Kill switch wiring

[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?

[Greg Amy]

What the heck am I missing?[/b]

GCR 17.27: "The master switch shall be installed directly in either battery cable and shall cut all electrical circuits..." (my emphasis) I was describing above with this requirement as an assumption.

You don't have a choice: the battery must be completely isolated from the rest of electrical system. If you did it as you describe, then the remainder of the car's electrical system is still getting juice from the battery.

Remember: it's not JUST about getting the car to shut off; it's about getting the car to shut off AND have no charge to ANYTHING else in the car. Picture running fuel pumps while you're upside down, or battery cables chafing against smashed sheet metal with dripping fuel, or sparking instrument cluster wires 2 feet from your fasce, and so forth.

You will have to remove those two wires from the battery, attach them to a common block, run a cable from that block to the kill switch (or run the two wires on ring terminals to one side of the switch), then from there to the battery. Alternately, you can bisect the battery ground wire with the kill switch. THEN, you run either the ignition or alternator field circuit through the smaller terminals, and now you're golden.

[JoshS]

GCR 17.27: "The master switch shall be installed directly in either battery cable and shall cut all electrical circuits..." (my emphasis) I was describing above with this requirement as an assumption.
[/b]

Good point, okay, now we're done. What that also means is that the other thread from this forum that I referred to earlier was giving bad advice, as it recommended connecting the alternator directly to the battery.

So thanks. Sorry that it took us so long to get there.

[Z3_GoCar]

GCR 17.27: "The master switch shall be installed directly in either battery cable and shall cut all electrical circuits..." (my emphasis) I was describing above with this requirement as an assumption.

You don't have a choice: the battery must be completely isolated from the rest of electrical system. If you did it as you describe, then the remainder of the car's electrical system is still getting juice from the battery.

....

[/b]

Greg,

Though I'd mention one exception which is electrically actuated fire systems must be hot at all times even when the BCS is off. If it's a pull cable like mine then all dead is good.

James

[raffaelli]

Can the lines which pass through the fire wall be continuous or do they need to be interrupted by one of those bulkhead terminals? I poked around a bit but could not find the answer. Should the line be continuous, what is commonly used to seal the opening?

[Flyinglizard]

You could break the negative side. WFM. MM

[Streetwise guy]

Just to make life a bit more interesting, most Bosch alternators are internally regulated, and will continue to charge unless you disconnect the main power feed, which will have unpleasant effects on the alternator diodes. I have hooked my kill into the trigger circuit on the main efi relay (Dodge calls it the ASD relay), which on my Neon kills alternator field, ignition and injector pulse. The advantage to using a low amperage circuit like a relay trigger wire is there is no chance of burning up the kill switch. The disadvantage, as I discovered when I installed an unknown used kill switch, is that not only does the car quit charging if there is a problem, the injectors and ignition quit too. Post up the wiring diagram of your cars injection relay, and I'll show ya which wire to cut.

[keilysmith]

It can be as easy as simply adding a little child and a transient (normally open) switch. Look for the sign "hot" exhaust - is a cable that goes from the center of the leg volume control, and ends at the end "tab on the outlet. Now, add a cable, for example, during the volume control, and welding of this cable to switch (either legs up). Then another wire bond for the remaining leg (the two who want to use) the switch and solder the other end to ground.

[tom91ita]

try looking at how i wired my 86ish honda crx.

http://www.flickr.com/photos/tom91it...7603191663037/

basically, i tried to show on the car's wiring diagram and the switch wiring diagram how they relate to each other.

so connection "A" or "D" on the switch is shown as "A" or "D" on the wiring diagram.

hope this helps. btw, i have done the annual tests as well race weekend tests and never hurt the alternator. (weekend tests are wondering why the car won't start when i arrive at the track and then remembering i keep the master switch off in the trailer......)

[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

[Z3_GoCar]

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
[/b]

Josh,

You'll want to wire the two pole switch to the two wires that run from the battery to the engine compartment on the passenger side. There should be an ignition relay in your circuit, I don't have the Bently manual in front of me right now, but I do remember it being there, it's in the immobilizer part of the curcuit. Anyway, the problem is that the switch just removes the 12v. it really needs to ground out the alternator and computer, to make the circuit dead, this is what the relay does ground the curcit when it's off, applies 12v. when it's on. This is where the alternator feedback can keep the motor running with the battery disconnected when there's no path to drop the voltage in the field wire. I wired the Battery Cutoff Switch (BCS) in series with a toggle switch on the small wire, this energises the relay and ungrounds the alternator field, computer key switch wire, and fuel pump wire. This way the BCS and the toggle both kill the motor, but the BCS removes all electricity from going into the engine bay. I can tell you the wire numbers you want to look for and the colors if you like, but the BCS should be along the main battery wire running along the passenger side. Good luck,

James