running in order to work on our machines or as a theft deterrent. For this purpose, installing the electrical disconnect switch on the negative side is a more appropriate solution as you are shutting off the source of electrical current flow through the single line without worry that there is some single circuit (probably cobbled in by a po) still connected or available to create a light show. So:

Cutoff switch: get the right one, install it in the positive circuit including the alternator to stop the engine when it is running (btw, some racing bodies require the external switch to be installed on the back of the car).

Disconnect switch: single pole, install it between the - battery terminal and chassis to isolate the battery when the engine is not running.

[Marc-André]

In fact, I'd like to get rid of the original ignition key and install a cutoff switch: If I were to simply replace the ignition key with a rotary on-off and additional switches for, say fuel pump, ignition and push button for starter, which would replicate the original ignition key functions, I guess this wouldn't affect the alternator since it doesn't actually cut it from the battery circuit, right? But this type of setup wouldn't be approved by any racing organization... ?

[A horse with no name]

And for those that don't know, on all newer cars, it's 'a must' to take the ground off first, and to reverse the procedure when putting the terminals back on...+ then - ...The same goes when jump starting.

By taking the ground off first, then the positive, you will not damage the computor(s)

[Algernon]

A lot of these replies are not really accurate, or not truly helpful. The fact is, if either terminal is disconnected, the battery is no longer involved. If the engine is able to be run (how?) without the battery, then damage may result, of course. But the choice for disconnecting either terminal with an isolation switch is (ideally) of no significance. In our non-ideal world however, safety in the event of battery shifting, etc. probably makes negative switch preferred. But this will always also stop all accessories, whereas with a positive switch you can have a low-value fuse by-pass directly to clock, alarm, etc., which might be convenient (as they would not stop). Hope this helps.

[larrym]

X2 on the guys above who actually understand this issue from a track-car perspective

Originally Posted by KTL

Definitely have to make sure your switch is wired to stall a running engine, without damaging any electrical components.
Many racing organizations will require you to demonstrate the switch's proper operation as part of a pre-race or annual tech inspect requirement.

- our racing clubs will tell you that an "FIA" kill switch is required & they will test your switch in Tech to see if the engine stops running when the kill switch is theoretically switched off -

secondly, as pointed out by a few above - if you don't have the correct FIA switch with alternator protection - you will kill your alternator - maybe worth the $$ risk in an old chevy (a $35 item), but not for my Porsche, nor even for my alternator equipped Manx buggy

i don't fathom why Porsche guys with expensive cars & expensive parts want to save a couple $$ by using a cheap HF or Walmart switch???????

you can find the correct wiring instructions HERE - - https://www.pegasusautoracing.com/group.asp?GroupID=SWITCHBATT

or just google it -
https://www.google.com/search?q=FIA+kill+switch&ie=utf-8&oe=utf-8