Scope usage and comprehension

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  • maven
    Senior Member
    • Mar 2014
    • 269

    Scope usage and comprehension

    How many guys here use a scope?

    How many use it all the time? To record properly functioning vehicles or just on broke ones?

    Do you have a personal library of good waveforms?





    I think scopes are great. I have and use multiple versions. I cant wait til I get the newest PICO one, Im excited about it. It took me years to become any kind of skilled at it. I read about usage and techniques and tests at least once a week. I spend time to test good vehicles to develop procedures for diagnosing bad ones whenever I can.

    That said....when I get asked for in person, or offer assistance anywhere, I almost NEVER ask another tech if he has "scoped" something, nor do I recommend that same tech do so. Even if its established this person has a scope.

    Just knowing how to hook a scope and get a waveform is a skill, and then knowing what that waveform is supposed to look like when its good, and what a "bad" waveform actually means are all additional proficiencies that this person needs to have to make hooking up the tool even worthwhile.

    Its my personal belief and experience that with rare exception, that the guys who could actually use a scope to diagnose a problem, do actually use it.

    And that the people who are asking for diagnostic assistance on a problem where a scope would help, don't know how to effectively use one. Most likely unless you hooked up, set up, captured the waveform, and then analyzed it would a scope be useful in such situations.

    It happens on S/T, it happens in my shop, it happens in other local shops. I get called by professional friends who've been told to "scope" something, and thats it...no explanation. No instruction for what to look for, no information about what a bad waveform would be, etc... I see it on guys faces when they get told it....they just go blank. You might as well have just told them to read Japanese.
  • greasybob
    Senior Member
    • May 2008
    • 1590

    #2
    I use it for automotive and other electrical uses. Got my own library of wave forms.

    Comment

    • SPS10.4
      Member
      • Aug 2012
      • 39

      #3
      I started with a two channel Pico almost 15 years ago. Very few in my area had a DSO at the time. A few shops had Counselors. Most of the shops wouldn't even consider the "expense" of owning a scope. These same shops would also glaze over when I mentioned iATN. iATN was my lifeline for known good patterns. I also started saving my own good/bad captures.

      These days there are way more shops that have tooling such as the Modis and Vantage PRO. It would make it easier to compare waveforms, but as you mentioned, anybody that has one usually can figure it out on their own.

      Beside auto use, I also use the Vantage PRO to save known good patterns for various outboard motors. I miss the Picos' incredible resolution and the huge PC monitor, but I love the Vantage PRO for portability and the database.

      Comment

      • Crusty
        Senior Member
        • Dec 2007
        • 2450

        #4
        Something that is FUNDAMENTAL is that a scope is merely a voltmeter that also displays time and it displays both in a graphical form.

        Without understanding what a circuit is supposed to do, a scope displaying voltage and time is useless.

        The advantage of a scope is that it shows you WHAT the circuit is doing and also WHEN it is doing that.

        The proverbial WHO, WHAT, WHEN, WHERE, WHY, HOW.

        The speed at which electricity travels is a tad on the fast side (LOL) and a scope that can capture the signal, display the signal and keep the evidence of the signal on screen long enough for us to recognize it can be the enlightenment of WHY something is amiss.

        Any scope that can't save and store that visual evidence in this digital age is lacking.

        There are a lot of pieces of kit out there but precious few even recognize the capabilities of an accurate scope. They think that the magical machine will solve the problem for them. Nothing could be farther from the truth.

        Just because someone HAS a scope, doesn't mean they SHOULD have a scope. It really goes back to fundamentals.

        I was able to PROVE what I already SUSPECTED based upon other evidence, by using a scope that was in a monster of a machine (by today's standards) when I first opened the shop back in 1993. I would have given my first-born for a scope when electronic fuel injection started becoming popular back in the early 80's.

