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Thread: Detecting when engine finished cranking?

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    Maximum Bitrate LiquidKernel's Avatar
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    Question Detecting when engine finished cranking?

    I always wondered this. Seems that many aftermarket alarms (with autostart) can do this. What method do they use?

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    Tach signal or voltage sensing...

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    Maximum Bitrate LiquidKernel's Avatar
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    Quote Originally Posted by dmurray14
    Tach signal or voltage sensing...
    Gotcha. What would happen to the tach signal once the car started? I'm assuming that there is no signal until the engine starts.

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    Quote Originally Posted by LiquidKernel
    Gotcha. What would happen to the tach signal once the car started? I'm assuming that there is no signal until the engine starts.

    More of a constant versus a non constant signal. When it's cranking, there is some voltage. The system waits for it to reach a certain voltage based on the amount of cylinders in in the engine, and then waits probably a second or two to make sure that the engine is running and it can stop cranking.

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    you could do it relatively easily with the combination of the two signals from the ignition (ACC line powered when the car is on, and the other powered during cranking)

    just wait for the cranking, then wait a second or two after the cranking stops.

    also, in most cars the ACC line is not powered during cranking... therefore if you have a shutdown controller that waits until the ACC line has been active for a certain amount of time, if you crank the engine immediately when turning the car on, the SDC won't see the ACC line stay high long enough until after the cranking is done.
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    Maximum Bitrate LiquidKernel's Avatar
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    Thanks for the tip evandude. I am thinking of making an MCU controlled device to turn on my headlights (with an HID projector retrofit) only after crank is finished. That way I don't have to worry about having to manually turning them off while the car is cranking (otherwise they turn off and on again when cranking is done).

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    Low Bitrate happpyg's Avatar
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    I used the alternator charge light signal.
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    Constant Bitrate
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    Yeah the alternator has a 3rd lil wire, usually called D wire (in German at least) and that tells you once it is turning. So that will be the way to go I think
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    cheap custom title JC-S60's Avatar
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    In older cars: oil pressure light...

    Or voltage measuring (when running, the charging-cirquit provides a little more than the battery when idle). I don't see what the number of cylinders has to do with it though...(?)

    Or just measure the alternator output.

    Many options

    Oh and btw, before you search the 3rd wire on the alternator, that could well be the 7th wire or something, as modern alternators have multiple phase outputs and other more complex configurations...

  10. #10
    Maximum Bitrate fantomas's Avatar
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    the number of cylinders has to do with the tach. not sure EXACTLY how, but the information is out there if you want to search and read up on it.
    rebuilding carpc... kinda..

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