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Daox 05-08-2009 11:45 AM

Kill switch signal cutting options
I've been contemplating doing a kill switch for a while now. I'm just not sure the route I want to take with it. There are several members who have done kill switches and everyone seems to do it a different way! I'd like to take some time to explain the different ways, their pros and cons, and hash out what we think might be the best way.

Cut ignition signal
Cutting the ignition signal is fairly easy. Its a single wire job, but doesn't necessarily ensure that fuel injection stops as soon as you activate the switch. Also won't work for diesels.

Cut fuel injector signal(s)
Cutting the fuel injector signal is a bit more complicated on most vehicles. On throttle body injection this isn't a great option as you still have fuel in the intake manifold and the engine won't die immediately. On batch fire injection, you have two or more wires to cut into and stop signals. On sequential injection you have at least four wires to cut into and stop signals. However, this approach ensures no fuel is used immediately after the switch is activated.

Cut fuel pump power
This is IMO not an ideal way to kill an engine. You have to wait for the engine to die as the pressure is bled out of the fuel system by the injectors. It is, however, a simple one wire option.

Cut the crankshaft position signal
I think dcb recently posted about using this method. I'm not sure if it cuts fuel and ignition all at once? If so, this would be great one wire solution.

So, add your method and/or discuss existing options so we can get an idea of what is best.

doviatt 05-08-2009 11:51 AM

Perfectly timed post for me. I plan to do this to my Geo this weekend. I picked up a Haynes manual (from public library) last night so that I could have access to the electrical diagrams. I have considered all of the above options but, like you, could use some wisdom from those that have experienced this before us.

some_other_dave 05-08-2009 03:36 PM

Most modern fuel injection systems supply power full time to the injectors. The ECU will supply a ground when it wants the injector to open, and then disconnect the ground to close it. There is usually a single source for the power to the injectors, which then gets split into individual power leads for the injectors. Find that single source, and you can kill all of the injectors by interrupting one wire.

Some cars use resistors to drop the voltage going to the injectors; usually the single wire goes in to that box and is split in there.

That's where I'm planning to hook in my kill swtich on doing to my CRX.


dcb 05-08-2009 03:51 PM

As mentioned, the best place I've found is either the camshaft sensor (if equipped) or the crankshaft sensor. It is low current, doesn't require extra relays, and has the effect of disabling the injection and the spark. Basically this tells the computer that the engine has stopped and it responds by turning off the spark and the squirt.

I've tried about every kill switch scheme imaginable and I like this solution best so far.

ceej 05-08-2009 03:55 PM

If you cut the crank postion sensor circuit, won't you throw a code putting the emission system in open loop operation on restart?


dcb 05-08-2009 03:59 PM

No codes, not in my case anyway. A stopped engine is a normal state of being for a car.

Daox 05-08-2009 04:21 PM

On the engines you've used the cam or crank position sensor cut, does the engine have both sensors? I assume the engine uses one as a check for the other in most cases?

MetroMPG 05-08-2009 04:59 PM

My switch is inline with the coil pickup in the distributor. Is this what you're referring to as "camshaft sensor", dcb?

The engine occasionally stumbles when it's used. That could be dieseling from fuel still coming through the throttle body - I don't know. I've never tried any other method.

My repeated advice - regardless of method chosen - use a momentary switch, not an on/off one. (Safety implications. You would think after hundreds of uses that you'd always remember to turn it back on, wouldn't you? ;))

dcb 05-08-2009 05:03 PM

My saturn didn't have a real camshaft sensor, so I tried the crankshaft sensor and it worked without a hitch.

I tried the camshaft sensor on the metro after trying other schemes. It also worked flawlessly. It is also tbi, as well as using a distributor. Dunno if that matters.

This is empirical evidence. I do not know every nuance of every car, and for all I know some cars might complain if you interrupt the injector or ignition circuit for that matter. I know the saturn would throw a code if you left the FI fuse interrupted for too long, and that the metro would run on if you killed just the fuel.

Edit: yup metrompg, the two wires coming out of the distributor, just interrupt one of them with a nc momentary contact switch (micro). It kills ignition and fuel at the same time and does not throw any codes.

Daox 05-11-2009 11:00 AM

Anyone else on any other options?

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