Kill switch wiring

[JoshS]

My confusion about the Pegasus diagram is that there is no simple wire from the ignition switch to the coil in my car. My car has all sorts of fancy electronics, six ignition coils, an electronic anti-theft immobilizer, a multi-pole ignition switch with several wires on the output side, etc.

It's just not obvious what wire to disconnect to shut the car off. I was hoping that the simpler thing would in fact kill the engine, based on, among other things, this thread.

[JimLill]

My confusion about the Pegasus diagram is that there is no simple wire from the ignition switch to the coil in my car. [/b]

Locate the group of wires that connect to the "run" position on your ignition switch, route that group through the kill switch "ignition" path.

[JoshS]

Locate the group of wires that connect to the "run" position on your ignition switch, route that group through the kill switch "ignition" path.
[/b]

I'm sure I could find the right thing. There is a wire at the coil packs that feeds all six, I could probably pick it up there instead of at the switch. At the switch, there is a line that's hot in run, and a line that's hot in both run & start. Seems like to kill the ignition it would be the second one. Or I could get them at the coils.

Between the switch and the coils is so much electronics that you really can't tell what's going on.

But ... what I really want to know is, will the first approach work? It seems like it. The alternator output side (the B+ terminal) would be connected directly to the battery, even when the switch is off, so it would have somewhere to drain. However, with the switch off, there would be no contact between the battery/alternator and the ignition. Isn't this simple approach the best approach?

If you isolate the battery with the main circuit, and run the alternator field circuit through the normally-open circuit with the resistor, you'll be fine.
[/b]

Forgive me Greg ... I don't understand a word you just said! I don't know what a field circuit is, and I don't really understand what you mean by "main circuit" either.

Can you draw me a picture?

I understand the principle of protecting things with the resistor ... I just don't understand why the first approach doesn't do that.

[Greg Amy]

I don't really understand what you mean by "main circuit"...[/b]

The main circuit from the battery to the car, just as in the drawings above.

I don't know what a field circuit is...[/b]

VERY basically put, alternators work on the concept of comparing a circuit in the car to what voltage it's putting out, always trying to keep them equal to its setting. In the case of a 12V car, that's about 13.5 volts. If that secondary circuit, the field circuit, gets low on voltage the alternator puts out more; if it gets high, less.

Therefore, with an alternator, if you cut off the battery only then as the field circuit voltage drops the alternator tries to out out more and more voltage. Eventually the field circuit voltage will drops too low and the alternator goes offline, but it's not instantaneous, and in the interim the alternator continues trying to charge pu the system to ~13.5V.

However, if you open this field circuit with a switch or relay then the alternator sees zero voltage and drops offline.

So, you can wire the car one of two ways: wire the kill switch's smaller secondary contacts such that it disconnects the field circuit and the alternator goes offline, system voltage drops to zero, and the car quits running due to lack of electrical power; or, you can wire the ignition through those secondary contacts so the car quits running and the alternaotr stops producing power due to lack of rotation.

Either way works fine, simply choose the method that is most convenient for your car and kill switch location. - GA

[JoshS]

VERY basically put, alternators work on the concept of comparing a circuit in the car to what voltage it's putting out, always trying to keep them equal to its setting. In the case of a 12V car, that's about 13.5 volts. If that secondary circuit, the field circuit, gets low on voltage the alternator puts out more; if it gets high, less.

Therefore, with an alternator, if you cut off the battery only then as the field circuit voltage drops the alternator tries to out out more and more voltage. Eventually the field circuit voltage will drops too low and the alternator goes offline, but it's not instantaneous, and in the interim the alternator continues trying to charge pu the system to ~13.5V.
[/b]

Thanks! I get it. The point of the secondary kill circuit is to reduce the amount of time it takes to get the alternator fully off-line.

Now, just out of curiosity, what harm is caused during this duration of time if the alternator main circuit is still connected to the battery, as my original question asked?

[Greg Amy]

...what harm is caused during this duration of time if the alternator main circuit is still connected to the battery, as my original question asked?[/b]

The "generally accepted" problem is one of damage to the alternator; alternators REALLY WANT to have some kind of accumulator to dampen the positive and negative spikes it produces, and that's the battery's job. You disconnect that battery from the equation and you run the risk of alternator damage or possibly runaway.

"Runaway"? From a safety aspect, theoretically an alternator-equipped car can continue to run with the battery disconnected. The car will run as long as the electrical system has voltage; the alternator will continue to put out voltage as long as the field circuit is "excited"; the field circuit will remain "excited" as long as the alternator is putting out voltage. See the Catch-22 there?

I've never tried shutting of my car with the kill switch without the alternator/ignition properly wired, so I can't say that's what will happen for sure. But I certainly don't want to try it on my car. Maybe this is a good one for Mythbusters?

The goal is to get the car to stop running, with all electrical equipment de-energized. You'll do that either by stopping the alternator from turning (by shutting off the ignition) or getting the alternator to stop putting out electricity by opening the field circuit (thus killing the ignition due to lack of electricity). Either way works.

