Kill switch wiring

[mossaidis]

Posting for a friend that is prepping his car for SCCA in NARRC ITE..

We're trying to wire the killswitch. He has a late 1.8T VW motor in a 84 Scriocco.

"The problem with the altenator is that it takes it's feed for the exciter wire directly from the battery cable wire from the back of the car. I will asssume I have to cut all power to the main feed cable before the altenator? Not sure how to isolate it completely from the system so the car shuts off when switched."

[Greg Amy]

How "late" of a VW 1.8t?

Most VWs use Bosch alternators with internal field circuitry; ergo not even with the sheisrocket would there be a field wire to bisect. In that case, I'd recommend bisecting the wire from the battery to the starter with the main cut off circuit, then bisecting the main ignition wire through the secondary normally open circuit of the switch ("Z or 2" in the diagram on page one.)

For the 1984 Scirocco, the secondary wire to bisect would be the "15" or "switch on" circuit that supplies power to the rest of the car, a 16-gauge black wire coming off the ignition switch. Cut that wire and the car dies.

GA

[GTIspirit]

I encountered similar situation on my '87 VW GTI 16V when wiring the kill switch. Alternator keeps the engine running quite fine when battery power is cut..... So I plan on interrupting the "T15" to the ignition because that will kill the spark which will in turn kill the fuel pump. Most vehicles should have some kind of fuel pump enable that's controlled by the ignition. So when the engine stops running the fuel pump stops pumping, no need for a separate fuel pump cut. This should also dissipate the charge from the alternator so there is no risk of blowing the regulator with a sudden shutoff of the alternator output.

[dhrmx5]

This is a subject that is a real pain to me. Yet another vaguely written rule in the GCR that is subject to interpretation.

I have always wired my switches to kill battery power and the fuel pump. Cars will run just with just a battery or an alternator but if you take away the fuel I guarantee the engine is going to die. Sure, it takes a few seconds for the engine to shut off and the alternator to lose it's energy but at that point in time ALL circuits are dead and there are no fuel lines pressurized to 60-90 psi just waiting to burn through or crack and cause a flash fire. Fuel pressure is gone because the engine burned it off. Certainly a much safer scenario for the turn worker who is trying to assist you or put out an underhood oil/gas fire.

Why is the rule poorly written? There are no criteria for time, testing, or wiring for most cars. The fact that the GCR is specific about wiring a SRF but not any other vehicle leads me to believe there are options as to how the results are accomplished. The way I wire my car has all circuits dead with the car running after a few seconds and the circuits are all dead with the car not running. Given that the rule simply states that all circuits must be cut would mean that my wiring accomplishes that goal.

[CRallo]

best first post ever?

This is a subject that is a real pain to me. Yet another vaguely written rule in the GCR that is subject to interpretation.

I have always wired my switches to kill battery power and the fuel pump. Cars will run just with just a battery or an alternator but if you take away the fuel I guarantee the engine is going to die. Sure, it takes a few seconds for the engine to shut off and the alternator to lose it's energy but at that point in time ALL circuits are dead and there are no fuel lines pressurized to 60-90 psi just waiting to burn through or crack and cause a flash fire. Fuel pressure is gone because the engine burned it off. Certainly a much safer scenario for the turn worker who is trying to assist you or put out an underhood oil/gas fire.

Why is the rule poorly written? There are no criteria for time, testing, or wiring for most cars. The fact that the GCR is specific about wiring a SRF but not any other vehicle leads me to believe there are options as to how the results are accomplished. The way I wire my car has all circuits dead with the car running after a few seconds and the circuits are all dead with the car not running. Given that the rule simply states that all circuits must be cut would mean that my wiring accomplishes that goal.

[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.