Topic: Kill Switch Question

So our car is 99% complete. We are only having problems with the kill switch. I imagine its because Im a lawyer and not a mechanic.

Here's my issue: I interrupted the engine's power from the battery. Hooked up this way, the engine continues to run when I switch it to off (but the car wont start in the off position). What am I missing? is their something telling the alternator to keep the engine going even without battery power?

The car is a 2001 bmw 3-series.

2 (edited by Spank 2021-03-01 01:06 PM)

Re: Kill Switch Question

AustinSTX wrote:

is their [sic] something telling the alternator to keep the engine going even without battery power?

Yes.

The car can get 12v from either the battery or the spinning alternator. As long as the alternator is spinning fast enough to disable the alternator/charge warning light on the circuit, then it is spinning enough to provide 12v power to your car. Sorta like how you can push start a car with a dead battery. There are multiple ways to tackle this. I'll let you either search the site to find it or let those with more intellect explain the methods. I'm not equipped to discuss electrical stuff with any more authority than I just have.

Re: Kill Switch Question

AustinSTX wrote:

...I interrupted the engine's power from the battery. Hooked up this way, the engine continues to run when I switch it to off (but the car wont start in the off position).

What kill switch do you have? I have this one from the Lemons store: https://24hoursoflemons.myshopify.com/c … ill-switch  See the image below for the wiring diagram that comes with the switch.

When the switch is closed:
-The main battery + connection is closed
-'W or 1' is open
-'Z or 2' is closed

When the switch is open, it is all reversed:
-The main battery + connection is open
-'W or 1' is closed
-'Z or 2' is open

The key to your issue is the 'Z or 2' connection (I'll call it Z). You need to find a circuit in your car that, when interrupted, immediately kills the engine when it is running. To find this on my car, I started the car and went to the fuse box and started pulling fuses one at a time until I found one that killed the engine. I then wired that circuit up through the Z connection on the kill switch.

If you can't find a circuit that kills the engine with my fuse pulling method, then you need to break out the wiring diagram for the car and find something there. Some people have suggested that interrupting the fuel pump may be a good option here, but most cars will continue to run for a few moments after pulling the fuel pump circuit, so that might not pass tech.

You also need to wire up the W connection as shown in the diagram so that you don't kill your alternator (so I'm told).

Make sure you tape up all the connections on the back of the switch so that you don't have any exposed battery + connections. This is an instant tech fail.

Also be sure that all power goes through the switch. Meaning that a cable should run from your battery positive to the switch, and nothing else should be connected to battery positive or draw any power from the cable running from the battery to the switch. All power to the car should be connected somewhere on the other side of the switch from the battery.

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4 (edited by cheseroo 2021-03-01 01:45 PM)

Re: Kill Switch Question

The simple way I do it is to route the battery positive and the alternator output (big wire out of alternator) to one side/pole of the switch and everything else on the other side/pole.  You will get 27,000 different explanations as to how to do it but this one works, is simple and is Lemons head of tech approved (and the way he does it).

FWIW, those smaller poles are to dump the alternator output so that it doesn't smoke the alternator when it's output suddenly has nowhere to go.  By putting the alternator output on the same pole as the battery, it's allowed to continue dumping output into the battery until it stops spinning.  And yes your car is still running because the alternator is continuing to feed it's output into the car's system.  That's how you can do a hillbilly battery charge by removing your car's battery while it's running and replacing it with a dead battery that needs charging.  Essentially you've wired it so that switching it off is no different than removing the battery while it's running.

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Gone bye-bye
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Re: Kill Switch Question

This has been discussed ad nauseum in several different threads, for example: https://forums.24hoursoflemons.com/view … p?id=39587

As others have said, you must do more than just disconnect the battery via the main part of the kill switch (large lug terminals). If you choose the setup where the alternator isn't interrupted, then you must use one of the smaller switches (Switch "Z") to interrupt the 12V wire going to the fuel pump/coil/ecu, etc.

If you choose the setup where you tie the battery and alternator on the same side of the main kill switch, then that will be enough since you have eliminated all sources of 12V to the fuel pump/coil/ecu. This also eliminates any potential of damaging the rectifier diodes in the alternator, as the alternator never becomes disconnected from the battery (see the discussion about load dump). All good things. The only downside of this approach is that the 12V from the battery is always present on the alternator wiring. For our car, which has the battery in the trunk and the alternator in the front, that means the always-hot, heavy gauge, 12V wire runs the full length of the car. If that wire is pinched during a crash, there is no way to turn off that short circuit condition. Our alternator hangs  off the nose of the engine, and is located in a low, vulnerable spot. A heavy front-end crash would very likely compromise that alternator output wire. Not a place and time that I want lots of sparks happening.

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Re: Kill Switch Question

RSB wrote:

For our car, which has the battery in the trunk and the alternator in the front, that means the always-hot, heavy gauge, 12V wire runs the full length of the car. If that wire is pinched during a crash, there is no way to turn off that short circuit condition. Our alternator hangs  off the nose of the engine, and is located in a low, vulnerable spot. A heavy front-end crash would very likely compromise that alternator output wire. Not a place and time that I want lots of sparks happening.

In that situation couldn't you put an automotive circuit breaker next to the battery?  Summit has a 150 amp for $25.

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7 (edited by Limonaid 2021-03-02 02:38 PM)

Re: Kill Switch Question

rb92673 wrote:

In that situation couldn't you put an automotive circuit breaker next to the battery?  Summit has a 150 amp for $25.

I've been thinking about that, but have not done it.

We put in a cutoff on the battery positive so that we have a way to completely kill the electrical wire leading to the main kill switch, but I guess if the car is on fire then we will not be able to use it in practice.

We got one like this: https://www.amazon.com/Ampper-Battery-D … 07JYYXJH7/

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Re: Kill Switch Question

rb92673 wrote:

In that situation couldn't you put an automotive circuit breaker next to the battery?  Summit has a 150 amp for $25.

Yes, I use one of these in my builds: https://www.amazon.com/gp/product/B01F76VJ3I/
Easy and cheap protection against pinched wires or misplaced wrenches.