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Five ways to use a scope for diagnosing vehicles

Sunday, January 1, 2017 - 09:00
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For years, I was amazed at how few technicians I worked with had ever used a scope, let alone owned one of their own. In every shop I’ve worked in over the last 20 years, I was it. But I’ve been pleased to see that scope use is on the rise, as evidenced by the number of techs who raise their hands in the presentations I make around the country and the growth of online support groups – even on Facebook! And no matter the reason for the growth, I’m glad that more and more of you are seeing just how valuable this diagnostic tool can be. Here are a few of my favorite uses for the scope. I hope you find them helpful.

As a note before we get started, I am going to share the scope settings I use. I admit I am no “guru” in scope use and learn continuously from those that are. I’m sure there are many of you who can offer advice on even more efficient ways of setting up and performing these tests, and I hope you will – in the comments section on MotorAge.com!

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The relative compression test

Many years ago, I was tracking down the cause of a slight misfire on a Chrysler minivan V6. After checking fuel and ignition, I found myself having to dive deeper into the mechanical health of the engine. All of you know how time consuming performing a conventional cranking compression test on a transverse V6 can be, and I also checked cylinder leak down on all six at the same time! Needless to say, I spent a few hours on the job that I never did get paid for.

The relative compression test is a way of assessing the overall health of the engine with a lot less trouble and in a matter of minutes, instead of hours. It is one test I perform on every engine that I find myself troubleshooting a drivability concern on.

Scope settings:

Channel 1 – High current clamp attached to either negative or positive battery cable. Scale set to read to at least 600 amps to capture initial inrush current expected.

Channel 2 – Standard lead or secondary clamp attached to cylinder ignition source (coil or secondary lead). Choose cylinder that is easiest to access. Adjust voltage scale to match connection and be careful to use attenuator if needed.

Time base – 500 milliseconds per division OR 5 seconds total screen time

Trigger – Single capture with trigger set to rising slope and +1 amp.

These settings allow me to capture all the info I need on one screen, and to do so without worrying about starting or stopping the scope manually. As soon as I turn the key “on,” the scope will begin to capture data. Figure 1 shows an example of what you can expect to see.

Figure 1

The green trace is the current pattern captured. The focus is on the repetitive sawtooth pattern, where each peak represents the amount of current it took to push a cylinder through its compression stroke. Even on my old UEI scope, I could distinguish as little as a 10 percent drop in an individual cylinder’s compression using this method. In this capture, the cylinders are uniform in appearance. But remember, this is a “relative” view, relative to all the other cylinders. If there is a mechanical issue that is causing low compression across the board, you’ll see a very similar picture, with only one noticeable difference – the current levels of the peaks will be lower than you’re used to seeing on a healthy engine. I can also see that ignition timing appears to be OK, since the ignition reference is intersecting the current pattern on, or just before, the peak. Spark occurs just before TDC, right?

Figure 2
Figure 3

Take a look at Figure 2 for an example of a “known bad” capture. Though hiding behind the ignition reference, it is clear that the peak for that cylinder is not reaching the same level as the others. By following the firing order as shown in Figure 3, I know which cylinder I need to take a closer look at. With that information in hand, I have no problem going back to the customer and asking for the additional time I’m going to spend isolating the exact cause.

Battery/charging system test

A common reason I hear from techs who own (or have access to) a scope but don’t use it is the time it takes to set up. If you fall into that category, then let me ask you this. Don’t you turn the lights and coffee pot on every morning when you get to the shop? Don’t you start up the computer work station you use so you can access your service information and work flow for the day? Starting up your scope should be on that same list of items, and if you use it to perform this next test on every car you get, you’ll also become more comfortable with the tool. A side benefit is that you’re performing a service for your customer that just might make you a little more money in the process.

Scope settings:

Channel 1 – Standard leads attached to battery positive and negative terminals. Scale set to read up to 2 volts per division or 0-20 volts total screen range.

Channel 2 – High current clamp attached to either negative or positive battery cable. Scale set to read to at least 600 amps to capture initial inrush current expected

Time base – 500 milliseconds per division OR 5 seconds total screen time

Trigger – Single capture with trigger set to rising slope and +1 amp.

Figure 4 shows an example of a typical system test result. But for clarity, let’s break out the voltage and current patterns and review the key points individually.

Figure 4
Figure 5

Figure 5 is the voltage alone. Just before “A” is the same Open Circuit Voltage you would read with your voltmeter, while “A” itself is the slight drop in voltage when I turned the key on. But I didn’t stop there, did I? I continued to the “Start” position and engaged the starter motor. “B” is the inrush voltage drop caused when the starter motor is just beginning to turn. After all, I have to get the starter moving and the starter is acting against all that mass in the engine. This loaded voltage reading is going to be lower than what you’re used to seeing when performing a conventional loaded voltage test so rather than use the 9.5-9.6 limit you learned in school, use 8.5 volts as your minimum here.

The time between “B” and “C” is the engine cranking. The little hills you can see are caused by the individual cylinders coming up on their compression stroke, just as we saw in the relative compression test. At “C”, the engine has started and is running on its own, with the key returned to the “Run” position. The upslope is the charging system replenishing the battery before settling on a more stable charging voltage at “D”. Now on Figure 6, the current pattern.

Figure 6

As we saw in the voltage pattern, “A” is where the key is turned on and the scope starts its trace. As the key passes to the “Start” position, current flows through the solenoid contacts and then into the starter motor. Again, the inertia of the starter motor and the engine have to be overcome before it will begin to move and that brings us to “B” – inrush current. In this example, inrush current is reaching nearly 600 amps! But it’s only for a microsecond and is not an indication of any problems in the system.

