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Automotive How electron flow is in a car system

  1. Feb 22, 2012 #1
    Hi, I would like to know how electron flow is in a car system? I ask this because when looking at a car amp install it say connect to the positive terminal, a negative to ground "chassi". I understand conventional flow holes ect, but realistically I can't see how this wiring method works.
  2. jcsd
  3. Feb 22, 2012 #2
    Re: Car battery

    Most every car made in the last 40 years has the negative battery terminal being wired to the chassis.

    I make a good connection to the body for my negative feeds to the components as well as ground the cases to the same connection.
    Last edited: Feb 22, 2012
  4. Feb 23, 2012 #3
    Re: Car battery

    as vague as the question was, i think i may understand. After knowing electron flow of current and then going to school for automotive technology, i got totally lost when working with multi-meters and ohms law just because of the fact that us AUTOMOTIVE guys OBSERVE the CONVENTIONAL flow of current.

    If you want to make sense of current flow/voltage using a multi meter, what took me a week to understand was the fact that when you place a lead on the on a part of the electrical system to check for voltage with the other lead on the negative side of the battery ( in most cases. your observing one direction of flow. as soon as you put that lead on the positive terminal as opposed to the negative, your voltage reading will change. Therefore observing a different side of the circuit. it all depends on your frame of reference. when you touch meter leads backwards, you get the same reading always, but with a (-) sign.

    If you were to hook a car battery up backwards, at least nowadays, that could be potentially lethal to your cars power control module, alternator and so on/so forth. this is strictly because of the fact that these ELECTRONIC things have diodes and transisters that are current flow direction-specific. (allows flow one way, blocks flow another way.) The reason you can hook a flashlight up backwards (incandescent light), is because there are no electronics involved.

    This should explain why the direction of flow matters and where it is significant, even the electrical engineers in the auto industry all know to observe this conventional flow and design electronics with respect to the power and ground sides of the electrical system.

    In a nutshell, this is the significance of flow direction.
  5. Feb 23, 2012 #4

    jim hardy

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    Re: Car battery

    When trouleshooting, first thing i always do is this and it seems wierd at first:

    Connect voltmeter's negative lead to a good point on the body.
    Connect positive voltmeter lead to battery's negative post, that is push lead against the lead post that the battery cable clamp grabs around not the battery clamp itself..
    Have somebody turn on headlights, meter should read very slightly positive...

    More than a few millivolts means i need to clean up connections at ends of negative battery cable or fix engine to body ground wire.

    On old Fords look for somebody replaced negative cable and didnt realize that the little metal tab in middle of original factory cable WAS their negative connection to body for both battery and engine. Melted braid ground strap at back of engine is a giveaway..
    A natural enough mistake, it looks innocuous.
  6. Feb 29, 2012 #5
    I think I see where you are coming from now, so there is a complete circuit for the current to flow, the body?
  7. Feb 29, 2012 #6


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    Yes, the body provides a return path to the battery. This is true for both electron and "current" flow.
  8. Feb 29, 2012 #7
    I think if you have a solid understanding of how your multimeter/voltmeter works, it can help a lot when trying to learn these things about electricity flow, with the redundancy and whatnot. Study ohms law and just make it strictly intuitive that automotive industry observes electricity through CONVENTIONAL FLOW which is from positive terminal of the battery, to the chassis/negative.

    This method of wiring works and it has for decades. Think of it this way, electrons want to flow from the positive to the negative, but they want to find the shortest path to ground/negative. So when something like the fuel pump is being energized, current goes through the motor windings and out to a terminal thats bolted to the chassis. Electrons can then find their way back to the negative terminal of the battery. No one knows the exact path of the electrons through the chassis itself, (cause it has such a large area) BUT it doesnt matter, cause this occurrence doesnt negatively affect the performance of the circuit. Resistance is what can cause complications, which is usually in the case of bad connections or light-gauge wire... Especially if your interested in stereo wiring. Biggest thing is to get a heavy gauge wire to power your amplifier and there are no exceptions to fusing that wire.. unless you want a carbeque. Speaker wiring is a different story however, they are driven from and to the music box, because they are responsible for interpreting the variations of frequencies in which electrons are sent through their coil windings.
    Last edited: Feb 29, 2012
  9. Feb 29, 2012 #8

    jim hardy

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    If negative wire from battery goes only to engine and not to body, and there's no other wire from engine to body,
    then any current through let's say the headlights or radio can't get back to battery.

    If negative wire from battery goes only to body and not engine, than current from starter and alternator can't get back to battery.

    That shows up as bizarre symptoms, everything looks crazy - lights are dim, alternator wont charge, gages pegged... if it'll start at all .

    And you dont want that current finding its way home through, say, wheel bearings or speedometer cable..

    So i always check. Only takes a second and you dont have to admit you were dumb enough to look for somethiing that simple.
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