Kill switch question

Last year I added a kill switch to my 73B roadster. The switch interrupts the positive side of one of the batteries instead of the path to ground.

I've read that a ground interruption is the recommended method. Can anyone tell me why interrupting the ground is better then breaking the positive feed to the system?

As an aside this is not a "normal" kill switch. I wanted the switch on the dash console instead of on the battery compartment wall behind the passenger seat. To do this I added an appropriately sized solenoid relay between the battery positive terminal and the positive cable to the starter. I then powered this relay through a spare hazard switch I put in an empty space on the console. As such, when turned off the only "live" power in the car is in the 16 guage line from the battery to the spare hazard switch. Turning on the hazard switch completes the circuit to the solenoid relay, turning it on and closing the battery positive side circuit to the starter.

It's a great system and fun to use too! Only downside is that the solenoid is fused for safety (a 2 amp fuse for about a 1.2 amp solenoid draw). If the fuse blows for any reason (like it did on the way to British by the Sea last weekend due to my hooking the radio memory power source to the spare hazard switch and not increasing fuse size) the car will run on the alternator without a battery connection. This I'm told can be harmful to the alternator, and will let the car stall at low idle (which it did) due to not enough power to the plugs.

Thanks,
BH

BH, everyone will dive in and tell you their method. The only reason to do the ground side is that is lessens the chance that you might short the switch terminal or battery postive to ground if you're fooling around with it.

Other than that, no. I put mine in the positive side, the cable routing was easier, it made for a cleaner, neater installation, and that's how we've done it in industrial and marine for 30 years.

my kill switch to the positive side blew a diode on my electronic ignition set up. The power surge can be bad and it also messes up the radio settings, mainly power surge when you turn it on. MHO

I learned that a fuel cut off with a double switch is the best....the car starts and runs 100 feet and dies, the thief tries to start and fails and leaves the car....your car is about a block away and you can start it up...

the + vs - is covered extensively in Rick Ashley's "MGB Electrical Systems"--I do not have the book in front of me so cannot recite it...nor even verify correct spelling of his name and the title.

if you do not have it, you should get it. it is cheap and readily available (Amazon stocks it) you will learn more usefull stuff from this book than from any other MG book you might read

Now...in this installation the thing is not a kill switch. it will not kill the engine when it is running. even a + installation will not kill a running engine, because there is alternator flow to consider.

for that you need to refer to SCCA spwcs or procedures.

I believe that a proper SCCA kill switch will prevent starting as well as running. but the mounting is crucial.

I cannot advise what this connection scenario is...I don;t know. But when I get around to installing mine I think I will go the SCCA route, even though I am (no longer) a racer...unless someone can argue well against it.<G>

Current flow is current flow. If it is going to flow then it doesn't matter whether you start it by hooking the ground up or the positive side. All the current flowing through the negative cable has to flow back through the positive cable.

I called the manufacturer and I was told the kill switch was to be mounted on the positive cable. I have read somewhere that you don't ever want to break the connection while the engine is running because it can damage the alternator.FWIW.

I have read somewhere that you don't ever want to break the connection while the engine is running because it can damage the alternator.FWIW.

Haven't seen any damage from accidently turning the battery cutoff while running but if you turn the switch back on while running it will blow the alternator AND the headlights.

Not fun at 3:00 in the morning

The kill switch on a race car has 2 posts, one for the positive power from the battery and one for the alternator. This effectively kills all power to anything electrical (especially the fuel pump) and stops the car running off of the alternator. If the switch is thrown, something bad has happened!

If you're trying to stop theft, I recommend a fuel shutoff as previously mentioned or a coil short circuit to ground. Either one can be wired to a hidden switch anywhere on the car and won't be visible to a thief like the big red keyswitch.

I know this style cutoff switch is not ideal, but I mounted it on the shelf, just behind the battery and painted it black. It's pretty much hidden by the boot cover and passenger seat back. Oh, and hooked up negative.

(click on image to enlarge/hosted offsite)
[URL=http://img367.imageshack.us/my.php?image=cutoffswitchtb8.jpg][/URL]

ted

If anti-theft is what you want, why not just throw the coil wire into a hiding place in the car or your pocket???
Just a thought

LearningMGB Wrote:

BH Davis Wrote: