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Thread: Cut off switch.

  1. #1
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    Cut off switch.

    Hello,

    What is the best way to wire a cutoff switch to a Classic Impreza.
    I just want this installed to prevent draining and a a sort of safety measure.
    Last week (after 18 yers) I had to remove the alarm because it was totally knackered.
    No need for a new alarm, my car is now only a weekend (read monthly) toy.

    I read some things about burning ECU's because of peak voltage and resistors to prevent that.

    Also what is a good brand of cutoff switch.
    They go from 5 to 200 and look all the same to me...

    Thanks

    Wouter
    Subaru Impreza Turbo : Ecutek, AST tarmac coilovers, SW Motorsport brakes....

  2. #2
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    depends what you are trying to achieve
    if you want to cut all power, you need to cut the positive or negative side of the battery, like done on race cars

    if you are trying to DIY anti-theft, cut the ground of the fuel pump relay

  3. #3
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    If you want it to cut the engine once it's been started you'll need to cut the ignition, otherwise it will keep running on the alternator, as I found out the first time a scrutineer tested the cut off switch on my race car!

  4. #4
    Senior Member TimH's Avatar
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    If you use anything that cuts the battery positive or negative DO NOT use this to actually stop the engine. If you do there is a good chance that the energy in a running engine will use the ECU as the path of least resistance to dump the energy and, most likely, destroy it.

    If it's just a security measure, then fine. Disconnect the battery *after* you have turned off the engine using the ignition key.

    An MSA-type kill switch is there to ensure that an engine can be stopped and that the battery is isolated. So, in the event of a crash, the marshalls can reduce the likelihood of fire. They have two features over and above the main high-current battery isolate function:

    1) a separate switch contact that's used to feed the engine ignition; this is then and ignition cut, to stop the engine, when the kill switch is used.
    2) a separate switch contact that switches in a 3 ohm, 11W, resistor to ground when the cut switch is activated, to provide a discharge path for the alternator ti avoid damaging it. It also provides a discharge path for energy in coils and injectors, etc....hopefully!

    There is still a risk of ECU damage though so the advice is still for this to be used *just* as an emergency kill switch: use the ignition key to actually stop the engine in normal operation. Please!

    There are also more expensive electronic kill switches, such as the ArmTech "hybrid" type, CarTek isolators, and so forth. These are neat, and give nice push button operation (rather than cable operation) and can also send a kill signal to the ECU. But they still abruptly remove power or ground, and they are a little lacking in information as to what discharge paths they actually provide for energy in the system (other than a discharge path for the alternator to make sure the engine actually stops).

    Advice remains the same - they are emergency kill switches, not day-to-day engine on/off switches!

    As said above, if security is the only issue, a hidden switch to remove power to the fuel pump is an easy solution!
    Last edited by TimH; 25th September 2018 at 19:13.
    aka JT Innovations www.jti.uk.com No scoob any more Outlander PHEV GX4h for us now !!

  5. #5
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    Quote Originally Posted by TimH View Post
    If you use anything that cuts the battery positive or negative DO NOT use this to actually stop the engine. If you do there is a good chance that the energy in a running engine will use the ECU as the path of least resistance to dump the energy and, most likely, destroy it.

    If it's just a security measure, then fine. Disconnect the battery *after* you have turned off the engine using the ignition key.

    An MSA-type kill switch is there to ensure that an engine can be stopped and that the battery is isolated. So, if the event of a crash, the marshalls can reduce the likelihood or fire. They have two features over and above the main high-current battery isolate function:

    1) a separate switch contact that's used to feed the engine ignition; this is then and ignition cut, to stop the engine, when the kill switch is used.
    2) a separate switch contact that switches in a 3 ohm, 11W, resistor to ground when the cut switch is activated, to provide a discharge path for the alternator ti avoid damaging it. It also provides a discharge path for energy in coils and injectors, etc....hopefully!

    There is still a risk of ECU damage though so the advice is still for this to be used *just* as an emergency kill switch: use the ignition key to actually stop the engine in normal operation. Please!

    There are also more expensive electronic kill switches, such as the ArmTech "hybrid" type, CarTek isolators, and so forth. These are neat, and give nice push button operation (rather than cable operation) and can also send a kill signal to the ECU. But they still abruptly remove power or ground, and they are a little lacking in information as to what discharge paths they actually provide for energy in the system (other than a discharge path for the alternator to make sure the engine actually stops).

    Advice remains the same - they are emergency kill switches, not day-to-day engine on/off switches!

    As said above, if security is the only issue, a hidden switch to remove power to the fuel pump is an easy solution!
    Thanks for this very informative answer.

    It is indeed not a rally car so I opted for the switch on the ground of the fuel pump.
    Eventually for the looks I can still use a cutoff switch for this.

    Wiring in a real cutoff switch is indeed slightly overkill for me.
    My car is garaged 11 months a year and mostly covered in mud and grime, nobody wants to steal that pile,

    Thanks again.
    Subaru Impreza Turbo : Ecutek, AST tarmac coilovers, SW Motorsport brakes....

  6. #6
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    Just shows it's still worth visiting here even though I don't have a Subaru anymore!
    Found this thread very useful when fitting an FIA kill switch to my new Clio 182 'race car conversion if it ever gets done project'

    On the Impreza I had the main and ignition wired through the switch, but no resistor to dump the load on switch off.
    Guess what, eventually I had to replace the alternator......OK it had done 200,000 miles so probably died of old age, but you never know!

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