Author Topic: Where my electricity went  (Read 24598 times)

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Offline s ǝoɾ

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Where my electricity went
« on: June 3, 2009, 12:17:29 am »
People asked for it, here it is.

Voltage and Voltage drop testing.


Here is the basic. Test for available voltage to the device (headlight, fuel pump, sensor, ect ect)

The voltage is not always 12.6. This is only on a fully charged battery and to a 12v device. Other devices (coil, sensor, led, ect ect) may be designed to run on a lesser voltage. Common to find 5 and 8v sensors on Mopars.

Notice the multimeter setup. Leads FIRMLY plugged into proper jacks. Leads inspected for damage. Meter verified on known test source (battery/wall outlet/whatever) Placed on DC or AC voltage depending on needs. You can't expect valid results with improper setup or faulty test equipment.

Notice the black lead placed on KNOWN good ground source. If you do not know which wire is bad, you should place this lead on the negative post.

Red lead at test point.

In this illustration #1 is the positive wire. #2 and #3 are both located on negative wire. Common sense will tell you if black lead is placed on negative, and red lead is placed anywhere on #2 #3 or other place on negative wire, you SHOULD have a reading of 0v.

We are also assuming you found full voltage all the way to the end of the positive wire.

The deviced remains plugged in and TURNED on during all testing here.

You cannot expect valid testing if there is a switch on EITHER the ground or hot side, which is turned off (open circuit)


In this illustration, you see we have full voltage all the way to the load.

At point #2, which is ground-ground voltage drop testing you see a reading of 11v.  :o Bingo, we have a problem. (remember 1st illustration, no voltage should be present on ground side)

This device may or may not even operate. It may rapidly click, blink, dim, or not operate at full speed.

Testing location #3 shows a correct reading of 0v.

This indicates the problem MUST be located between these 2 points. Usually the wire is simply replaced, but you may do further testing to pin point the exact location in order to perform a splice repair. (same test may be performed to load test your solder/crimp splice)

As you see, there is a rheostat located between #2 and #3. Rheostats provide resistance in a circuit. Common example is a fuel tank sending unit. Here variable resistance on the ground side is DESIRED. The float will move, adjusting resistance, thereby adjusting voltage output, which is shown on your gauge.

Other times, the resistance may not be variable NOR desired. This would be the case of a frayed ground wire. Enough strands remain to power lights or stereo, but the moment the starter is engaged, rapid clicking is heard, nothing happens, or lights go out.

If the main ground cable is suspect (many/all devices fail to operate) that cable may be tested. Keeping in mind it must have a load applied. Turn on all lights, fans, ect ect. Always have black lead on negative battery post. Place red lead on ENGINE BLOCK. Obviously in a place that should have clean ground.

Have an assistant attempt to start vehicle, or use a remote starter button to do the same.
« Last Edit: April 1, 2010, 02:58:12 pm by s ǝoɾ »
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    Offline s ǝoɾ

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    Re: Measuring Voltage
    « Reply #1 on: June 3, 2009, 12:56:48 am »
    Only after confirming the ground side is good, can you move on to the positive side.

    NOW, the black lead remains on the positive battery post while red lead is moved along different points of the positive cable.

    Same thing applies, switches on, load applied.

    Checking voltage especially on both sides of any connectors along the way.

    No more than about .2/200mV at any particular connector, and no more than a total of .2-.5 along the ENTIRE wire.

    A individual connector/switch that shows excess drop, indicates corrosion or loose connection or high resistance/open in the case of a switch.

    Some factory circuits in new condition may still show excess undesireable drop. Such is the case with dim factory head lights where all current is routed directly through a undersized switch/too small of a wire. The situation only gets worse, as each time the switch is toggled, it arc, burning away a little more of the contact, until no more lights period. This is where relays are employed. Cheap, easy to replace, and take the load off the factory switch.

    SOMETIMES a drop is desired. Such may be the case of blower motors and ignition coils, where full power may damage the device, or speed control is desired.
    Avoid replacing any part that you have not proven to be faulty through extensive testing.

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    Offline Hybrid

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    Re: Measuring Voltage
    « Reply #2 on: June 3, 2009, 12:58:04 am »
    Thanks for the write-up, Joe!

