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Emergency battery cutoff & alternator protection

I am wondering the correct way to wire in an emergency battery cutoff switch, or even multiple switches (One on dash, one on outside of car) in reference to switches with alternator protection. The car in question is a 944 series car. I have seen these E-switches with "alternator protection" circuits, which add an alternate set of contacts that disconnects the alternator from the circuit to avoid damage. My question is, is this actually needed? I swear I have seen cars with emergency switches with just the two posts wired to the battery circuit, nothing else. Are these alternator protection switches mainly meant for newer cars with mroe complex charging systems?

Any direction would be appreciated.

Also, anyone have pictures of where they have mounted their cutoff switches inside the car? I obviously would need to be able to reach it from my seat while strapped in, which limits it's location. I have seen cable actuated remote switches but have a distrust/dislike of them for some reason.

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Old 11-18-2008, 07:08 PM
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bump....?
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Old 11-23-2008, 02:54 AM
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I am by no means an expert on this. I can only tell you how not to do it, since we have not yet installed our replacement kill switch CORRECTLY

We decided to install a kill switch on our LeMons BMW and did exactly what you said- spliced it inline on the battery lead. It doesn't do what you need it to do. What you've accomplished by doing this is simply a way to quickly disconnect the battery but not kill the car. If the car is running and you turn off a switch wired this way? The car will keep running on alternator power!

You need to wire it into the alternator, ignition coil, or fuel pump to kill the car. And you don't want to use a two pole switch to wire both the battery and coil/pump/alt. through it because you'll have 12V constant wired to the coil/pump/alt and then be overriding the keyed ignition switch. That's why they make the 4+ pole switches.

And it's location is typically required by racing organization rules to be OUTSIDE the car in a prominent location with a lightning bolt decal to call it out, so TRACK SAFETY PERSONNEL can kill the power to your car following an accident. The goal here is not for it to necessarily be in a convenient location for the driver.
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Last edited by KTL; 11-23-2008 at 06:31 AM..
Old 11-23-2008, 06:25 AM
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Two reasons not to use the basic 2-pole. As Kevin said, overrun from the alternator can keep the car and other electrical circuits running. The 6-pole (?) ones provide a large resistor to bleed off alternator generated power when shutting down. This not only shuts the car off but prevents damage to the alternator if the power has 'no place to go'.
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Old 11-23-2008, 08:32 AM
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I put a write up on doing this at http://944spec.org/944SPEC/content/view/77/25/

You don't need the alt bleed off resistor if you are willing to risk a surge taking out your DME. Your choice. You do need to break the coil circuit (or similar) to kill the engine and electrics when the switch is activated
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Last edited by jerryw; 11-23-2008 at 03:30 PM.. Reason: spilling
Old 11-23-2008, 03:30 PM
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I read Jerry's writeup quickly, but I think that is the same way I wired my switch. Worked flawlessly all season. Bought the wires/cables from Auto Zone, and the switch itself from Ogracing.com.

I made this diagram of the wiring, hope it helps!
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Old 12-03-2008, 07:05 PM
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Yes! that is a big help. I'm all for getting this correct- I was in the paddock during a scare once when some spec racer ford that decided, for no reason whatsoever, to arc some wires and try to burn itself to the ground because of an electrical fire. The car did not have a properly wired cutoff switch and the only way to cutoff the battery was to take part of the bodywork off and pull the connector from the battery. Everyone jumped in and got it off in time but it could have been worse if someone hadn't noticed the smoke.
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Last edited by Schumi; 12-04-2008 at 12:25 AM..
Old 12-04-2008, 12:16 AM
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Porsche alternators typically don't require the resistor, but it cannot hurt. I have seen many race cars in our area use two pole switches, I opted for a six pole. One key thing is to triple check all of your crimps and your routing (to ensure there can be no chaffing of insulation on hot wires).
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Old 12-04-2008, 10:27 AM
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Quote:
Originally Posted by #39 View Post
I read Jerry's writeup quickly, but I think that is the same way I wired my switch. Worked flawlessly all season. Bought the wires/cables from Auto Zone, and the switch itself from Ogracing.com.

I made this diagram of the wiring, hope it helps!
Yup -- that's how I did it.
If I can grab that diagram I'll add it to the write up if you don't mind.
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Old 12-04-2008, 12:52 PM
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Quote:
Originally Posted by jerryw View Post
Yup -- that's how I did it.
If I can grab that diagram I'll add it to the write up if you don't mind.
I don't mind at all.
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Old 12-04-2008, 07:03 PM
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Quote:
Originally Posted by #39 View Post
I don't mind at all.

Great -- thanks
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Old 12-04-2008, 07:40 PM
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where on earth is best to mount a cut off switch, my rules say on the cowl, there is no cowl on the 944, the fender doesnt seem like a good place
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Old 05-21-2009, 12:13 PM
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On the 944, we have made a bracket to mount it adjacent to the battery, as low as possible below the hood line. then we attach a pull cable that is long enough to be exposed on the base of the windshield.
Remember to use proper insulation on all connectors so you don't start a new welding project!
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Old 05-21-2009, 01:08 PM
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Dave's spot on. You can go down to your local auto-parts store and buy a "manual choke" cable to use as a pull from a remote location. I had my switch on the outside with a cable that also leads inside in case I need to pull it while belted in.
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Old 05-22-2009, 07:19 AM
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Just put in a kill switch on my 911. Porsche makes a 6 pole that was installed in the 964 Cup Cars... about $90.00. My shop wired it so it kills the DME relay and the fuel pump, along with disconnecting the battery.
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Old 05-22-2009, 07:40 AM
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Is it necessary to wire in the coil to the cutoff switch? Is it acceptable to leave the two terminals for the coil in the diagram below blank? It seems to me the cutoff would still function properly as the main power supply would be cut AND the resistor would bleed off the spike?
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Last edited by JohnJL; 09-23-2009 at 04:58 PM..
Old 09-23-2009, 04:53 PM
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Like this?

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Old 09-23-2009, 05:34 PM
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The coil part of that circuit doesn't have to be used on the coil. In fact it is usually used on the ignition box (switched lead of the MSD or other CD box). It can also be used on the fuel pump relay or DME relay. It can be connected to anything that will make the motor stop running. Without it the engine will keep running. In the example you show the engine will keep running using the alternator power to run the fuel and ignition systems. The resistor to ground will just get hot since the alternator will be powering it as well. I suppose if you made the resistor such a low resistance that it was a short to ground the alternator wouldn't be able to maintain enough voltage to keep the engine running but the resistor would sure get hot while the engine was spinning down and I would think the overload on the wires and alternator would be harmful.

-Andy
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Old 09-23-2009, 06:56 PM
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Thanks Andy. It would be more convenient physically to cutoff the fuel pump but I dont think that would work well...this car has Webers and it would seem to me the car would continue to run for at least a few seconds off the fuel left in the bowls?

Instead of the coil wire, which I would think would be particularly sensitive to any voltage drop experienced across a 15 foot run of cable from the engine to the front and back again, I'm thinking maybe of running the MSD "on" wire through the kill switch. I run a JBRacing twin-plug dizzy with two MSD boxes. The MSD 6A boxes have a "light" red wire that gets switched power and a "heavy" red wire for its mains power. In this case maybe I'll use the kill switch to interrupt the MSD light red wire?
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Old 09-23-2009, 08:27 PM
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Does anyone have a REALLLY good write up for this? One that the home mechanic can follow?

Old 09-25-2009, 12:08 PM
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