Scope usage and comprehension

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  • maven
    Senior Member
    • Mar 2014
    • 269

    #16
    Originally posted by Witsend
    I agree . A fuse is not supposed to be any sort of a load to be tested across on one side or the other for a voltage drop unless you are testing for an open fuse or suspect a voltage drop due to corroded contacts or if the current flow is approaching the limits of the fuse rating that it starts to get hot and then it might cause enough measurable voltage drop to be considered a load if it doesn't blow like it should. Maybe if fuses experiencing a voltage drop because of resistance changing because the temperature rise of the fuse, might be detected by a good infra red temperature probe? Seems to me some of the newer VW mini fuses were sealed and were actually un probe able. I'm wondering if a LINE SPI left in the DLC ,the LED location might shed light on what modules aren't going to sleep.
    Its not that its "not supposed" to be a load to be tested across.

    Any circuit with current flowing in it has voltage dropped across every component of the circuit. And knowing that no good, 8mm section of any circuit in a typical vehicle with less than 50mA running through should ever have more than .0008V on it is actually useful information. So if youve got a vehicle fully powered down and youve got a fuse thats dropping a millivolt(.001V) or more you can be pretty sure theres significant current flowing in that circuit to kill the battery prematurely.

    The issue is that with the small sections of circuit, small currents, and even smaller voltages we are dealing with, any inaccuracies in your meter(lots of guys use cheap *** DMMs)any imperfections at the measuring point due to corrosion or other poor contact issues dramatically skews the results.

    Its still useful test, its just(like anything)that you need to fully understand what youre measuring and what it means. If your ammeter reads that youve got 35mA to hunt down....youre not gonna find it by voltage drop testing across fuses, however if your ammeter is broken or otherwise unavailable or is attached to a Counselor cart that isnt convenient to roll outside then you can make some quick useful measurements with a high quality resolution DC voltmeter. If youve got 1mV across a circuit you can be pretty confident theres at least 50mA flowing in that circuit.

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    • Witsend
      Banned
      • Nov 2012
      • 2942

      #17
      Its not that its "not supposed" to be a load to be tested across.

      Any circuit with current flowing in it has voltage dropped across every component of the circuit. And knowing that no good, 8mm section of any circuit in a typical vehicle with less than 50mA running through should ever have more than .0008V on it is actually useful information. So if youve got a vehicle fully powered down and youve got a fuse thats dropping a millivolt(.001V) or more you can be pretty sure theres significant current flowing in that circuit to kill the battery prematurely.

      The issue is that with the small sections of circuit, small currents, and even smaller voltages we are dealing with, any inaccuracies in your meter(lots of guys use cheap *** DMMs)any imperfections at the measuring point due to corrosion or other poor contact issues dramatically skews the results.

      Its still useful test, its just(like anything)that you need to fully understand what youre measuring and what it means. If your ammeter reads that youve got 35mA to hunt down....youre not gonna find it by voltage drop testing across fuses, however if your ammeter is broken or otherwise unavailable or is attached to a Counselor cart that isnt convenient to roll outside then you can make some quick useful measurements with a high quality resolution DC voltmeter. If youve got 1mV across a circuit you can be pretty confident theres at least 50mA flowing in that circuit.
      Reply With Quote Multi-Quote This Message
      Thanks Maven , it looks like there's not many options to try if pulling one fuse at a time and reinstalling generally will wake up the computer network and blow the DVOM 2A fuse for the Milli- amp setting. Another reason a low amp probe, though not as accurate is still a better idea than blowing a fuse in the DVOM . Makes you wonder why all DVOM fuses cant be eternally located on the meter.

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      • maven
        Senior Member
        • Mar 2014
        • 269

        #18
        Originally posted by Witsend
        Thanks Maven , it looks like there's not many options to try if pulling one fuse at a time and reinstalling generally will wake up the computer network and blow the DVOM 2A fuse for the Milli- amp setting. Another reason a low amp probe, though not as accurate is still a better idea than blowing a fuse in the DVOM . Makes you wonder why all DVOM fuses cant be eternally located on the meter.
        I know that a quality meter is a big expense especially when youve already got a functioning one, but my Fluke 28II will read in tenths of a milliamp(.0001A) when on the 10amp scale if I place it in highrez mode, it displays milliamps by default. The Fluke 8X series should all do this as well. You cn probably pick up a really niced used or refurb Fluke 83 for around $100(thats not much beer....) Im sure other meters available from Fluke, Flir, Keysight/Agilent,Gossen,(and many othersm including some actualt quality offshore stuff)

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        • scottmechanics
          Member
          • Jun 2013
          • 42

          #19
          Volts & current

          Hello
          I find most of the fluke multi meter are good and well made mine about 10 years old I also bought the hook from power probe it to is good for checking current flow.