    Offline Elwenil

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    Re: Measuring Voltage
    « Reply #3 on: June 3, 2009, 01:02:56 am »
    Good stuff!  +1
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    Offline projectW

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    Re: Measuring Voltage
    « Reply #4 on: June 3, 2009, 01:11:32 am »
    This ought to be stickied  :)
    Great work Joe.
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    Offline s ǝoɾ

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    Re: Measuring Voltage
    « Reply #5 on: June 3, 2009, 01:14:07 am »
    This ought to be stickied  :)
    Great work Joe.
    Still working on it...this is only the beginning.

    I have some horribly corroded batteries, sketchy wiring, and other impromptu training aides. I have just been to ashamed to take pictures of them yet.  ;D
    Avoid replacing any part that you have not proven to be faulty through extensive testing.

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    Offline oldmanram

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    Re: Measuring Voltage
    « Reply #6 on: June 3, 2009, 01:39:38 am »
    Good write up Joe, and good start.

    I'm a bit confused though - in your second picture as shown, there should be a voltage drop at position #2 since the circuit shows that the light bulb is 'on' and what ever resistance is in the bulb will lower that voltage ... I read it over a couple times ... maybe I have too much soap on the brain  ;D ;D

    Ok - unless the rheostat you show is supposed to be in the off position ....
    « Last Edit: June 3, 2009, 01:43:37 am by oldmanram »
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    Offline s ǝoɾ

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    Re: Measuring Voltage
    « Reply #7 on: June 3, 2009, 02:24:27 am »
    Good write up Joe, and good start.

    I'm a bit confused though - in your second picture as shown, there should be a voltage drop at position #2 since the circuit shows that the light bulb is 'on' and what ever resistance is in the bulb will lower that voltage ... I read it over a couple times ... maybe I have too much soap on the brain  ;D ;D

    Ok - unless the rheostat you show is supposed to be in the off position ....


    In a parallel circuit ALL voltage should drop after the load (ground side). #2 should read 0v, since it is on the ground side, and other lead is also on the ground side. Another way to put it...the light bulb should "consume" all electricity, with none left over.

    The only way for there to be some left over (11v in this case) is if it is SHARING current with another load whether intentional (resistor block/rheostat) or unintentional (corrosion/loose connection)

    What is happening in picture #2 is another load is consuming most of the voltage....leaving only 1.6v for the light bulb. This would produce a dim light (might be desirable in a dash cluster...not on head lights)

    #2 shows a rheostat which is an intentional second load...but it could just as easily be corrosion.





    If there is an open...such as the case with disconnected wire or switch off, there will be a voltage drop equal to source voltage.



    Instead of a lightbulb Picture #2 could just as easily be a starter circuit where one lead is on engine block, other on negative post. Instead of a rheostat, this could be a bad chassis ground cable.

    The high resistance area acting as a load, actually produces heat (through consumption of remaining electricity)

    This is how a loose/corroded/high resistance connection eventually turns into a melted bulkhead connector, a landrover dash fire, a 70s ammeter fire, or other melted wires. That electricity has to go somewhere, and it does. It changes to heat and burns things.

    This is the same reason electrical components get warm/hot. Things like fan motors, ignition coils, stove elements, cd players, ect ect. The resistance is transforming electricity into heat. Same thing with a rear defroster, or your cummins grid heater, or a scalding hot ballast resistor.
    « Last Edit: June 3, 2009, 09:58:34 am by joe s »
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    Offline Gerhart

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    Re: Measuring Voltage
    « Reply #8 on: June 3, 2009, 05:25:17 am »
    Very good HOW TO!!
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    Offline Bueller

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    Re: Measuring Voltage
    « Reply #9 on: June 3, 2009, 09:32:56 pm »
    Good write up Joe!  The only thing I'd say different, is ALWAYS go to the negative battery post when testing a circuit.  When you go to a "known" good ground, you're going through all kinds of connections, alot of times even going through the frame itself.  It's another chance for error that doesn't need to be there.  You just can't get more grounded than a negative battery post.

    I'm not trying to jack your post, Joe, just trying to help ;)

    Frank
    « Last Edit: June 3, 2009, 09:35:35 pm by Bueller »
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    Offline AZ-Trailduster

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    Re: Measuring Voltage
    « Reply #10 on: June 3, 2009, 10:54:24 pm »
    sweeet, i've just been watching how bright the test light gets...  ;D ;D ;D just kidding

    +1 for ya man, good write up
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    Offline s ǝoɾ

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    Re: Measuring Voltage
    « Reply #11 on: June 4, 2009, 12:23:28 am »
    Good write up Joe!  The only thing I'd say different, is ALWAYS go to the negative battery post when testing a circuit.  