I can assure you that, upon presenting your car to an SCCA tech inspector for its logbook, one of the very first things the inspector will do is ask you to start the car then he/she will reach over and flip off the master kill switch to see if the car stops. If you're comfortable having someone do this using just the battery circuit, and it works, then you're all set... - GA

[JoshS]

I can assure you that, upon presenting your car to an SCCA tech inspector for its logbook, one of the very first things the inspector will do is ask you to start the car then he/she will reach over and flip off the master kill switch to see if the car stops. If you're comfortable having someone do this using just the battery circuit, and it works, then you're all set... - GA
[/b]

I understand that. But somehow we're not understanding each other. It's probably me because I didn't stay at a Holiday Inn Express last night.

Based on the diagram in teh first post, the car is wired such that the positive connection of the battery has two leads, one which connects to the alternator (let's call it "wire 1") and one which connects to the ignition system and the rest of the car's electrics ("wire 2").

If the car is running, and the alternator is producing output, that output will be going straight to the battery on wire 1, and then from the battery to the ignition system on wire 2.

If I put a switch in the middle of wire 2, then when that circuit is broken, there will be no voltage on the field circuit, either from the battery or from the alternator, because both the alternator and the battery are upstream of the switch. Further, the alternator isn't left hanging with nothing to absorb its remaining energy, because it can still reach the battery on wire 1. Seems to solve all of the problems. And yet I keep reading between the lines of what you are saying, in which you are implying that either this will not stop the engine, or maybe you're saying that it will stop the engine but will harm the car's alternator or other systems.

What the heck am I missing?

[DavidM]

"Runaway"? From a safety aspect, theoretically an alternator-equipped car can continue to run with the battery disconnected. The car will run as long as the electrical system has voltage; the alternator will continue to put out voltage as long as the field circuit is "excited"; the field circuit will remain "excited" as long as the alternator is putting out voltage. See the Catch-22 there?

I've never tried shutting of my car with the kill switch without the alternator/ignition properly wired, so I can't say that's what will happen for sure. But I certainly don't want to try it on my car. Maybe this is a good one for Mythbusters?
[/b]

The car keeps running. My car had a single pole switch on it when I got it that just disconnected the battery from the car. I didn't know any better at that point. I go to my first ever annual tech and the first thing the inspector does is throw the kill switch. The car keeps on running. My comment was "It's not supposed to do that is it." I have no idea how Bob passed annual. I replaced it with a three pole switch that disconnects the computer signal to the ignition coil. The car now stops. Moral of the story: You need something other than just disconnecting the battery from the car.

David

[JimLill]

My confusion about the Pegasus diagram is that there is no simple wire from the ignition switch to the coil in my car. [/b]

Locate the group of wires that connect to the "run" position on your ignition switch, route that group through the kill switch "ignition" path.

2nd post by accident...... how to delete??

[JoshS]

Locate the group of wires that connect to the "run" position on your ignition switch, route that group through the kill switch "ignition" path.
[/b]

Yeah, there is no "run" terminal of the switch. There is one terminal hot in acc/run/start, and another that's hot in run/start. (There are others too, but those are the ones used that are hot in run). The point is that there are two of them, and the switches on the market have only one.

I still don't get why the simpler approach won't work or could cause any harm.

[raffaelli]

Hey fellas. Can you check me on my wiring before I apply power? The sketch is attached.

[chuck baader]

Quick and dirty way....red battery wire to switch. Jumper from red wire to one side of aux switch. Other side of main switch to starter, etc. Other side of aux switch to wire going to feed fuse box. Done. Start and stop car with key...charge battery...kill everything with kill switch. Chuck

[Greg Amy]

...kill everything with kill switch.

And, possibly, your alternator diodes and some electronic components as well due to the surge...that's what the resistor bleed circuit is for: to slowly bleed down all potential energy (voltage), rather than simply shutting the door in its face...

All that voltage gotta go somewhere, and if the battery isn't there to absorb the shock (its secondary purpose is to dampen voltage spikes and act as an electrical accumulator) then something will... - GA

[chuck baader]

Greg, not to be argumentative, but I have wired 5 e30 BMWs that way with no issues. The only problems with the BMWs is when you remove the cluster..you must add a bocking diode to the circuit to replace the one removed from the alternator light. Chuck

[cjb25hs]

What size wire do I need to use?

[Tristan Smith]

Ok, I am finally wiring up the 300zx. It's been 10 years since I wired up the 240sx, and I am embarrassed to say I have forgotten what I did exactly.

So I get the battery part. The secondary circuit will cut the alt. field wire where it circles back to the alternator, to disconnect the alt. circuit.

The only problems with the BMWs is when you remove the cluster..you must add a bocking diode to the circuit to replace the one removed from the alternator light. Chuck

Chuck, I will not have the cluster in the dash, so I would need to add a bocking (sp?) diode in addition to the Bulb Check Relay, which would just be grounded to slowly bleed off any surge or spike at the alternator. Correct?

Greg Amy, jump in on this.

Thanks.

[Greg Amy]

Tristan, I'm not familiar with "the BMW problem" and its systems, so I really can't speak to that. However, I consider that circuit with resistor for slowly bleeding electrical potential to ground pretty important (see first post, second diagram)...