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Installing batery disconnect switch

Discussion in 'Technical Q&A' started by jacques, Sep 18, 2009.

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  1. jacques

    jacques Formula Junior

    May 23, 2006
    877
    Los Angeles/Florida
    #1 jacques, Sep 18, 2009
    My 1980 400i needs new battery cables and grounding straps. Where is the best place to install the battery disconnect switch, pos. or neg. ? All help greatly appreciated.
    Thank you very much.Jacques
     
  2. h00kem

    h00kem Guest

    Dec 15, 2004
    734
    Texas
    #2 h00kem, Sep 18, 2009
    Negative side only.
     
  3. fastradio

    fastradio F1 Rookie
    BANNED Professional Ferrari Technician

    Apr 26, 2006
    3,664
    New England
    Full Name:
    David Feinberg
    #3 fastradio, Sep 18, 2009
    Why?
     
  4. PhilB

    PhilB Formula 3
    Owner Silver Subscribed

    Feb 17, 2004
    2,303
    Southern New Jersey Shore
    Full Name:
    Phil
    #4 PhilB, Sep 18, 2009
    You run the risk of completing a circuit if you put it on the positive side. Same logic when replacing the battery - negative cable comes off the battery first, negative cable goes back on the battery last.

    Phil
     
  5. h00kem

    h00kem Guest

    Dec 15, 2004
    734
    Texas
    #5 h00kem, Sep 18, 2009
    Exactly. Thanks Phil. Read the instructions on the disconnect. If it doesn't tell you to use it solely on the negative side throw it away! BTW, same logic when jumping a battery in your daily driver. AND it reinforces why the grounds in the car are so important for the car to work properly.
     
  6. jacques

    jacques Formula Junior

    May 23, 2006
    877
    Los Angeles/Florida
    #6 jacques, Sep 18, 2009
    Neg. side is how I've done it all my life, but I recently read an article stressing to the contrary, I could not find the logic, so I just thought that I'd ask here. Thank you all again. Jacques.
     
  7. technician F

    technician F Karting

    Sep 15, 2009
    57
    #7 technician F, Sep 18, 2009
    Install negative side only and nearest to the battery is the best.The switch install at negative is safty because it has resistance.When it connect series at postive side,poor connection or a poor switch will cause high temperature at the switch.
     
  8. Paul_308

    Paul_308 Formula 3

    Mar 12, 2004
    2,345
    #8 Paul_308, Sep 19, 2009
    Reason for removing the negative terminal from the battery first is that swinging the wrench back and forth will always inadvertently touch metal at ground potential. If you remove the positive terminal first you will nearly always inadvertently cause a short with your wrench.

    Reason for placing the disconnect on the negative side is traditional and rare, there is no logic to support it. By the 30s many cars switched from 6v positive ground back to negative grounded battery after unsubstantiated claims of body corrosion and radiation plugging with positive ground.

    In all other areas of electronics, the + has the favored disconnect device. This stems from the days (20s) when the battery had no negative terminal, the case housing the battery was itself negative and strapping it in formed the connection. Hence the only disconnect was positive.
     

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