Pegasus Kill Switch

Hey gang, I've been stumped a bit by what is probably an easier question than I am making it out to be. I'm trying to install the Pegasus Master Battery Cutoff Switch with Alternator Protection (or the Pegasus Master Battery Cutoff Switch with Alternator Protection for short). This unit provides a resistive path to ground for the alternator when you switch it off, and it also provides, for reasons that I'm not really sure of, a way to cut the ignition system signal, too.

My question is really about that last part--the ignition interrupt. There are two spade connectors on the base of the switch that basically act to break the connection in a wire that is meant to go from the ignition switch to a coil or an ECU. Because I'm killing power to the ECU anyway, by running the chassis electric cable to the main post on the switch, doesn't this seem unnecessary? This is a modern car (2003 Audi), so there is no single ignition coil. I probably have to use them anyway--so what's a good wire to tap into? I'm sure many of you have installed these switches before, so I'm curious what you tapped into.

If the engine is running, you aren't killing power to the computer because the alternator will still be working.

Run the ignition switch through the kill switch, this shuts everything off when you disconnect the battery.

In my car, the alternator and starter seem to be connected to the battery on one cable, while a second cable provides power to the rest of the car. In testing, I unplugged the "body-electrical" cable, and the car died immediately. So, it seems redundant to kill power from a now-dead ignition switch to a relay that's also dead. Regardless, I'm having a hard time identifying which of the 30 or so cables that come out of the column are the right ones, or where to cut them.

It seems to me that older cars have a single wire coming off of the positive terminal of the battery, and that this cable leads to the starter. Then, from the starter, power would be distributed to the rest of the car, including to the ignition switch. Sometimes, it's hard for me to put the whole electrical system into my mind's eye, so I am obviously missing something. Or am I? Does somebody see where I'm going wrong in my way of thinking about this?

I have used these switches in several cars.  On modern cars I use those terminals to cut supply to something important, like the main relay on a Honda, or ECU power or fuel injector common power or something like that.  While a single pole switch might well work, I like the added security of these to be sure the alternator doesn't keep things going. 

Good idea, Sonic. The engine can't run if any of those systems are dead. It would be really easy to tap into something in the ECU. I can't seem to find any references for this car to tell me which relays do which things, but I do have a diagram for the ECU lying around somewhere. I'll find a 12V power source there, maybe, and see about using that.

Pegasus Kill Switch would be a cool band name

Huh. It totally would be! 

I've gone so far as to pull potential fuses one by one with the car running to see which one killed it, then run that wire to the kill switch. 

I mean, the fuel pump relay wire would also be a no brainer.

The thing with having multiple battery positive cables is that they must go though the kill switch.  The idea is not just to kill the engine but also to ensure that there are no ways for a positive cable to short to ground.  That's why the easy method of running the alternator output to the battery side of the switch is not acceptable anymore - that wire will still be "hot" no matter the position of the switch.

The expectation is that if you NEED to hit the switch, the car is now a mis-shapen lump of freshly balled up metal and you need to prevent a possible fire before you try to extricate yourself from it.

This is great advice. The idea that the wire can be something else, not just "the coil", is going to make this easier. Killing the device that may be actively feeding a fire that I am simultaneously trying to put out seems smart. Plus, I think I know where the fuel pump relay is. Thanks gang!

Sometimes, going home and going to sleep helps you see things the next day from a different perspective. It just occurred to me that installing the switch correctly will cause a bridging of the alternator to the ECU (they'll be on the same lug on the switch now). So now the car really will run on the alternator alone if I don't hook up everything. This search for a killable circuit isn't redundant--the installation of this switch makes it necessary.

I use one because it insures there is no hot lead in the engine bay when the switch is turned off. I also have(well, now had) an engine that runs without a battery