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Thread: Engine runs on with key out

  1. #11
    Senior User Iain's Avatar
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    I wired the fuel pump and coil directly to the car ignition live and it behaves fine. Run it through my relay and fuse box and it misbehaves.... It sticks at 5v when the keys out

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    I have a theory but may be questionable. The battery light goes out when the alt is charging when there is 12v on both sides of the ignition and therefore no current flow energising the lamp. In reality you have for example 12v from the batt and 14v from the alternator. Because you are using the switched live from the ignition to only power the relay you are getting the difference in voltage eg 2v back feeding the ignition and pulling in the relay. 2v (your 5v) is enough to pull in the relays as there is no load on it.

    Your options are. Fit a resistor or resistor wire from the switched live to the coil of the relay or a diode on the alternator to batt light wire. If you had more resistance on the switched live you would loose the 5v and not have enough power to energise the relay. The diode will stop the 5v being back fed to the switched live.

    Im in sure that's right but I can't remember where I fitted the diode so my theory might be wrong.

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    I think you've pretty much nailed it.
    The charging circuit doesnt switch on or off against ground. The regulator raises the potential at the excitation coil as it charges so the potential across the lamp is equal, no current flows and it goes out.

    To stop the back-feed, the diode needs to go in series with the excitation wire.

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    Senior User Iain's Avatar
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    It worked fine like this with the injection set up and same alternator and relay configuration though ;(

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    Quote Originally Posted by Iain View Post
    It worked fine like this with the injection set up and same alternator and relay configuration though ;(
    Yes, but you also said it was fine until you fitted the alternator so it must not be a current path on the rest of the loom.

    To test it, turn the key off so that its running on and pull temhe excitation wire off the alternator. If the engine stops, thats the problem.

    The inj system probably had enough load that the voltage collapsed and the relays dropped out. Without seeing a diagram of your loom before and after, its up to you to work out the reason. But hopefully doing the above test will diagnose it.

  6. #16
    Senior User Iain's Avatar
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    Ok thanks. Sounds like a potential solution would be to run the coil and fuel pump off the biggest existing ignition live (via a small fusebox) that I'm currently using to trigger the relay and use another spare ignition live to trigger the relay for the rest of the gear (epas, gauges, etc etc)

    That way it'll kill the coil and pump and thus the engine which will flick the relay for the rest of my gear.

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    You must have had more load on the switched live with the injection setup.

    You could probably wire the battery light to a switched live your relay is not on so that the battery light is completely separate from the relay that remains energised.
    Last edited by therealnovaboy; 29-03-15 at 09:35 PM.

  8. #18
    Senior User Iain's Avatar
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    Done what I said above and all is working well. Coil and fuel pump run with no relay off the ignition live. Rest is fused that gets killed as the engine actually dies now. Thanks all.

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