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Kill switch wiring with a relay

Old 08-15-09, 04:55 PM
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Kill switch wiring with a relay

So I opted for the relay kill switch rather than the old standard giant switch. Main reasons were 1) able to kill power closer to the battery so there is only a short live wire when the kill switch is flipped 2) less heavy wire to run all over the car. I also like the switch mounted within reach when belted in (as well as within easy reach for corner workers), why you would put a switch you couldn't reach I don't understand.

During my recent 40-50 hour wiring job on the car (there is not a stock wire or connector left on the car) I wired up the car like the black lines/text below. The kill switch is good in that there is no power to the car unless the switch is flipped, however the kill switch does not kill the car because of how the alternator is wired. The kill switch kills the power supply from the battery but everything continues to run off the alternator power.

The only 2 choices I see now are to run the alternator wire all the way back to the relay on the battery side or add a second relay. Rather than run another 10ga wire all the way from the engine compartment to the back of the car and have a long hot wire even when the kill switch is flipped, what do you think about adding an 80a relay like the red lines/text in the picture and tying it into the existing switch?

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Old 08-15-09, 05:27 PM
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its not very healthy for the alt to be dissconnected form the battery with the engine running,it can also cause the alt to overcharge without a battery to sense,ive always ran a seprate alt wire back to the battery and used a fuse link at the battery for it,so in the event of a crash if the alt wire shorts out it just blows that fuselink,your idea of the second relay should work but you may find its decreases alt life!
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Old 08-15-09, 05:41 PM
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Originally Posted by rotatrix View Post
its not very healthy for the alt to be dissconnected form the battery with the engine running,it can also cause the alt to overcharge without a battery to sense,ive always ran a seprate alt wire back to the battery and used a fuse link at the battery for it,so in the event of a crash if the alt wire shorts out it just blows that fuselink,your idea of the second relay should work but you may find its decreases alt life!
I always shut the car down by killing the fuel pump which is on its own switch, that also depressurizes the lines.

Most inline fuse holders I have seen are only rated at 30a. I have looked for bigger and did not find for the blade type fuses.
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Old 08-15-09, 10:57 PM
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i wouldnt use blade fuses but you should be able buy a holder for the fuse blocks(like the factory ones used in the fc3s under the bonnet)and mount it close to the battery,these can be rated up to atleast 120amp!i dnt know what the regs are over there but the governing body of motorsport here(motorsport nz) requires that the kill switch should stop fuel and spark,so all my cars have just had switches that shut off the main relay which powers the fuel pumps and coils,which is really easy to sort out!!
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Old 08-18-09, 09:33 AM
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The standard 3 pole kill swith works 3 ways. When you put the switch in the off position, the large lugs disconnect EVERYTHING from the positive post. One small lug grounds the alternator via a resistor. This gives the alternator energy somewhere to go without damaging the alternator. The other small lug cuts the ignition path.
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Old 08-18-09, 09:49 AM
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For anyone interested this is the 200A relay I use: http://www.alliedelec.com/Search/Pro...C=&SEARCH=&MPN

And this is the 100a relay I purchased to kill the alternator power http://www.alliedelec.com/Search/Pro...07&MPN=120-901
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Old 08-20-09, 07:31 PM
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SCCAITS - I'm confused, if your switch kills the fuel pump then the car should die when fuel runs out right? Of course it might take a few seconds to empty the lines too, but still, as long as the alternator is disconnected from the battery when the switch is tripped, you should be fine. Just get the car to die.

My thing would be, how long is it taking the car to starve of fuel after the switch trips. Cutting the ignition kills spark, and the motor dies right now, regardless of hwo much power the alternator puts out.

Rotatrix,

I don't have a lot of experience with fusible links, but don't you have to replace them when they fail? Isn't that kind of a hassle when you throw the switch to test the system (i.e. when the tech guys decide to pull your car for inspection after a race?) And once the link fails, isn't the alternator now powering nothing and the potential for killing the alternator an issue? That's why the 6 pole switch includes a set of poles for a resistor to absorb the power the alternator puts out after the switch is thrown.

Is there any potential for the fusible link to 'not' blow out? Then you have a live power line from the battery to the alternator, and while I don't have a GCR handy, I thought that was a no no.
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Old 08-20-09, 07:37 PM
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Ah, I looked at your chart, so right now you have nothing to kill the motor in the circuit, right? The car will run all day long on the alternator cause you haven't actually killed the ignition or fuel so the motor will stop.

Sooooo, do that! Then you've basically replicated a standard 2 pole kill switch.

PaulC
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Old 08-22-09, 10:50 AM
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Originally Posted by Silkworm View Post
Ah, I looked at your chart, so right now you have nothing to kill the motor in the circuit, right? The car will run all day long on the alternator cause you haven't actually killed the ignition or fuel so the motor will stop.

Sooooo, do that! Then you've basically replicated a standard 2 pole kill switch.

PaulC
Would you splice into the little ignition wire on the starter or the connector on the back of the alternator to help kill the car?
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Old 08-22-09, 02:57 PM
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Originally Posted by mazda6guy View Post
Would you splice into the little ignition wire on the starter or the connector on the back of the alternator to help kill the car?
You have to kill the wire running from the alternator. Cutting the little wire that has a spade connection on the starter would do nothing.
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Old 08-22-09, 07:09 PM
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Neither, I kill the wire going from the ignition circuit providing power to the coils.

No spark, no run. Easy as that.
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