        A couple of bugaboos about my Snap-On equipment is that the scanners MORPH the engine run time pid which then takes all the other parameters with that change in WHEN something is happening, and, my low amps probe hooked into my Vantage-PRO is inaccurate below about 12-milliamps (to which the technical people at Snap-On said that is negligible-!!) which makes it almost useless for non-intrusive parasitic draw testing. This is where the people making the tools don't understand what the circuit is supposed to be doing which hamstrings us in the field, in the real world, on the bench, in the trenches.

        Maven; YOU KNOW how low the normal parasitic drains can be in today's vehicles so you can understand why a low amps probe that jiggles around between minus 11-milliamps and plus 11-milliamps skews the measurements. Some modules only draw 3 or 5 milliamps-!!

        Comment

        • Modis500
          Senior Member
          • Sep 2012
          • 358

          #5
          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
          "If you aim for nothing, you'll hit it every time!"
          Zig Ziglar

          Comment

          • Crusty
            Senior Member
            • Dec 2007
            • 2450

            #6
            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
            Tom; I'm not sure what you mean about a shunt amp port. I'm sure it's just semantics on my side.
            I have a couple of ways of DIRECTLY MEASURING the current by using a "knife switch" to keep the circuit closed while I hook up an ammeter.
            The low amps probe apparently can be influenced by things such as fluorescent lights and the earth's magnetic field. SOME people have found that wrapping tinfoil and such can help the FLOAT of the probe but it's no where near good enough to get down to measuring the low milliamps required.....

            Comment

            • Crusty
              Senior Member
              • Dec 2007
              • 2450

              #7
              Parasitic drain testing wasn't popular back in the days of points and carburetor.
              AFAIC the low amps probe is only good for checking 100 milliamps and once you find that the amperage draw is less than that, inductive probes are unreliable.
              I have zeroed the probe holding it in the correct orientation hooking onto a circuit and measuring 30 or 40 milliamps then unlocking the probe where it SHOULD then return to the previously set zero and found the low amp probe and the Vantage PRO showing MINUS fourty milliamps-!!!! So was it really fourty milliamps or did the vehicle have EIGHTY ma of drain-??
              That "float" that the gurus at SnapOn say is "negligible" makes the equipment unstable and thus UNRELIABLE.
              With normal parasitic drains in the ten and twenty ma range these days the swings of twenty or fourty or eighty ma is simply unacceptable.
              The devil is in the DETAILS.
              It is nice to be able to hook up a knife switch, record the ma and save the data in a universal method by using SSC though.

              Comment

              • Witsend
                Banned
                • Nov 2012
                • 2942

                #8
                It is nice to be able to hook up a knife switch, record the ma and save the data in a universal method by using SSC though.
                Reply With Quote
                I agree , any movement of the low amp clamp around a wire, the readings are going to fluctuate. I was thinking you could have a half dozen small led light bulbs that each require a specific milliamp draw range with red bulbs for excessive draw to a tiny green ones for acceptable range of miliamps

                Comment

                • Modis500
                  Senior Member
                  • Sep 2012
                  • 358

                  #9
                  Crusty,

                  I've heard some call the amp port on top of Vantage and Vantage Pro called the shunt amp port, and there's a thread under the V Pro category. So you'll just measure the draw while in series, but then you have to disrupt the connection at battery to put it in series. I hope this clears up what I was talking about.
                  Tom
                  "If you aim for nothing, you'll hit it every time!"
                  Zig Ziglar

                  Comment

                  • greasybob
                    Senior Member
                    • May 2008
                    • 1590

                    #10
                    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.

                    Comment

                    • greasybob
                      Senior Member
                      • May 2008
                      • 1590

                      #11
                      Sorry, kind of got off of the scope usage topic and on to parasitic draw tests. Here is a video I made I year or so ago showing voltage drop through a fuse with a very low load on the circuit. https://www.youtube.com/watch?v=9KnL...gv3y5UdHGDr4pg

                      Comment

                      • Crusty
                        Senior Member
                        • Dec 2007
                        • 2450

                        #12
                        Originally posted by Modis500
                        Crusty,

                        I've heard some call the amp port on top of Vantage and Vantage Pro called the shunt amp port, and there's a thread under the V Pro category. So you'll just measure the draw while in series, but then you have to disrupt the connection at battery to put it in series. I hope this clears up what I was talking about.
                        Tom
                        I was sure that's what you were saying but the word "shunt" gets me thinking other things.
                        I've been measuring parasitic drains in series since the 70's. There are ways to ensure you don't lose the drain (such as sticking relay contacts) when you hook up the series connector, then with the knife switch in position, you can then measure the current, and close the switch again, change scale on your meter or scope and then re-open the knife switch to take a second reading.

