newbie problem, engine kill switch

Hi All I was wondering if somebody out there could offer some help with a problem I have just had arise with a Trial car I have just built. Today at a safety check during a practice day my battery kill switch was turned off whilst my engine was running - this did not kill my engine. I was told I need to fit a relay to stop the feedback via my alternator. This is what I need to ask - how and what? This is my set up. The battery is in the back of the car the Battery isolator switch just opens the positive lead which runs to the starter/car electrics. I need to fit some kind of relay may be with a resistor to stop voltage spike.(this is what I have been told). One problem I have is that the main battery cable is run through the chassis this give me the problem that I can not run the alternator output back to the positive terminal of the battery. Also it is not possible to run wires back to the rear of the car so I can not use a FIA type kill switch that has a built in relay. I will need to fit and wire up a remote relay at the front of the car where all the cars electrics/fusebox etc is positioned. What I need to achieve is that whilst the car is running turning off the battery isolator at the rear will kill the engine and stop the back feed from the alternator. If this is possible and you know how it might be done any help will be very gratefully received. Fingers crossed! Cheers Here is the beast   

Here's a simplified diagram using a relay and a single pole on-off switch. The black wire to relay 85 can be a thin gauge and the switch can be low current. The quenching diode, actually not needed in this method though for safety etc. is a 1N4004. The relay is a standard automotive relay I showed a 4 pin but the more readily available 5 pin can be used, just don't use 87a. The 40* amp fuse is mandatory, you can earth the black to any good local earthing point though I've shown it going back to the battery for convenience. Car parts suppliers, Maplin etc. hold all the parts. The switch must be on/off latched NOT momentary. This will shut down your engine immediately. *Measure the current draw across your ignition circuit. starter_cut.png

Hi Howie Thanks for the quick reply. With this we would have a open / closed isolator switch at the positive battery terminal. By opening this isolator would that kill the engine? In your diagram where you have 'starter' would that be the Alternator? Would you pass the return from the Alternator through a resister to earth?

No it's the starter did I mention alternators? Forget the alternators it's irrelevant. To stop a petrol engine, interrupt the ignition. That's it. Why add complications? Resisters, alternators? Why over complicate things?

I need to clarify the above. What I described above is an engine kill. An isolator should be next to the battery and for safety reasons the NEG (-) side.

What I need to do is to stop the engine whilst it is running by turning off the isolator at the rear of the car. The isolator is fitted on the positive side of the battery. http://www.electricalcarservices.com/FIA-Battery-Isolator-Switch-with-Resistor-ECS-RSK596/p-234 This is what I should have fitted. But now it is very difficult to retro fit this switch. This is how the above should be connected. I need to try to achieve similar by mounting one or two remote relays at the front of the car. Can this be done? My fate is in your hands!

For various reasons, NO wrt using relays - there are various reliability (safety) issues. And from a sire I saw recently, suitable relays cost over (AUD) $800. But check with whatever applicable Regulations if acceptable and what then is required. Additionally, whilst the 3-pole FIA approved isolate & kill switch FINALLY solves issues with combined isolate and kill switches (read my posts elsewhere for the stupidity of using a traditional single pole battery isolator switch to ALSO kill the engine), most wiring diagrams I have seen still use +12V (hot side) isolator which IMO is sheer stupidity unless the isolator is adjacent to the battery only. The hot side +12V interruption is ironic since the 3-pole FIA switch enables the far less risky GND (earth) interruption that is so preferred. Below is a diagram that shows the FIA switch using the normally accepted "SAFE" GND isolation of the battery -ve terminal. Other than its caveat, its notes can be ignored if you are happy with the resistor method of killing alternator output (that's the "step load" change I sometimes refer to in driving without a battery connected).    

PS - I forgot to add that many of the sites that had the +12V interrupting 3-pole FIA switch also referred to oil-pressure control of fuel pumps. IMO that highlights their stupidity & I have covered that issue elsewhere - ie, ignition kill is the engine protection method; to use oil pressure can cause death or cause lean out that can do far more engine damage; the best oil pressure warning is warning lights etc and the driver decides to kill ignition immediately or after completing the corner or overtaking or clearing the railway crossing. Tho some Regs allow up to 6 seconds for engine run on, I reckon 6 seconds of unhindered fuel pumping over the engine bay is somewhat more undesirable than the essentially immediate fuel kill using alternator charge lamps or air flaps or some tachometric relays, though most VAG tachometric (spark) fuel pump relays take no more than 1 second to de-energise after engine stoppage (eg, in the primitive 555 timer types, the same delay as the initial IGN-on prime time).

What this guy needs is when the Tech Inspector turns off the battery cut off at the rear of the car with the engine running, the engine dies, AND the battery is simultaneously disconnected from the car's electrical system. That's all he's after. The systems I've set up for NHRA rules (same requirement as yours, the engine must die) utilize a second set of nominal 20 amp contacts in the battery cutoff switch. These contacts are wired in series with the alternator FIELD, excitation, or 'turn-on' circuit. Depending on the alternator type and model, this may be easy , difficult, or damn near impossible to implement. It depends on the construction and connections of the regulator / alternator system. Howie's circuit will also work, with the addition of a dual-circuit switch, the big contacts for the battery, and the little contacts for the relay. You would only have to run a single small gauge wire forward for the relay control. As far as not being able to run additional wires to the rear of the car, you're going to have to find a way to do so. There's no way to accomplish what you need to do without additional wires, or the second set of contacts to interrupt the alternator field or turn-on. (Or the "howie method" ignition interrupt relay). Running the alternator B+ output directly to the battery B+ terminal, and fitting a "zap-stop" diode to the alternator output terminal would work, BUT the alternator wire will always be hot, and not switched by the cut-off. Not ideal. There's some ideas. I like your rig. Interesting.

I pointed out that it has traditionally not been a "that's all". In my competitive days, GND break systems were knocked back because the idiots thought the isolation has to be on the hot side. I (or others) managed to have the relevant authority add "may be in the GND side" after demonstrating how hazardous their implemented hotside isolation switches were. And that was when the switches only needed to be in the cabin - NOT is a rear corner which was typically the opposite end to the battery. Later came the even stupider single switch combined isolation and engine kill. That require hotside switching with the alternator heavy (B+) still connected to battery hot (+12V) which was not regarded as isolation by many. A diode IMO is not an appropriate solution for various reasons. The FIA-approved isolator & kill switch is the first commercially offered "isolation" switch I have seen that enables BOTH isolation and engine kill in accordance with safety requirements in a reliable and safe manner. My only surprise is that over one third of a century down the track we seem to be back to hotside isolation, hence the GND isolation method offered above. The DPST switch you linked is another (even if it doesn't kill self exited alternators and hence their engines). Multiple FIA-approvesd switches can simply be daisy-chained in series just as the original single-pole isolators could be. (Yes, I have seen hotside +12V dragged from the front-right battery to the rear left corner of the vehicle and then back to the front starter motor!) The exception is the alternator kill resistive circuit but I'd expect it's acceptable to use a relay that connects a (parallel) resistor actuated from the additional 'isolators'.