By now, you recognize what the peaks represent on the way to “C,” or the time the engine starts running and the alternator starts putting back what the starting system took out. Notice the short but rapid increase in current that quickly drops off and becomes stable. Since the current clamp is around all of the negative (or positive) battery cables, the current graphed on the screen represents “net” current flow; that is, the final total of current demanded by the system and the current being supplied by the alternator. This net amount will run around 3 amps or so. Any significantly higher amount should be cause for further investigation and may indicate a sulfated battery that is placing a burden on the alternator.

Running in-cylinder pressure diagnostic

This is one test that I truly believe was ground breaking, changing the way we test engine mechanical systems forever. By adding a pressure transducer to your scope, you can now “see” the pressure changes in a cylinder throughout the entire 720° engine cycle – and with no more effort that performing a cranking compression test with a mechanical gauge. In Figure 7, I’m showing you a normal cylinder with the ignition and injector events included for reference.

Figure 7

Scope settings:

Channel 1 – Pressure transducer is connected to channel 1. Follow the manufacturer specific set up instructions for your scope. On this Pico, the transducer scale is selected from a drop down menu and in this example is reading -25 to +100 psi.

Channel 2 – Connected to ignition event based on ignition system used by vehicle.

Channel 3 – Connected to injector event, by backprobing ground side (control side) of injector and using a 10:1 attentuator to protect the scope from voltage overload. Scaling is -100 to +400 volts to allow for inductive kick of injector that occurs on turn-off.

Time base – 20 milliseconds per division OR 0.2 seconds total screen time

Trigger – Set to “auto”, rising slope of channel 1 with capture to begin at 4 psi.

While this capture was taken using the Pico WPS500 transducer, there are others on the market notably those offered by Snap-on and Automotive Test Solutions (ATS). I have to offer props to ATS and its head, Bernie Thompson, because I sincerely believe he is the one most responsible for bringing this test method to the industry. There is so much information in this one capture that I can’t begin to do it justice in this short space but luckily, Bernie has written a series of articles on this technique for us and you can access them all on the website. But to whet your appetite, consider that this method can find problems with poorly sealing valves, variable valve timing issues, timing belts that are out of time, and a lot more – all without tearing anything apart for a visual check. Think that will save you time?

Fuel system testing

The pressure transducer can be used for a lot more than in-cylinder testing, too. One way I like to use it is to deploy it on my scope for monitoring fuel pressure in place of my mechanical gauge. It allows me a quick way to check for injector issues that otherwise would take a lot of time to test. But I’m getting ahead of myself. Let’s get to the settings I use to capture the patterns you see in Figure 8.

Figure 8

Scope settings:

Channel 1 – Connected to injector event, by backprobing ground side (control side) of injector and using a 10:1 attenuator to protect the scope from voltage overload. Scaling is -100 to +400 volts to allow for inductive kick of injector that occurs on turn-off.

Channel 2 – Low current clamp wrapped around a “Fuse Buddy” that has been placed in substitute for the fuel pump power feed fuse in the junction box. Scale set to 0-10 amps.

Channel 3 – Pressure transducer installed in fuel test port instead of mechanical fuel gauge. Scale set to -25 to +100 psi.

Time base – 50 milliseconds per division OR 0.5 seconds total screen time

Trigger – Set to “auto”, rising slope of channel 1 with capture to begin at 50 volts.

Testing fuel pump current has been around for as long as I’ve been using a scope but there are still lots of techs that have never heard of this technique before. The pattern shown is not clear enough for diagnostic use, so let’s take advantage of the scope’s ability to zoom in and take a look at the pattern shown in Figure 9. With very little effort we can see how much current the pump is drawing (a little more than 5 amps here) and what the speed of the pump is (roughly 5300 rpm). Consider that this approach works well on older style systems and you may have to adjust it a bit when looking at pumps that are pulse-width modulated. Even so, the pump is a motor, just like a starter motor, and it’s current will be impacted by the amount of work it’s doing – or not doing. And using this method beats banging on the fuel tank with a hammer!

Figure 9

Consider a pump that has a low current draw and a high pump speed. That indicates a pump that isn’t working hard. Ever have a customer get towed in for a “no start” concern where his car said it had a half a tank of gas but it was really empty? How about the opposite scenario – where pump speed is low and current demand is high? That indicates a pump trying to overcome a restriction – maybe a clogged filter?

Figure 10

On to Figure 10 for a closer look at the pressure transducer pattern. Notice the six lowest valleys between the injector events. We’re looking at a six-cylinder engine, and the injector pressure drops across all six injectors. How long would it take you to perform this kind of test conventionally?

In this example, they all appear even so I don’t suspect a problem with a sticking injector. However, there are funny little squiggles on every other valley. What causes those, do you think? In this case, the three squiggle captures are all on the same bank and closest to the test port. This is normal “ringing” in the fuel line as each injector opens and closes. The rear are less so because the distance allows the rail to dampen the effect before it gets to the transducer.

And No. 5 is

Actually, my fifth favorite test is any test I can perform with my scope that I haven’t tried before. The four examples I’ve provided are great examples of tests that once, had never been performed before! But some enterprising technician asked “What if?” and tried it. Now, they are all pretty standard for all of us!

So ask yourself the next time you face a diagnostic challenge how you could apply your scope to the situation. And if you come up with something, let me know. I’d love to hear about it!

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