Dust off that scope

Oct. 30, 2015
The DSO can be an invaluable diagnostic tool if you’re comfortable using it. 

Do you own a Digital Storage Oscilloscope (DSO)? You do? Great! Next question – are you using it?

The DSO can be an invaluable diagnostic tool if you’re comfortable using it. And that’s the hard part, isn’t it? Taking the time to become familiar with the tool you own, trying all those cool testing procedures you’ve read about in our magazine on those “known good” cars so that when a problem does arise, you’ll be confident in your ability to accurately perform them. But when you do have a problem that the scope would help you fix, the time and trouble it takes to set everything up is often enough of a deterrent to its use that you go back to the testing methods that are convenient (but not necessarily the most effective).

The good news is that we are creatures of habit, and habits can be learned so, first things first. When you go into the shop tomorrow morning, make setting up your scope part of your usual routine like turning on the lights, compressor and coffee maker. Then, pick some of the every day scope applications I’m going to share with you and perform them every opportunity you get.  Some you can do on every car you touch, others are more specific to diagnostic challenges you face routinely. All will help you gain proficiency with your scope of choice, preparing you for the time when you really need the diagnostic power the DSO can offer.

Figure 1 Figure 2 Figure 3

Testing the basics – Battery/starting/charging systems
The very first one I want to share with you is a fundamental test of the battery, starting and charging system. Nearly every scope on the market has the ability to sample (“sample” references how often the meter/scope reads the input voltage – the more samples per second, the more accurate the actual display on the screen) much faster than any other tool you’d use to perform this basic test and will capture problems that may otherwise go unnoticed. It is also a test you can perform on every car you are assigned, regardless of the original complaint.

Performing this test gives you practice in adjusting time and voltage scales, the use of the high amp clamp and in learning how to set up a trigger and to capture a recording for later review. It also gives you a great printout you can share with every customer and just may add to your numbers by catching system failures before they have the chance to strand your customer.

This test is best performed with two channels. What’s a channel? Think of your scope as a Digital Voltmeter that displays a reading over time, in the form of a graph. That is the capture you see on the scope’s screen. A channel is one DVOM capture. Additional channels are like adding additional DVOMs, with the ability to see all of the data on one screen. And, just like your DVOM, you can add accessories to measure nearly anything you want – voltage, current, vacuum, pressure, and more. But I’m getting ahead of myself.

Each channel’s graph is divided vertically and horizontally. The vertical divisions are voltage divisions and they can be adjusted to display very small voltage changes or very large ones. Start your set up for this test by setting the first channel on your DSO (the red trace on Figure 1) to read a total range of 0-20 volts. On my older Pico, that means I select the -20 - +20 volt range from the dropdown menu. Your scope may ask you to set the amount per division so in that case you’d count the total divisions your scope can display (usually 10) and divide that into 20 to get the per division setting of 2.0 volts. The leads for this channel will be attached to the battery’s positive and negative battery posts.

The second channel (the green trace in Figure 1) is going to be used for measuring current. But since a scope can only receive a voltage input, I need a tool that will convert current flow to voltage. That’s what a current clamp does and any scope user should have a high current and low current clamp in their collection of scope accessories. Most scopes have the ability to select an appropriate scale, based on the accessory being used, but if yours doesn’t, you’ll need to convert the voltage signal on the screen to current using the scaling included with the clamp. In my case, I’m going to set up the second channel to read 0 – 600 amps, and then I’m going to place the clamp over the negative battery cable. You can use either, but usually the negative is the easier to get to.  Keep in mind that current flows in one direction, so if the capture you get is not looking like the one you see in Figure 1, you may have to reverse the clamp on the cable.

That leaves the horizontal scale. This scale is used to set the amount of time per division the scope will display and is the real power of this tool. Most scopes today are capable of accurate displays in the thousands of a second, allowing an expert user to see even the slightest anomaly. Most of us, though, will never need that kind of diagnostic power for every day use. For this test, I like to set my scope to .5 seconds per division, for a total “sweep” of 5 seconds per screen. But my Pico allows me to record numerous screens so I am actually capturing a lot more data than that. It also gives me enough resolution that I can capture so much on one screen that it looks like a giant smear, and then zoom in without loss of detail so I can keep it all on one screen if I prefer. Many scopes allow deep recordings so this is a great chance for you to experiment with your scope to see what record options you have.