                        It's nice to use the scope and the low amp probe for situations where the drain is high enough and or it cycles between perhaps 60 or 80 ma up to 200 or 400 ma or even higher in some cases.
                        It's even nicer on the rare occasions when it behaves itself...

                        As greasybob is indicating, you don't want to influence the condition of the vehicle by our techniques of measuring.
                        What witsend is suggesting wouldn't be a totally bad idea, except that even led's use some current, albeit very low, and if it's powered by the vehicle it would add to the draw. Another problem is the cycling of some drains which is normal depending on the vehicle and build options.

                        greasybob also mentions using voltage drop across the fuses. I think Volkswagon put out a TSB around 2002 or so which does a good job of describing that technique, and the scales for the different fuses is also in that bulletin.

                        Comment

                        • maven
                          Senior Member
                          • Mar 2014
                          • 269

                          #13
                          Crusty, you hit the nail on the head and thoroughly reinforced my point. Knowing how a circuit is designed to function and how influences on this circuit(resistance,source voltage) affect its operation are key to proper diagnosis, whether youre using a DMM or a DSO, and its the guys who use DSO's successfully that understand this, have a firm grasp of Ohms law, and would most likely be able to figure out whats wrong even without the scope.

                          If someone is asking for electrical diagnosis help, doesnt mention a scope and doesn't mention what good/bad voltage/resistance readings they've collected, theres a really good chance that telling them to "scope" something is gonna get them to glaze over....hell I find that a LARGE percentage of guys dont even understand HOW to do a voltage drop test, let alone WHAT results its shows or WHY they may be getting a reading.

                          You guys bring up some excellent points and all stuff Ive run into as well.

                          The low amp probe in my opinion/experience is worthless below about 25mA. Same goes for my go to Midtronics PDF40, it starts to suffer from random interference below 20mA or so as well. Ill admit one bad habit I have though...on non sensor based voltage control vehicle Ill disconnect the neg cable with vehicle running to install a switch for direct series amp measurements.

                          Voltage drop across the fuses "works", BUT I havent been able to get truly replicable results across multiple vehicles/vehicle lines, I have difficulty even getting repeatable results on identical vehicles. For example on the bench I can pretty readily read a .001V drop across a mini fuse with a 50mA draw. However, in the car a similar 50mA draw only results in a .0004V drop. Im up over 100mA draw before I get truly stable usable mV readings. Corrosion on the tops of the fuses? most likely, but cleaning would be excessively tedious and could easily result in disturbing the circuit. I leave the fuse voltage drop readings for bench and forum sessions, discussions and teaching of theory.
                          Last edited by maven; 02-02-2015, 08:05 AM.

                          Comment

                          • hhong12dlm
                            Junior Member
                            • Mar 2013
                            • 22

                            #14
                            "I think Volkswagon put out a TSB around 2002 or so which does a good job of describing that technique, and the scales for the different fuses is also in that bulletin."

                            great discussion,
                            I find the vdrop on the fuses is a little shaky too on the lo ma drains
                            but here are the links to the VW info and also at power probe too if anyone is interested.


                            Attached Files

                            Comment

                            • Witsend
                              Banned
                              • Nov 2012
                              • 2942

                              #15
                              voltage drop across a fuse?

                              I find the vdrop on the fuses is a little shaky too on the lo ma drains
                              but here are the links to the VW info and also at power probe too if anyone is interested.
                              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.
                              Last edited by Witsend; 02-02-2015, 03:07 PM.

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