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Thread: E36 battery disconnect/kill switch

  1. #1
    Join Date
    Jul 2009
    Location
    Bellingham, WA
    Posts
    364
    My Cars
    1998 M3, 2001 530i

    E36 battery disconnect/kill switch

    I'm putting a QuickCar switch in my track car, mounted on a gusset on the roll bar, and it needs to
    1. disconnect the battery
    2. kill the engine under all circumstances

    I found some old threads with folks talking about using a multi pole switch on the positive side to include the alternator, but that doesn't float my boat because it adds a bunch of wiring that can potentially be shorted to the chassis during a big impact (my interior is stripped out behind the seats).

    I want to use the switch to kill the negative battery cable to chassis ground, which leaves the alternator circuit to take care of. The alt is grounded to the engine, so I want to use a big relay that I have laying around to deactivate the alternator field wire.

    Is there a circuit that isn't fed by the alternator that I can use to control the relay? Has anyone gone this route before?

  2. #2
    Join Date
    Jul 2009
    Location
    Bellingham, WA
    Posts
    364
    My Cars
    1998 M3, 2001 530i
    After reading some articles on the subject, I think I need the relay to short the alternator output to ground through a resistor after the kill switch is opened. Going to sleep on it now.

  3. #3
    Join Date
    Jul 2009
    Location
    Bellingham, WA
    Posts
    364
    My Cars
    1998 M3, 2001 530i
    Now I found some more old threads saying that the shunt resistor is unnecessary on BMWs. I couldn't figure out how to shut the alt down with a relay, so looks like I'm getting a different kill switch with a separate post for the alternator.

  4. #4
    Join Date
    Jun 2008
    Location
    Lombard, IL
    Posts
    215
    My Cars
    1999 M3, M235i, X5D
    It's very easy on E36. Need 6 pole kill switch. Intercept large and small battery red 12v+ cables on 4 of the 6 poles. Done.

    Edit: Read your post fully. Where is your battery/wiring now? With factory wiring, I only needed to add few feet of cable to add kill switch.

    You could also run Cartek battery isolator which allows minimal large gauge wire additions and ease of use when implementing multiple shut off switches (ie. exterior).
    Last edited by r0nd3L; 04-04-2019 at 10:36 AM.

  5. #5
    Join Date
    Nov 2006
    Location
    oskaloosa,ia
    Posts
    898
    My Cars
    '01 3.0 ZCoupe
    +1 on the Cartek. Just installed one, and yes, it is expensive, but it makes things really easy, and should be very rugged.

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