Kill switch wiring

[Greg Amy]

Don't overthink it; it's all about safety.

Think of it this way: what if you were upside down in a Miata and the damn thing caught on fire, still running thus the alternator still juicing, and it was porking out fuel from a ruptured fuel line; what would YOU think about "just a few seconds" of additional chassis electricity after hitting the kill switch?

After all, what's "just a few seconds" of ruptured fuel line between friends?

That's the kind of stuff I think of when I'm tech'ing a car. If I tested your kill switch and it continued to run "just a few seconds" after I hit the switch, I'm dingin' ya, plain and simple. You can argue all the technicalities you want - and you'd be right, from a purely technical standpoint - but when you hit the kill switch the engine should stop, right then and there, with no power to anything at all, no delay, no dwell time, and certainly not within a few seconds.

And that's really the way it should be.

And when it comes down to it, how is splicing into the alternator field circuit wire (or the ignition circuit wire) any more difficult than splicing into the fuel pump wire? Just do it "correctly" and everyone's happy, yes? - GA

[Flyinglizard]

I wire all of the new builds to cut the fuel pump power. The modern car has so many relays, field wires, diodes, etc.
The only sure way to stop the fuel fire( and that is the reason for the power switch),is to drop the power to the fuel pump.
Remember Greg, AT idle the stop may be 2sec, at RPM the stop is right now.
Never had any issues at tech. Actually they like my cars, cuz they shut off.
MM
pS the VW does not need a field wire.. Most ,if not all, will self field @ about 3500rpm, engine speed.

[Z3_GoCar]

As long as the alternator self-energises any thing, all circuits are not dead. I ground out the ecu key wire, the alternator field wire, and the fuel pump wire, with a immobilizer relay. Imagine that, I use a relay that's already in the wiring, and on the car.

The rule flexibility is because not every car has a coil wire, or it may be a fuel cut-off valve on a carburetor, or in my case it's the ecu wire. Some cars didn't come from the factory with an immobilizer relay, but they're easy to get from your local alarm/sterio shop.

[Matt93SE]

Quick and dirty method:
wire the alternator output to the battery side of the cutoff switch. no need to fiddle with field wires, ECUs, fuel pump relays, nothing.

when you flip the switch, there is ZERO power anywhere under the hood. Zip. Zilch. Nada. No fuel pump, no ignition, no ECU. Absolutely foolproof and there's no freewheeling alternator feeding back into the circuit to fry the ECU.

Even if the alternator is self-fielding, all it's doing is charging the battery that is already disconnected. Took me about 6' of #8 wire and 30 min to accomplish the job.

[JoshS]

Quick and dirty method:
wire the alternator output to the battery side of the cutoff switch. no need to fiddle with field wires, ECUs, fuel pump relays, nothing.

when you flip the switch, there is ZERO power anywhere under the hood. Zip. Zilch. Nada.

Uhh ... except for that big wire going to the alternator ... Doesn't cut "all circuits."

[Flyinglizard]

That is the only way to do it with a 2 post switch. It is the common way, not the ideal way but it is done.

[Matt93SE]

Uhh ... except for that big wire going to the alternator ... Doesn't cut "all circuits."

since the field coils on the alternator are cut, it effectively cuts "all circuits". All sources of power are on one side of the switch, all users of power are on the other. You flip the switch and the car stops. right now. What else do you want?

[JoshS]

since the field coils on the alternator are cut, it effectively cuts "all circuits". All sources of power are on one side of the switch, all users of power are on the other. You flip the switch and the car stops. right now. What else do you want?

Maybe that's good enough, and as I think about it, maybe I'm colored by the fact that my car has its battery in the back. I was trying to avoid having any electricity in the engine compartment. But I suppose if the battery is in the engine compartment, that's not a realistic goal.

But it says "all circuits" and as long as there's a circuit from the battery to the alternator, to the engine ground, back to the battery, it's hard to argue that it meets the letter of the rule.