    I think I covered that in post #1.  ;D :P

    Always have black lead on negative battery post.

    BUT, always is a very subjective word. Always as in, if I want to know the available voltage, yes go for it.

    This does not apply to hot side VOLTAGE DROP testing, as both leads are placed on the same side of circuit. This also does not apply to negative side voltage drop testing IF post to chassis has already passed, nor does it apply if I am targetting a specific ground side connector/junction/wire.

    Thought I should clarify, just in case any body is confused.

    MOST people understand the concept of placing one lead on hot/ground battery terminal and expecting a voltage of around 12v. It is understood, one lead on negative, and one on positive should = expected voltage.

    What happens when no voltage is present? What if the reading is lower (maybe 10.8v)?

    You could replace ALL cables, fusible links, switches, connectors, sensors, and wires within the circuit. You could disconnect every thing, run a resisntace test on each piece (only after disconnecting) but then again...who knows how accurate your meter is dealing with smaller resistance values.

    ENTER the voltage drop. The voltage drop is used AFTER doing the simple available voltage test, to diagnose and isolate the exact problem area of a circuit. The piece may be cleaned, spliced, or replaced as needed, w/o excess parts shot gunning.

    As mentioned before, this process is also used to test a solder joint, a crimp connection/repair, UPGRADE factory circuits, or even to ANALYZE new customized circuits to evaluate if wire size is appropriate. Surely 16ga wire is not sufficient for a starter motor, but do you really need 00ga welding cable? No need to spend extra $$$ if it is not needed. What about that custom winch/stereo/lighting job? Is the provided wiring of sufficient size?

    The results are very conclusive.

    Less than .2-.5 volts is sufficient for MOST automotive circuits. This means a loss of approx 5%, well within reason.

    More than that but LESS THAN SOURCE (battery or what ever power source) indicates high resistance.

    A reading of source means a resistance reading SO HIGH it is practically a open circuit OR the circuit is open (completely cut off)

    Avoid replacing any part that you have not proven to be faulty through extensive testing.

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    Offline Bueller

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    Re: Measuring Voltage
    « Reply #12 on: June 4, 2009, 10:10:39 am »
     {fencing}

    Frank
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    Offline s ǝoɾ

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    Re: Measuring Voltage
    « Reply #13 on: June 4, 2009, 11:28:14 am »
      {fencing}

      Frank
      :P

      Some common applications for Voltage Drop testing

      • Hot side testing for dim head lights (positive post to head light terminal)
      • Windshield wiper ground issue (wiper motor body to ground post)
      • Chassis ground (negative post to numerous chassis/engine/body points)
      • Starter main hot cable (positive post to solenoid)
      • Starter solenoid testing (main input to starter motor output)
      • Battery clamps (post to clamp)
      • Sensor ground wires (ground pin to negative post)
      • Sensor 5v/8v VREF wire (vref pin on pcm to vref pin on device)
      Summary
      Any device you can turn on AND keep on. This does not work with pulsed devices, on the pulsed side of circuit. You cannot test the ground side of coils or injectors with this method. You cannot test mag pickups with this method.

      This is essentially a resistance/continuity test that can be done while a circuit is attempting to operate under normal conditions, therefore nothing needs to be disconnected. Performing a ohm test on a powered circuit may result in damaged meter or inaccurate results. Save the meter, save time, get conclusive results quicker.



      POINTERS
      Keeping in mind that the device must remain plugged in and turned on, this some times makes for difficult access to connector pins. Not to mention testing a tail light harness may be difficult while trying to hold a lead to battery post.

      Not to worry, you are free to buy/build custom leads.

      The common meter banana jack is available at radio shack. You may use this to build "custom" long leads to reach points at the rear of your vehicle/trailer.

      You may fabricate a variety of ends to include needle points, small alligator clips or large battery charger style alligator clips.

      You may be required to "back probe" a connector with a small tee-pin or sewing needle to create a test point.