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          • Crusty
            Senior Member
            • Dec 2007
            • 2450

            #20
            Originally posted by greasybob
            If you disconnect the battery while looking for a parasitic draw you'll mess up the test. You should try to leave everything as it would be when you normally shut off and leave the vehicle. Use a temporary battery cable that can be disconnected after you put the amp meter in series. Have you ever tried check for a parasitic draw by checking for a voltage drop across the fuses.
            That's why I'm so disappointed with my low amps probe. When I get a response that 11-ma is "negligable" someone obviously isn't aware that some components only draw 1, 2, 4 and 5 milliamps. When they add up a normal draw can be in the 20-ma to 35-ma range on a great many vehicles. The Old-School value of 50-ma is out of date by several decades because the miniaturization of electronic integrated circuit chips has greatly reduced their keep-alive power consumption.

            There ARE ways to disconnect a battery to hook up a knife switch without fully removing battery power but you have to be aware of how to supply a supplementary power source while you do that.

            That isn't any different than back in the 1960's or 1970's when a relay would stay stuck and the inductance flowing through the points would keep it closed until battery power was removed and POOF, the drain was gone.

            Now we need to ALSO be aware of initial turn on and wake up power until modules power down to their "sleep" state. MOST vehicles will power down to sleep state within a half hour or an hour but there are also times when modules can stay awake for 4 or 5 hours after you turn the key off and walk away. Some GM A/C control modules are one example.
            Another example of 45 minutes of normal battery drain is the EONV evaporative emission systems watching the fuel tank pressures after shut down where the ECM looks at the system build pressure then drop down to a vacuum as the entire vehicle cools down.

            Scopes and non-intrusive testing is very desirable but not fully understood by some equipment developers.

            Comment

            • DonWright
              Junior Member
              • Mar 2015
              • 6

              #21
              Its not just snap on but everyones low amp clamps just don't measure that low accurately, and for that reason i never use them for parasitic draw. The voltage drop testing at the fuse box is my favorite way for sure! But your comments about how this field hasn't got a good hold on scopes,low amp clamps,voltage drop testing etc etc is very sad. With all these CAN systems and 30 on board computer cars coming into our bays it just makes me wonder... lol??? it also seems to be the more you know the less you make, especially if your a diagnostic guy. A problem child car comes in and you get stuck with it knowing your going to lose your butt while they tech next to you who can't run a volt meter is going to clock 12 hrs doing brakes,hoses,flushes and a water pump?? Its a screwed up industry. I hate government and regulations but i really wished you had to be a certified tech to work on cars. Its the only thing i see that would give this field the respect it deserves.

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              • diesel71
                Senior Member
                • Feb 2008
                • 921

                #22
                all good points, Im just going to throw this out here. check out the eect74-amp hound, I find this a very useful tool.
                EECT74.jpg

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                • Foden
                  Junior Member
                  • Mar 2011
                  • 27

                  #23
                  got one and agree

                  got one of these myself and totally agree very useful
                  admittedly I don't use it every single day as work type varies but it has save me a lot of time when I do use it , even if your just checking fuses for bust and not checking the draw its a time saver.

                  Comment

                  • dbarnes5654
                    Senior Member
                    • Apr 2007
                    • 123

                    #24
                    Originally posted by Modis500
                    Crusty,

                    You were mentioning to Maven how the low amp probe jumps around from -11 mA to +11mA... I too have run into this while doing draw testing and was wondering if I was to blame or if my amp probe was wigging out. I guess I now know the answer. Sometimes I'll reattach the clamp and start the testing all over because of this issue. So my question is this: will using the shunt amp port remove this discrepancy on the measurent side? And does a Verus have a shunt amp port? Or just the Vantage and Vantage Pro?
                    Tom
                    Yep, just the vantage and vantage pro have the amp port. Why snap-on removed this from their newer scopes is a mystery to me. Personally for me, if I'm trying to measure parasitic draw, I only use the amps feature. There is just to much fluctuation when using a low amp probe. I have recently purchased a low amp probe from Aes Wave that has additional shielding that may eliminate that problem. I had to call AES to get it made for me but it was still way less expensive than snap-on's low amp probe that has the problem. When I hit the zero button, it zero's out and stays there. I haven't tried it out yet for draw testing but will soon.

                    Don

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