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Electricity 101

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  • Electricity 101

    When making electrical checks you can be measuring voltage or amperage. Voltage is the pressure behind the electron flow. Think of a high pressure hose squirting water on the lawn. It might have 40 pounds of line pressure, but not a lot of flow. Amperage is the amount of flow. Think of a river which flows hundreds of gallons of water per second. It doesn't have much pressure, as you can hold the open end of a low pressure gauge against the flow, and the needle won't move.

    It takes pressure to move the electrons, such as thousands of volts to make the electrons jump the gap of the spark plug, but it takes current flow (amperage) to do the work. Think of the high amperage needed to spin starter armature to turn the engine over.

    When you do electrical checks for voltage loss, you need to have current flow, or at least try to have current flow. A very poor connection can show the right voltage, but it won't flow the current. If every wire except one is broken in the starter cable, you can still measure the correct 6.3 volts at the starter switch, but it won't flow the current needed to run the starter. If the starter spins fine, then you can assume the battery connections and ground strap are fine, so if you are trouble shooting for poor lighting, horn, or ignition, you can start at the starter switch and move upstream towards the part that isn't working.

    Remember, you need current flow or at least trying to flow current to make these checks, so if the lights don't work, turn them on and measure the voltage at the starter switch. If it's good then move up to the terminal box and check each wing nut. If both are good, then you know the terminals, wires, and ammeter are OK, so you move on to the cutout, where the lights harness picks up it's power. If you have good voltage there, move to the light switch, and from there to the headlamp sockets. If you still have good voltage at the headlamp sockets, then you could have a poor ground. Use a clip lead as a good ground bypass.

    It only takes seconds to check voltage along these points. It's always a good idea to carry a couple of 36" clip leads to fix a poor connection while on a trip. The clip leads are also handy to use as a temporary bypass for a good ground. Be sure the clip leads are good quality with soldered wires at both ends. I wasted a lot of time once making checks and wondering why things weren't making sense. It turned out my cheap clip lead wasn't making a good connection because the wire wasn't soldered. Another time I was left wondering why things weren't making sense, and found my multimeter had a loose connection inside the case, where the leads plug in.

    With electricity, if you have more than one problem at the same time in the same circuit, then it can really get confusing trying to make sense out of it.
    In the end, just carry a good test light, and a good analog multimeter, plus a wiring schematic of your car, and it won't take long to find the problem.

    BTW, when you see someone walking around checking batteries at a junkyard by using just a voltmeter, they can't really don't know much about the battery.
    The correct battery tester puts a load on the battery as the meter checks the voltage. This is the only way to know how good the battery is.

  • #2
    OK, get your Lucas replacement smoke ready . . .

    The stock A harness is all 16ga. Does this mean a 20 amp load will burn it up?

    I was helping a friend swap out a dead alternator on his modern and he commented about how the 100 amp alt was connected with "only" an 8ga output lead. Most car guys seem to magically defer to house wiring rules of thumb for gauge vs. amps. He thought it should be at least 3ga, like the feeds to a 100A breaker box in a house. That's not the only game in town, there are lots of factors involved.

    Here's a few other differing views on ampacity.
    Caveat Utilitor.



    Mechanical engineering 101: If you put an adjustment knob, screw, bolt, or tolerance specs on something, some people will immediately fiddle with it. If you mark it DO NOT TOUCH everyone will mess with it.

    Comment


    • Mike V. Florida
      Mike V. Florida commented
      Editing a comment
      It can get confusing when you figure AWG and SAE for the same gauge wire have two different values. The length of the wire and if it is in a bundle also affects the value one uses.

    • jmeckel
      jmeckel commented
      Editing a comment
      The load / amp capacity of a wire at 12 volts DC is much less than at 110 Volts AC assuming the same % of loss of voltage. The chart below shows the ratings for no more than a 3% loss. A 12 ga wire 15' long can carry 10 amps at 12 volts DC. There are a lot of factors in the calculation, however to keep it simple the higher the voltage the smaller the wire size can be to carry the amp load.


      http://docs.engineeringtoolbox.com/d...ximum_amps.png

  • #3
    Series and Parallel circuit basics!



    Scan0037.jpgScan0038.jpgScan0039.jpg
    4~ Tudor's
    1~ Coupe

    Henry Ford said,
    "It's all nuts and bolts"


    Mitch's Auto Service ctr

    Comment


    • #4
      I'll try to read the above information when I'm in the mood. Some people were given electrical knowledge at birth, at 67 it will be a mystery right up till I'm in a box with dirt on top. Bob

      Comment


      • CSPIDY
        CSPIDY commented
        Editing a comment
        Electricity is PFM (pure fn magic)

    • #5
      Scan0311.jpgScan0312.jpgScan0313.jpg
      4~ Tudor's
      1~ Coupe

      Henry Ford said,
      "It's all nuts and bolts"


      Mitch's Auto Service ctr

      Comment

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