      In a last resort scenario, you may be forced to "pierce probe" a wire. This requires physically damaging the insulation in order to access a test point. I prefer to avoid this option, opting for a special alligator clip known as "bed of nails"

      When building a new circuit from scratch, it may be hard to dynamically test, BEFORE taking the time to install it. You can bench test a repaired harness or new circuit with a few items. A load of similar rating and an external power supply such as a booster pack, ac/dc adapter, or spare car battery. Often a cheap suitable load may be a simple sealed beam lightbulb of similar/greater wattage than the expected load. Of course you are not going to test a repaired starter cable w/o a carbon pile load tester or installing it on the engine, so scratch that thought. An old blower motor/cooling fan may be used as a suitable load as well.

      You can wire it up on the bench and then test, if installing it in car provides to be too difficult.



      Intermittent failure
       {flame}
      Enough to cause a stroke.   {cross}
      This can be used for intermittent issues as well.

      A circuit may be stable under certain conditions but when heat or vibration is applied, you suddenly lose headlights, car dies, ect ect.

      Instead of waiting til you get caught w/o lights 3 hours away from home on some dirt road, you can often simulate these tests within the comfort of your own home.

      Need a screw driver, multimeter, and heat gun (blow dryer may not be durable enough) optional is gloves and a infrared temp measuring gun.

      With this process, it is a bit more time consuming because you have to either test each individual point OR get real lucky at guessing.

      You simply install meter lead in voltage drop form, on each side of a connector in the harness. You proceed to apply heat with the heat gun while shaking/pulling/tapping the connector with screw driver handle.

      Sometimes it may be hard to see/hear the issue occur, as may be the case with some discrete sensor, so the meter provides a visual display that you just lost connection.

      Of course when lights suddenly go out, or engines die, it may not be necessary to have a meter to tell you something just quit.  {idea} Then again, how many people are so in tune with mechanical function as to notice a PCM ECT sensor intermittently fluctuate?[/list]
      Avoid replacing any part that you have not proven to be faulty through extensive testing.

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      Offline s ǝoɾ

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      Re: Measuring Voltage
      « Reply #14 on: July 20, 2009, 08:56:47 pm »
      Starter Circuit Voltage Drop
      Avoid replacing any part that you have not proven to be faulty through extensive testing.

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      Offline KThaxton

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      Re: Measuring Voltage
      « Reply #15 on: July 21, 2009, 09:38:24 am »
      Good stuff. It's a good visual to understanding voltage drop.

      As Joe mentioned earlier, most people are familiar with placing a the meter leads to a positive and a negative, while many do not grasp the concept of testing voltage drop by placing the leads at two different points on the positive side of a circuit. One thing to help this make sense is to understand that the two leads on a multimeter measure the voltage difference between the two leads, i.e. there is a 12.6-ish voltage difference between the positive and the negative posts on a battery.

      If you are measuring voltage at two points on a positive circuit and get a reading of say 2 volts, that means that there is only 10.6 volts at that point which obviously means that there is a lot of resistance somewhere between those points.

      « Last Edit: July 21, 2009, 10:12:14 am by KThaxton »
      STOP PLATE TECTONICS!

      You're absolutely correct, Kendall. My mistake  ;D

      Offline Unclebull

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      Re: Measuring Voltage
      « Reply #16 on: July 21, 2009, 11:31:58 am »

      Posting so I can find it again.   ;D


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      Offline KThaxton

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      Re: Measuring Voltage
      « Reply #17 on: July 21, 2009, 12:38:29 pm »
      You can click the "NOTIFY" button in the black bar under the last post of any thread to get notification of replies if you haven't replied in that thread. If you have replied and click it however, it will disable your notifications.  ;)
      STOP PLATE TECTONICS!

      You're absolutely correct, Kendall. My mistake  ;D

      Offline s ǝoɾ

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      Re: Measuring Voltage
      « Reply #18 on: July 21, 2009, 02:05:00 pm »
      Posting so I can find it again.   ;D



      This is a sticky for a reason.  ;D
      After doing this test only once or twice, you should have the hang of it. I think I mentioned it earlier, this works for your old wiper motor issue. Place one lead on case and one on NEGATIVE terminal, any significant amount of voltage lets you know you have a bad case ground. Don't even have to plug anything in or unplug anything. Furthermore, by keeping one lead on negative terminal and probing around the firewall, you can possibly find a suitable mounting location, or atleast test your new location.

