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Thread: Battery Disconnect Switch with EFI

  1. #1
    Senior Member karlos's Avatar
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    Battery Disconnect Switch with EFI

    Been doing lots of research but cannot seem to find a consensus on how to safely incorporate a battery disconnect switch when using an EFI system. Not even sure what the specific concern is to be honest. The best explanation I've heard so far is that when the battery is suddenly disconnected from the alternator a voltage spike may occur, and if it does, it can make the EFI processor (ECU) most unhappy. I've contacted the maker of my EFI system who advised me to put only the battery on one side of the switch and all other electrical components on the other (switched) side. The problem with this approach is that I don't believe it will allow the disconnect switch to function as the intended safety device. In other words, if I disconnect the battery with the engine running I believe the engine will continue to run because the alternator will continue to power all downstream electrical components. It would also seem to play into the voltage spike scenario because the alternator would be isolated from the battery (no where else for a spike to go but into the electrical system).

    Confused as to what to do here. I hate to disregard the input of my EFI manufacturer, but it seems all wrong to me. Comments anyone?

  2. #2
    Senior Member BEAR-AvHistory's Avatar
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    I used the red kill switch that came with the car. Because if its switched off & all power from the battery is cut I have a jumper on the switch with a 10amp IIRC auto resettable breaker. This lets me kill power to the starter etc but maintain an always on low amp circuit for the ECU & Radio memories. If someone tries to start the car with the red key out the breaker will pop & all current into the car is dead till it resets.

    I have always viewed the red switch as an anti-theft device & a safe disconnect when doing electrical work.

    Have never tried to kill the engine while running so can't help you there. That said if I remember the main car circuits once the battery is cut out the car should still run off the alternator. If the alternator is dead the car should run off the battery till its reserve capacity is run dry. I know this works on my pickup truck which has an ECU managed system.

    EDIT: Agree using the switch as an everyday item will destroy it quickly. Mine is rarely shut off after the original one died.
    Last edited by BEAR-AvHistory; 05-16-2016 at 05:43 PM.
    Kevin
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    Delivered 2/7/14 - Plate "COYOTE NC1965" 3/25/15

  3. #3

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    You are correct on both counts.

    Using the master battery switch too much will wear it out. As it wears, it starts to create voltage spikes. Some electrical components can be damaged - like the ECU and the ignition module. I learned this the hard way. First, these components need some kind of buffer, like a 3A circuit breaker for the ECU.

    The distributor module can be protect by something like this: https://www.msdperformance.com/produ...rs/parts/29351


    Also, simply putting a switch in the main battery cable will not kill the engine. You also have to run the alternator power through the same switch. I prefer the 4 post type, but you can also use a two post. This is the one that I use. But, I don't use the switch very often. It will wear out rather quickly if you use it every time you stop the car. Another thing I learned the hard way.

    http://www.summitracing.com/parts/mor-74102
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  4. #4
    Member Arvin's Avatar
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    I installed the FAST EFI. I got power and ground for the computer directly from the battery per manufacturer. However, I put power to the fuel pump after the battery disconnect switch. Therefore, turn remove the switch - no fuel.
    FFR Complete MKIV Kit (#8271)
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  5. #5
    Tool Baron frankeeski's Avatar
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    I've never quite understood the fascination of these master cutoff switches. We have 8 vehicles and not one of them has a cutoff switch installed from the manufacturer. On top of that, as noted above, it will not kill the engine since the alternator continues to provide power as long as the motor is spinning. My Cobra has an ignition switch with a key, why would I need another key? I put the key in, turn it two clicks, the engine starts and the key springs back to the run position. I don't even have one of those silly push button start switches. Just one humble builders opinion but if you're not racing it within a sanctioning body that requires it, why bother.
    Frank
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  6. #6
    Senior Member karlos's Avatar
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    Pretty straightforward, really. The electrical system in my car was put together by an amateur electrician (me). Until I've accumulated some time and miles on the car, I want the security blanket of being able to shut the whole system down immediately via a single switch. Cheap insurance against a car fire, which, although admittedly unlikely, is still a real risk in these cars. At least more so than for a fully tested and professionally assembled production vehicle.

    In any event, something like the switch shown below might be the answer. Provides isolation between the battery and all other electrical components, and the dedicated alternator terminals allow the device to act as a kill switch as well. Can anybody explain why this couldn't be wired as shown in the second graphic (simply jumper the terminals on the switch rather than running another lead all the way back to the battery)?

    Thanks!







