The Trainer #30: Learning The Automotive Lab Scope

The Trainer #30: Learning The Automotive Lab Scope


is there a digital storage oscilloscope gathering dust somewhere in your shop you know an automotive scope can be a very powerful diagnostic tool in the hands of someone who knows how to use it the first thing you got to do is blow the dust off and power it up and I hope you’ll do just that after watching this edition of the trainer diagnostic uses of the lab scope are only limited by the imagination and skill of its user with some accessories the scope can also be used to view current pressure vacuum and more and like any other skill you’ve mastered it all starts with some fundamental knowledge and practice actually using the tool on cars that you know are ok any test you currently perform with your DV om can be performed with a scope think of the scope as a multimeter on steroids able to provide you with a visual display of what is happening in the circuit or component you are testing let’s try a simple example how about a simple battery charging system test now we’ve demonstrated how to test the battery starting charging system using the multimeter in another video so I won’t go into too much detail in this one so we’re going what I’m going to do is I’m going to set it on the voltage scale I’m going to connect it to the battery positive to positive of course negative to negative I’m going to set the record button and then I’m going to go in and start the car a couple of times if I’ve let it run for a minute or two before shutting it off now when I’m done I’m going to go ahead and hit the record function that’s going to give me the maximum reading in this case it’s 13.7 with the lights on that’s not too bad for a charting system that’s charging system voltage isn’t it hit it one more time now that’s the minimum reading that the multimeter took care of it’s reading nine point eight volts that’s above the nine and a half for a loaded battery test in it that’s the loaded voltage and then finally when I go ahead and clear the record function I’m going to get the open circuit voltage the current state of charge on the battery itself that gives me a really rough overview of the charging system let’s see what differences scope makes I’m going to use one channel to connect to the battery the same way I did with the multimeter I’m also going to take advantage of the additional inputs and use a second channel to record amperage that’s not cheating that’s just taking advantage of what the tool can do now let’s start the car and let it run for a few minutes the resulting graph or waveform it tells me everything the multimeter did and more I can see for example when the starter solenoid contacts actually closed and the current it took to make that happen I can see there’s a problem with the alternator diodes by zooming in on the voltage line and looking for the tell-tale signs of ripple I know how much net current is being put back into the system and more all in the same amount of time it took to use the D vom now a couple distinct advantages come to mind when I talk about the scope the scope samples many times faster than your multimeter can and it does so over a given period of time which it can then display here on the screen the multimeter is giving me little snippets little windows in time if you will that’s going to make a lot harder to catch any anomalies it might be going on with the circuit that I’m testing now no big deal if I’m checking something like the battery starting charging systems where things aren’t changing that fast but what if I want to check a faster signal like a camera frame sensor then it can make a difference candidate let’s find out I’m accessing the cam sensor right at the computer using Mac probe pins to avoid damaging the wiring any ECM input I want to check I can check here now let’s see what the cam sensor and crank sensor output looks like on the scope now let’s see what a cam and crank sensor signal look like using the same settings that we use for the battery starting charging system okay this is turning into just one long smear is making as I mentioned earlier the voltage divisions are along the vertical axis or the x axis the time divisions are along the horizontal axis axis so by changing that time division I can take a closer look and open up the patterns so let’s go from 10 seconds rays division 20 milliseconds for each division that’s a substantial difference what else can I tell using this waveform well if I have a known good pattern I can tell if the two signals are in sync with one another it beats tearing things down to find out doesn’t it and saves a lot of time in the process now here’s another example of how a scope can save you time it’s called the relative compression test and what we’re going to do is we’re going to check the overall mechanical condition of the engine by monitoring the starter current draw of each individual cylinder that’s right we’re going to use the time settings so that we can home in and look at how much current each cylinder is drawing or causing to be drawing I should say as it goes through its compression stroke and of course any time we make a motor work harder it takes more current flow doesn’t it so good healthy cylinder ought to make that starter work good and hard if the cylinder is not so good it should show as a lower current demand in that pattern so I’ve got the starter current or the current probe high amp probes still connected to the battery cable let’s give it a try and see what happens each hump is a cylinder as a passes through top dead center of its compression stroke the amount of current drawn by any electric motor is not only a factor of its internal resistance but the amount of physical resistance is trying to overcome healthy cylinders will take more current to rotate through than a weak one and we’ll see that here talk about doing a compression test quick do you need a big expensive scope to do a relative compression test not at all there is coax in the market place across the price range with cross different feature offerings and heck if you have a high scale scan tool in your shop you may already have a scope there and didn’t even know it here’s one example of products on the market this is the little AES u scope so one channel pocket-sized lab scope very powerful little scope let’s try the relative compression test using this and take a look at that now what if I did see a problem with that pattern what if one of the peaks was lower than it should be I don’t know what cylinder it is do i well that’s easy enough to find out with a scope I’m going to use another channel of the scope and find a reference that I can use to help me see where I’m at in relation to those Peaks now we can do that very easily using the ignition event hook up to the ignition system doesn’t really matter which cylinder you pick whatever is the easiest one to get to that will then show up on the pattern as your reference and if there’s a problem say two Peaks down the road use the firing order to find out exactly which peak is causing the issue to get a reference I’m going to use the igniter event on this number one coil that should give me a good reference morning scopes have a limit to the voltage input that they can handle attenuators are needed to safely measure voltages that are higher than this limit so be sure to check your scopes owner’s manual before you connect the high voltage components like ignition coils and fuel injectors now to make the pattern more stable I’m going to go ahead and add a trigger a trigger is just a point on the screen that you telescope when to start tracing the pattern so in this case we’re going to just make this figure point here and set it so it captures on a single capture and we should be all set to go now I know what cylinder goes with what peak what else kind of learn from this waveform take a look think about it did you say timing sure the timing were off wouldn’t that reference line be further away from the peak one way or the other and I didn’t even need a known good pattern to compare it to did I well these are just a few examples of how a scope can help you speed up your diagnostic process help you find those tricky diagnostic problems more quickly and more efficiently and I hope you’ve also seen that the scope is not as complicated to operate as you might have thought so hope you take the time to blow the dust off the one in your shop and give it a try and don’t sweat it if you need help we have lots of resources on the OTO pro workshop and the motor age comm website and you can also ask questions in the comment section of this video that’s all the time I’ve got for this month’s edition of the trainer I’ll see you next month you you


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