On to the actual test
If your scope allows you to save presets, this is one I’d keep close at hand. If not, then set up the two channels as part of your initial morning start up. You’ll find it takes less time to do than it does to read about it! With the two channels ready and the leads in place, it’s easy to attach them to the vehicle and perform the actual test.

To get an image similar to mine, you’ll need a trigger. A trigger simply tells the scope when to start tracing the signal and can be set as a continuous trigger or a single shot trigger. It can also be set to begin tracing when the voltage signal exceeds the specified threshold (rising slope), or as the signal drops below the threshold (falling slope). The advantage to having a trigger for this particular test is to allow you the time you need to move from the scope to the ignition switch to start the engine. I set the trigger to begin a trace as soon as it sees the current rise.

And that’s the test procedure. With your scope connected, start the engine and allow it to run for a few minutes. If you did everything right to this point, you should have an image similar to the one shown in Figure 1. Now to interpret the information you’ve collected:

            A – Look closely here and you’ll see a few short rises followed by a small plateau before the current takes off. This is when the key is turned “on”, through the “run” position and finally to the “start” position. The last plateau you see is the starter solenoid closing and you can use the zoom function on your scope to see it even more closely.

            B – Here I want to point out a few things. First, current draw is shown on the upper half of the “0” line. For some reason, we’re just more comfortable with it that way. Just keep in mind that any current below the “0” line is positive current flow back into the battery and not a draw. Second, the current peak is rising nearly to the 600 amp limit I have the scope set up to capture but only for a nanosecond. When we think of starter current draw, we normally think of specifications around 150-250 amps depending on the number of cylinders, don’t we? But the scope can react so quickly it picks up the total current used to get everything moving, and that can be a number 2-3 times the current needed to keep the starter (or any other motor) spinning. So don’t freak out – it’s normal. The voltage trace, of course, drops with the load of the starter applied. That’s the battery load test and should stay above 8.5 volts on a healthy battery. Yeah, I know, loaded battery voltage is taught in schools to be no less than 9.5 volts but again, the scope is so fast it captures the nanosecond drop you see on the capture.

            C – Those two peaks you see in the current pattern are two cylinders pushing their way through their compression strokes. That’s a great tool all on it’s own and I’ll share that with you in a bit. Look at the voltage pattern and you’ll see the same thing. This section of the pattern is the engine spinning over but it hasn’t yet started. A good engine should start relatively quick and this one does, running on it’s own in less than a second. How can you tell? Look closely where the pattern shifts slightly downward and the peaks are nearly unnoticeable. That’s the point of ignition! The slight drop is actually a positive current draw, and the start of the battery charging process. The battery has been depleted and it’s initial current demand is high but it quickly settles down to a more reasonable level. More on that when we discuss  “E”.

            D – Back to the voltage trace. You can see the same thing happening here as we did in the current pattern. As soon as the engine starts, output from the alternator starts replenishing the battery. It doesn’t take long to reach the charging system voltage level we’re all used to seeing; 13.5 to 14.5. volts. Take a moment to look back at the OCV (Open Circuit Voltage) recorded at the beginning of the pattern and you’ll have your three voltage points you are used to seeing for assessing the overall health of the battery and charging system.

            E – As the charging system replenishes the battery, you’ll see this section of the current pattern begin to return to the “0” point, but not quite. Remember, this is the overall current flowing in or out of the battery -  a net amount equal to system demand less system output. It should always be a net +3 amps or so, and should stabilize within a few minutes. Too much positive current flow (+5 amps or more) indicates a battery with a bad cell, a common problem that might not be caught using conventional testing methods but a problem that just might explain that car that’s been eating alternators. Why? Excessive current demand on the alternator results in higher heat loads and that’s a leading cause of premature alternator failure.

The test itself takes only a few minutes but provides a ton of information for the time spent. Take a look at Figure 2 as an example of a system that has a problem, and then imagine sharing the two images with your customer!