  7. #7
    Senior Member CraigS's Avatar
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    I haven't used a 4 pole switch but your idea seems very logical to me. I can't see any electrical difference between joining those two poles right at the switch vs joining them at the battery terminal. Sure will save on wire.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

  8. #8
    Senior Member canuck1's Avatar
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    The double positive wire/cable on the disconnect switch might be because the load on each side of the swiitch is different and each side requires different size wire?

    Sean
    MK 3.5 (MK IV body retrofit to MK III chassis), 351W stroked & poked to 396 c.i., custom EFI, 4 into 4's, 3link, 3.73, 15" Halibrand replicas, full bumpers, 2 X roll bars, IČ electronics, hydroboost, 5-lug, GT front, MK VIII/Cobra rear discs, Levy rear LCA's, Forte front bar, VPM rear bar, painted by SRP (again!)

  9. #9

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    That is the correct answer.
    .boB "Iron Man"
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  10. #10
    Tool Baron frankeeski's Avatar
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    Yea, I have a little trouble with some of these diagrams you guys pull off of the internet. Anyone notice the alternator is shown as an output. I've been doing electrical, both AC and DC for over 30 years. An alternator is a generator of sorts. It charges the battery, not the other way around. The alternator is not a "load" it's a "line". I still say that in most instances, the way these disconnects are wired routinely, it's an unnecessary piece of hardware that will not shut down the car in case of an emergency.
    Frank
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  11. #11
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    Why not leave the alternator connected to the battery and use a SPST from the battery to the positive rail? When the switch is thrown, the engine stops. Any additional charge from the alternator as it winds down will be dumped into the battery. You could always place a capacitor on the output of the switch on the positive rail if you're concerned about transients.

    If you want a dedicated line to the starter and keep the starter on the switch, you could replace the SPST with a DPST as described above. One output would go to the positive rail and one to the starter. In this case I would make sure to have a beefy enough cable on the feed from the battery to switch to starter. A fused bypass to the accessories would be a good idea too - unless you like resetting clocks and radio stations. Not to mention waiting for you GPS speedo to train on the satellites again.

  12. #12
    Senior Member karlos's Avatar
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    Quote Originally Posted by jceckard View Post
    Why not leave the alternator connected to the battery and use a SPST from the battery to the positive rail? When the switch is thrown, the engine stops. Any additional charge from the alternator as it winds down will be dumped into the battery.

    This is what I originally intended to do; however, my EFI manufacturer doesn't approve. This was their response when I asked about wiring the switch as you described:

    No…

    If you must use one, then EVERYTHING must go through the switch. Nothing should be connected to the battery side of the switch, except for the battery.



    As I indicated previously, I'm not really sure what the concern is. But they seem pretty adamant.

  13. #13
    bobl's Avatar
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    Seems like a perfect discussion to add my latest experience. I have the factory five cutoff switch installed. Somebody (me) left a battery positive cable loose and it touched ground. I flipped on the battery switch momentarily and then right back off as I realized something was wrong. This is where it gets ugly. The excess current draw caused the switch to short out, so it would not turn off. Fortunately the negative battery terminal melted and opened the circuit before any other damage. I always assumed if the switch were overloaded it would fail in the open position. I guess we learn something new every day. I bought a new better quality switch, but I still don't know how if it would fail open or closed.

  14. #14
    Senior Member cgundermann's Avatar
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    I used this memory guard from Speedway:

    image.jpeg

    Info

    If you'd like to use a battery disconnect switch but don't want your computer or radio memory to reset each time you cut the power, you need the Memory Guard. The Memory Guard hooks across the terminals of your battery disconnect and has a 3 amp fuse in it.

    This device permits the flow of the very small amount of energy necessary to preserve your computer's volatile memory, while simultaneously preventing a flow of energy sufficient to start the engine, so no one can steal your automobile and the battery life will hardly be affected! If anyone tries to start your car, it will pop the fuse.

    It's ideal for all vehicles with a radio with memory or clock and those running a fuel injection system.
    Last edited by cgundermann; 05-21-2016 at 09:16 AM.

  15. #15
    Senior Member Mesa Mike's Avatar
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    I have had my cut off switch on for 16 years. I use it all the time for any mechanical work and it really comes in handy and hasn't worn out yet. Also, if you track your car it is a requirement to have it mounted on the outside (not cockpit) for the track workers to get to if there is any problems on the track. Earlier post above are right on about keeping the computer powered up. Also, when my engine is running on the lift and I am under neath looking for leaks I can just switch it off without lowering the lift. It DOESN'T keep running.

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