I've got some really nice battery cutoff switches, with just the two contacts. I heard from a buddy that when you use it, inline with just the negative side of the battery (or pos, etc.) you can blow the diodes in the alternator when you disconnect. But even though he happens to be an engineer, he can't explain to me why. Or the mechanism that causes it to blow.

Can anyone tell me why I can't use these switches, or that this is indeed a real danger? Thanks!!!

With an alternator, you will need the extra connection module on the bottom of the kill switch that includes the big resister (I think it is) to ground so when the switch is thrown to kill the engine it will stop the engine. Otherwise the engine can continue to run on the alternator and it will burn up.

Or you can have the alternator and battery on one side of the kill switch, everything else on the other side. This way the ignition, injection, instruments, etc are on one side of the switch and your alt/battery on the other. Hit the kill switch, car dies.

Brando. That was how I planned on doing it. Will that damage the system at all when the switch is thrown.

I'm not going to use it day to day. But If I'm already in a situation to use it, then I don't need any more damage to my car.

It is my understanding that this will eventually destroy the alternator. The safe way is as John Rogers mentions. The main positive cable is broken by the large set of contacts. Separate contacts will shunt the alternator to ground via a load resistor.

The single pole disconnect switches are no better than disconnecting the battery with the engine running (not recommended as it will eventually destroy the alternator).

The multiple pole disconnect switches are the only safe way to wire in a kill switch.

Here is a PDF file with directions:

if you need just an ignition kill, and have an aftermarket cdi box, you can use the power lead to the BOX to kill

I figured it out. Kill the power input into the coil. BAM! Dead. And there's a wire to do just that at the ignition. And the fuel pump wire too... Perfect.