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Thread: anyone use a fuel pump kill switch?

  1. #31
    Registered Member WagonCrazy's Avatar
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    Laszlo.
    So this is what Ive observed happening with this circuit.

    the ECM puts out 12 volts when the key is first turned on. Then the engine starts and it continues putting 12 volts out thru the inertia switch and into the relay.

    When I bump the switch (and it closes), the engine stalls within a second. When that happens, the ECM wire loses all voltage. Since it's connected to the C terminal on the switch, and the switch mechanically transfers the connection from the NC wire over to the NO wire...that NO wire now acts as a ground to the light/buzzer.

    I first tried connecting the light/buzzer the "other way" (thinking the NO wire would now transfer the 12 volts coming from the ECM wire to the C wire within the switch, but since the engine dies that NO wire loses all voltage...so nothing powered up the light/buzzer with it connected in this manner).

    So the NO wire becomes a ground when the engine dies, the ECM quits putting out 12volts, and that allows the light/buzzer to turn on (because of the 12 volt connection to battery power)

    probably lost y'all on this by now.
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  2. #32
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    I understand how you have it connected up now, but it's kind of a fluke the buzzer/light works that way. It must be finding a ground loop through the ECM which might not be good. I'm not really seeing a easy way to hook it up since the 12v goes away when the engine stops. I would think it would work briefly during the pump prime cycle when you first turn the key on when the inertia switch is tripped? The simple solution might be to just skip the light/buzzer and just put in a small indicator light to know when the fuel pump is on or off.
    Last edited by 55 Rescue Dog; 01-15-2020 at 07:09 AM.

  3. #33
    Registered Member chevynut's Avatar
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    I would have thought the ECM would "float" the fuel pump output instead of grounding it when it's not trying to turn the pump on. I don't know what kind of output driver they use in the ECM or how much current it can sink or source, but the buzzer probably only takes around 20mA to work. It's a piezoelectric buzzer and LED and neither take much current. I would guess the ECM has a FET output but it could be a relay. If it was a SPST relay it wouldn't work as wired. If it's a SPDT relay with ground on the NO terminal (inside the ECM), it would work fine. I wonder if there's a schematic for the internals of the ECM anywhere. It would be interesting to measure the resistance of the ECM fuel pump pin to ground with the power off.

    The problem with your circuit is that if the ECM is trying to turn the pump on, but the inertia switch is tripped, the buzzer will NOT sound. And it looks like it would sound all the time with the key off and the inertia switch tripped, since the buzzer is always powered.
    Last edited by chevynut; 01-15-2020 at 11:34 AM.
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  4. #34
    Registered Member chevynut's Avatar
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    Paul, if the ECM can power the relay with the 12V fuel pump output, it should be able to power the buzzer. Both require similar currents under 50mA. I'd check your wiring and try it again the first way. Wired that way, whenever the ECM is calling for the pump to turn on the buzzer will sound with the inertia switch tripped. That makes a lot more sense to me.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

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    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  5. #35
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    I believe the fuel pump output won't work with just key on, and the engine off, which you wouldn't want. I think the engine needs to be cranking, or running to fire up the pump.

  6. #36
    Registered Member chevynut's Avatar
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    Paul, you might want to try this....I know it's possibly what you tried first, but it should work. Be sure to get the polarity on the buzzer/LED correct. Like I said, if the ECM can supply power to turn on the pump relay, it should be able to turn on the buzzer too.


    20200115_001.JPG
    Last edited by chevynut; 01-15-2020 at 12:04 PM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

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    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
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  7. #37
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    Quote Originally Posted by chevynut View Post
    Paul, you might want to try this....I know it's possibly what you tried first, but it should work. Be sure to get the polarity on the buzzer/LED correct. Like I said, if the ECM can supply power to turn on the pump relay, it should be able to turn on the buzzer too.


    20200115_001.JPG
    I think that's the way he had it hooked up the first time. The problem is the 12v goes away from the ECM when the engine shuts off. Good schematic though.
    Last edited by 55 Rescue Dog; 01-15-2020 at 12:10 PM.

  8. #38
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    I think that's the way he had it hooked up the first time. The problem is the 12v goes away from the ECM when the engine shuts off. Good schematic though.
    It's not really a problem. If the inertia switch is tripped, the buzzer is connected to the ECM pump output. When the key is turned on, the pump output goes to 12V for 2 seconds before it shuts off. The buzzer will indicate a tripped inertia switch then. If you try cranking, the ECM will turn on the pump output, again sounding the buzzer.

    The way he has it hooked up now it seems like the buzzer should be sounding all the time because the ECM isn't outputting 12V with the key off. The buzzer won't sound for the first two seconds after the key is turned on, until the ECM turns the output off. Then the buzzer won't sound while cranking.

    I suspect Paul may have had something hooked up wrong the first time. It should work.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  9. #39
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    Yes is should work that way trying to start the engine, but that is the only time it will turn on the buzzer. And it won't work after the engine shuts off from the inertia switch is all. You would need to try and re-start the engine to see/hear that the switch had tripped out.

  10. #40
    Registered Member WagonCrazy's Avatar
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    Some comments on your observations:

    Chevynut-
    It would be interesting to measure the resistance of the ECM fuel pump pin to ground with the power off.
    Yes, I plan to check that soon. I too would like to verify that there is some resistance in that "ground" circuit (when the engine is off...does that Pin09 really turn into a functional ground?)

    RD-
    I think that's the way he had it hooked up the first time. The problem is the 12v goes away from the ECM when the engine shuts off. Good schematic though.
    This is correct. So once that 12v signal goes away, there's no power to transfer (on the inertia switch) to the light//buzzer.

    Chevynut-
    It's not really a problem. If the inertia switch is tripped, the buzzer is connected to the ECM pump output. When the key is turned on, the pump output goes to 12V for 2 seconds before it shuts off. The buzzer will indicate a tripped inertia switch then. If you try cranking, the ECM will turn on the pump output, again sounding the buzzer.

    The way he has it hooked up now it seems like the buzzer should be sounding all the time because the ECM isn't outputting 12V with the key off. The buzzer won't sound for the first two seconds after the key is turned on, until the ECM turns the output off. Then the buzzer won't sound while cranking.
    This is correct. But I have altered it since...and connected the 12v source to an "12v ignition on" connection point, rather than the "12v battery-always hot" connection point. This way it only sounds once it's been tripped, and stays sounding until i turn the key off.

    RD-
    Yes is should work that way trying to start the engine, but that is the only time it will turn on the buzzer. And it won't work after the engine shuts off from the inertia switch is all. You would need to try and re-start the engine to see/hear that the switch had tripped out.
    Correct. So this brings up a philosophical question about what we're trying to accomplish by including this inertia switch at all. I want a way for the pump to shut off in the event of a hard accident or rollover. In the event that I cannot physically turn the key "off", I want to know that the pump is shut off (because of the buzzer noise...that will confirm it). And that noise and light is a reminder that its shut off, should it "falsely shut off" for any reason. So technically, the ECU stops giving the signal when the engine dies (for any reason), so in an accident or rollover when the engine dies...the pump quits pumping fuel because the ECU signal is not there. And that's fine. But if the accident isn't bad enough to stall the car, and it keeps running, and I'm incapacitated, it want that pump to shut off. Seen too many people burn up in vehicles where they are incapacitated, and it appears fuel is just constantly running out fanning flames.

    So for now, this is working as designed. But I will check that question about ground resistance back to the ECU Laslzo... because I'd like to know what's happening. Mabye I'll shoot and post a short video when I do that..
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

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