      I ve said it before, but I will emphasize, THE TESTED CIRCUIT MUST BE SWITCHED ON. Voltage needs to be able to complete the loop from battery to component back to battery. This works better for things that still operate, but slowly/inconsistently/dimly ect ect. If a bulb is burned out AND wire is bad, a voltage drop will not show a bad wire, because electricity does not have a complete path due to open circuit.

      I hope the video helps to understand the concept. Pay attention to the end where both leads are placed in proximity of positive post, note the reading, note the result.

      The test is not an end all be all replacement for everything, but generally tends to make troubleshooting turn from pain to pride. Saves money, saves time.
      Avoid replacing any part that you have not proven to be faulty through extensive testing.

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      Offline groundpounder

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      Re: Where my electricity went
      « Reply #19 on: April 1, 2010, 06:57:48 pm »
      how does you know so many stuff, guy??? {noclue}

      Offline s ǝoɾ

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      Re: Where my electricity went
      « Reply #20 on: April 2, 2010, 04:55:07 am »
      how does you know so many stuff, guy??? {noclue}

      The info is out there..... ;D
      Avoid replacing any part that you have not proven to be faulty through extensive testing.

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      Offline 88raminator

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      Re: Where my electricity went
      « Reply #21 on: January 9, 2011, 11:47:02 am »
      cant thankyou enough. fixed the bodyground to the rad support and was able to stop just shy of replacing passenger side power window motor in my 88 rc. turned out to be bad wire in drivers side door jam.
      80 W150 Clubcab, 99 4wd Extended cab, 57 dodge 1/2 ton 340 auto ,really miss 80 ramcharger and 98 v-10 3/4

      Offline buddman318

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      Re: Where my electricity went
      « Reply #22 on: July 31, 2011, 08:28:12 pm »
      you wanna take mine and find out where it goes.ive tried everythin ive learned over the years and in auto school to no avail. HELP  {lol} 24 hours from start to not enough power to run entry light on dash or its very very dim
      82 rc 318 holley 650 dp 727 np208 33 12.50 recently did a 4 in spring lift frnt (converting to aftmrkt tbi)

      Offline kcrosby

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      Re: Where my electricity went
      « Reply #23 on: August 22, 2013, 01:56:00 am »
      You can spent too much on a multimeter....but it is difficult.  Personally, I like my Fluke.  Don't even waste your money on the $4.99 special at Harbor Freight.

      Digital meters are perfect for precise readings.  Analog meters are better for seeing a trend or a transition.  For things like looking for a parasitic load (what drains your battery), an old-fashioned test light is still your friend.
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      Offline maachine

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      Re: Where my electricity went
      « Reply #24 on: January 29, 2014, 12:20:03 am »
      i love my Ideal voltmeter. runs about 60$. well worth the investment. i just did the CAD axle delete on my truck the other day. if anyone knows where i can buy a pre 84 transfer case light switch (switch that turns on the 4x4 indicator light) to replace the vacuum switch on a 208 t case that'd be great.
      '87 Dodge W-150 5.9L 727/208 3.23 gears & 33's - fun truck
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      Offline maachine

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      Re: Where my electricity went
      « Reply #25 on: January 30, 2014, 09:07:38 pm »
      haha, you know what, i posted about that switch thinking i posted in the CAD thread. oops
      '87 Dodge W-150 5.9L 727/208 3.23 gears & 33's - fun truck
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      Offline Killerbee

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      Re: Where my electricity went
      « Reply #26 on: March 9, 2017, 10:25:20 pm »
      Another way to check the amerage on a circuit w/o disrupting the circuit is to measure the voltage drop over a fuse. 

      Powerprobe has a link to charts.  I'm not completely convinced that different brand fuses will have the exact same resistance (and thus voltage drop).  But it's a nice way to see how much load in on each fuse quickly.

      http://info.powerprobe.com/fusechartsdownload

      -Ben

      Offline SuperBurban

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      Re: Where my electricity went
      « Reply #27 on: March 9, 2017, 11:48:46 pm »
      How many of us have a meter accurate enough to measure accurately down to a tenth of a millivolt? 0.0001 volt


      Harborfreight has a great little meter that plugs into the fuse, and reads in amps.

      http://www.harborfreight.com/30-amp-automotive-fuse-circuit-tester-67724.html

      « Last Edit: March 9, 2017, 11:55:47 pm by SuperBurban »
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