1 Topic by estomax (edited by estomax 2023-08-21 06:18 AM)

  • Registered: 2023-04-06
  • Posts: 7

Topic: Kill switch question with a bmw

Hey Guys,

Getting to the final stretch of Lemons prep with our E90, and had a kill switch question that I couldn't find a good answer for with search. The BMW has three power cables running to the front of the car from the battery - 1) starter/alternator, 2) engine electronics, 3) car electronics. A 6 pole kill switch would help me switch off two of them, and I was wondering if just killing the ECU and starter/alternator would be acceptable or if I need to find an 8 pole kill switch from somewhere to also kill the car electronics? I suppose I could also tie the engine electronics and car electronics lines to a single ECU rated fused line as the car wont need much electronics power anymore.

edit: one important detail is that circuit #2 and #3 have fusible links at the battery, so in case of a crash and short those will die safely on their own, which maybe helps justify leaving #3 unswitched.

A quick diagram of how the power is run from the battery to the front:
https://i.imgur.com/9nxXUhM.png

I am hoping to use the standard 6 pole kill switch sold here:
https://store.24hoursoflemons.com/produ … 4864832666

Thanks for your thoughts!
Marko

PS. See you all at the ridge!!!

  • From: Camarillo, CA
  • Registered: 2021-03-26
  • Posts: 386

Re: Kill switch question with a bmw

The 6 pole if I'm not mistaken is basically a 4 pole but has the added feature of dumping stray current into ground through a big resistor.

Pole 1) "Main/big poles" (the ones w/ nuts) should be a disconnect between the battery and ALL powered bits. In other words, when the wire that goes into pole 1 is opened, nothing can be powered... gauges, a dome light, the ECU, absolutely everything must be completed isolated from power.
Pole 2) "kill pole" (the one with spade connectors that opens when pole 1 opens) should open a circuit that would turn the vehicle off... engine fuse or ignition switch is what I typically do. But it basically needs to be a circuit that will kill the car when opened. This is not intended to be a power disconnect like Pole 1... it's a kill circuit
pole 3) "stray current protection" (The one with spade connects that shorts when pole 1 + 2 are open). This is wired just as shown in the diagram of the Lemons store page. It just gives area for stray current to go that won't let the magic smoke our of your ECU and other magic boxes.

I hope this clarifies it a bit. The additional poles are for additional features, not for additional circuits. A 2 pole has the function of the disconnecting the battery, 4 pole disconnecting the battery and killing the car, 6 pole disconnecting the battery/killing the car/stray current protection. Each pole has an independent function.

The primary pole (pole 1) must disconnect all power from anything and everything. So your best bet would be to relocate that box on top of the battery to a safe isolated location (similar to a sub panel in a house) and use pole 1 to interrupt the jumper cable between your battery and this box. You can also re-engineer this box as you mentioned but if I've learned anything with german cars, if the electronics are working, do the least amount of change possible.

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  • From: Tustin, CA
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Re: Kill switch question with a bmw

The way that I (and John Pagel) wires their cars is simple, Battery and alternator output on one pole and everything else on the other.  So in your diagram (1) is on one side of the pole and (2)(3) is on the other.  I suppose you could have a 27 post switch to handle everything but I don't see why you need that. 

Before the peanut gallery chimes in about how wrong that is tell it to the head of tech John Pagel, not me.

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  • chaase
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  • From: East Meadow, NY
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Re: Kill switch question with a bmw

cheseroo wrote:

The way that I (and John Pagel) wires their cars is simple, Battery and alternator output on one pole and everything else on the other.  So in your diagram (1) is on one side of the pole and (2)(3) is on the other.  I suppose you could have a 27 post switch to handle everything but I don't see why you need that. 

Before the peanut gallery chimes in about how wrong that is tell it to the head of tech John Pagel, not me.

We always use 4 pole switches but the same basic rule applies. Everything must be killed by the kill switch.

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5 Reply by estomax (edited by estomax 2023-08-22 11:45 AM)

  • Registered: 2023-04-06
  • Posts: 7

Re: Kill switch question with a bmw

Thanks for the input. I am onboard with how the 6 pole switch works, and it is a great idea to just move the whole power distribution box in the car forward too, after the switched circuit.

I am really liking the idea of combining 2/3 onto the 'ignition coil' part of the cutoff switch and then running the main circuit through the stray current protection set of poles, but the big gotcha there is that I don't know how many amps the secondary ignition coil part of the relay is rated for. The little contacts at the bottom are certainly not rated the same 100 amp continuous/500 amp peak that the main switch is. And an ignition coil is not drawing much current either. Maybe i could switch the ECU through that, but anything more i would say is questionable.

long story short, i think moving the power distribution box forward and cutting all power to the positive terminal is likely the easiest route. The other option being the remote mount isolator that disconnects the ground terminal on the battery remotely, from searching on the forums it seems the judges don't like that much? This thing - https://www.cartekmotorsport.com/battery-isolators/

  • Zacks
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  • Registered: 2022-10-17
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Re: Kill switch question with a bmw

Since our alternator meets the other power wires at the battery we basically have alt and battery on one side and everything else on the other side, so only a single pole switch.  But it's not bmw fancy. 

I imagine if you are worried about a power surge during kill operation smoking ecm ecr you could isolate the power for those computers (pre fuse) and run just those circuits through the low amp poles on the kill switch.  Sounds like a bit of work though.