Another look at starter current
Go back and take another look at “C” in Figure 1. Remember, I told you those two peaks were two cylinders pushing their way up against the pressure generated in the compression stroke. Electric motor current draw is dependent on two things; the condition of the motor itself and the load it’s asked to work under. So, doesn’t it make sense that a cylinder low on compression would make less work for the starter and reduce the current draw in the process? The only difference is that I’ve disabled the engine so it can’t start. This is called a “relative compression” test and it is a quick way of gauging engine health when you’re troubleshooting a driveability problem. Even my first scope, the UEI ADL7100, was fast enough to catch cylinder variations as low as 10 percent. Setting up for the test is the similar to the battery test we just went over but you only need one channel and your high current amp clamp. Disable the fuel system to prevent the cylinders from being washed down and hold the throttle wide open, just as you would when performing a conventional compression test.

Keep in mind the term “relative” too. It is possible to have what appears to be a good pattern with equal peaks, only to have a cam timing issue that is causing low compression across the board. Typically, in that case, the peak to valley current for each peak will be lower than normally seen. And yes, there is a way to use your scope to check the cam timing! Hang on, we’ll get to it.

Fuel delivery testing with your scope
One of the very first tests I learned to perform with my scope was a fuel pump running current test, often just called a “current ramp.” Rather than use a high amp current clamp, though, you’ll need to use one capable of reading smaller current levels – it’s low amp cousin. Make the investment. It is a tool with a lot of uses!

And so does this test. To get a current reading from the pump, you’ll need to get your current clamp around a wire feeding the fuel pump motor. The easiest place to get this access is at the fuel pump relay. Make yourself a fused jumper wire but be sure to use ends that match the pins on the relay to avoid damaging the socket. If you can’t get to the relay, look for the fuse that feeds the pump and use your jumper lead there in it’s place. You can buy “fuse buddies” designed for this job from a variety of aftermarket sources if you choose.

With the jumper in place and your current clamp around the jumper wire, set up  your scope to read 0-10 amps on the horizontal scale (remember, if your scope doesn’t allow scaling in amps, you’ll need to use the millivolt to amp conversion scale that came with the low current amp clamp), and set the time base at 10 milliseconds per division (for a total sweep of 100 milliseconds). Now turn the pump on by turning the key to “on”. On most vehicles, the pump will run for about two seconds – more than enough time to get a pattern.

You’ll see something like the capture in Figure 3.  The pattern should be relatively uniform and horizontal. If the engine is running, you may see it rise slightly up and down as the AC component in the charging system’s output influences the current flow. I’m focused on the shape of the peaks, though. They should look equal and sharp, as the example pattern does. If there are irregularities in the peaks, with any shallow or cut off, it’s an indication that the motor’s commutators and brushes are worn and is worthy of replacement before it fails entirely.

But that’s not all this pattern has to tell us. With a little effort, you can use the scope’s rulers to measure the time between revolutions by identifying the pattern’s repetitions, usually every 8 or 10 peaks. The speed of the pump, combined with the total current draw, can provide clues in the health of the fuel delivery system itself. A rough rule of thumb is 1 amp of current draw for every 10 psi of fuel system pressure. Let’s say the vehicle we’re testing has a specified fuel system pressure of 55 psi. I would expect to see a current draw somewhere around 5-6 amps. Pump speed is a learned value that you get from doing dozens of these tests but I would feel comfortable with a pump speed range of 5000-6000 rpm. Again, these are rough values and not every pump will fall into these numbers. The idea here is to perform a quick test that provides a lot of information and if I see something that doesn’t look right, I’ll dig deeper.

Back to the example. If the test pattern shows a slower pump with a higher current draw, I would suspect that there was some form of restriction downstream that was making the pump work harder. Perhaps it’s time to check that fuel filter! If the pump is spinning way fast and drawing little current, then I would think that the pump was starved for fuel. You know it is possible to have a car that starves the pump, even if the tank itself is full of gas, right? And how many times have you found an empty tank even though the fuel gauge said it was half full?

You can also use this method to see if the lack of fuel at the rail is caused by a bad pump or something else. Just grab the amp clamp and see if the pump is drawing current! Beats banging on the tank with a hammer, doesn’t it? And you won’t destroy a perfectly good pump in the process.

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