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Inside the Wave

Using a Digital Storage Oscilloscope to test fuel injectors adds depth to your fuel system diagnosis.
Monday, September 1, 2008 - 00:00

Using a Digital Storage Oscilloscope to test fuel injectors adds depth to your fuel system diagnosis.

Fuel Injectors fuel pumps fuel pump diagnosis fuel pump diagnostics digital storage oscilloscope DSOs using DSOs fuel systems repair shop repair shops automotive aftermarket

Modern scan tools, both OEM and aftermarket, have come a long way. More data parameters to use, faster refresh rates and better graphing all have played a role in making scan tool diagnostics easier. But there are still those occasions when I want to see what is happening directly without going through a "middle man," and those are the occasions I use my scope.

A Digital Storage Oscilloscope (DSO) is a high-powered voltmeter that allows a user to monitor a voltage signal over time, viewing those results in a graph format on the viewing screen. It is excellent for checking electrical circuits for intermittent failures, voltage drops and even mechanical function. It is also a good choice when monitoring a circuit that operates in milliseconds, like ignition systems or today's topic, fuel injectors.

A DSO is nothing more than a voltmeter that graphs the voltage signal over time. To set up your scope for viewing an injector pattern, set the time divisions (horizontal axis) at 2 milliseconds/division, and the voltage divisions (vertical axis) at 20 volts/division. One advantage of the scope is the ability to adjust these two scales to take a closer look at any particular part of the pattern, and we'll do that as we go along. Now, connect your scope's leads. The negative lead should attach to the battery negative post as close to the battery as possible.

Attach the positive lead to the groundside of the injector you want to test, as close to the injector as you can get. Does this lead setup sound familiar? It should, as it's the same lead setup you would use in checking this circuit for voltage drop. With the leads attached and the circuit running (engine idling), you should get a pattern similar to the one shown in Figure 1.

Let's break this pattern into sections and take a closer look at what is going on in each. In Figure 2, the highlighted section (A) is a measurement of system voltage to the injector and through the injector winding. Since the circuit is not closed at this time, there is no current flow and no voltage drop through the injector itself. This voltage should be within 0.50 volts of system voltage measured at the battery. If not, take a closer look at the power side of the injector circuit up to and including the injector coil windings.

Section B in Figure 3 is next. The ECM has now completed the path to ground, energizing the injector. This is the start of the ECM's pulse width command. You can see the voltage signal is pulled down to nearly 0.0 volt...but not quite. I'm going to jump ahead a bit and show you all of this a little closer. Take a look at the highlighted area in Figure 6. I've reduced my voltage division to 0.10 volt, and my time division to 500 microseconds to zoom in on section B from Figure 3. I first take a look at the end of the downward trace at the beginning of the pattern. Where it stops above the "0" line is the amount of voltage drop in the circuit — something happening too fast to be measured with a DVOM. Depending on the type of driver used in the ECM, this drop can be as high as 0.70 to 0.80 volt. Compare the drop on each injector, and look for values outside the average. If you find any, trace the cause as you would any other voltage drop.

Now look closely at the voltage trace. Keep in mind that we are looking at something that for all practical purposes is happening instantaneously. As the current flows through the injector windings, a magnetic field starts to build. See how the voltage trace curves slightly upward? As you'll see in a minute, this curve mimics the amperage pattern almost exactly. It is a result of the Counter Electromotive Force being created in the windings by the magnetic field that is building.

The CEMF adds "resistance" to the circuit and creates a voltage drop of its own. Variances in this curvature between injectors can indicate a weak magnetic field and possible shorted injector winding. As soon as the magnetic field is strong enough to overcome the spring pressure on the injector pintle, the pintle starts to open. On some injectors you will actually see a small "spike" in this trace indicating this event. This spike can be a diagnostic aid when looking for sticking injectors by comparing when they occur over time and how the opening time compares between injectors.

Back to Figure 3 and section B. The vertical rise at the end of this section marks the end of the ECM's commanded pulse width. With the current flow stopped, the magnetic field now collapses and creates flyback voltage in the injector coil windings. This flyback voltage, indicated by C in Figure 4, is important to proper operation of the injector. Some designs use a Zener diode to "clip" this voltage, some don't. In all cases, the design idea is to control the closing of the injector pintle to prevent premature wear that would be caused by the pintle hammering into its seat. It is also an indication of the health of the injector windings. Look for anomalies in this pattern between the injectors of the vehicle you are testing. Any traces outside of the average are cause for closer inspection.

Last is Figure 5 and Section D. The current is off and the magnetic field is dissipating. Notice the small "hump" in the downward trace? That is the actual closing event as the pintle passes through the injector windings, creating a small voltage of its own. If you have both the opening and closing spikes on your pattern, you can measure the actual opening time of the injector and compare it to the commanded opening time. Again, look for measurements on a given vehicle that are outside the average for all the injectors.

While a scope is nothing more than a graphing voltmeter, its ability to measure accurately at extremely fast speeds makes it an ideal companion to your scan tool. Not only can you complete circuit integrity tests that your DVOM can't match, you can actually see the mechanical aspects of the injector at work. And that may just help you nail down that fuel control issue a little faster.

Sidebar 1

Are You Ready For Piezo Injectors?

Piezo crystal injectors are in use today for both diesel and gasoline direct injection systems. The first gasoline direct injection injector was developed by Continental AG and uses a stack of Piezo crystal wafers that expand when a high voltage is applied. This expansion opens the injector and allows fuel flow.

The most notable advantage of this design is the ability to open the injector repeatedly in a very short time frame. The actual injection "pulse width" is typically 0.20 ms. Compare that to electro-mechanical systems that use a pulse width of 2.5 ms to 3.5 ms at idle.

Can you still test them with a scope? Sure, but I think experience will be the key here in knowing good from bad. You can still test the circuit for proper supply voltage and good ground control, and the current pattern may prove to be an indication of the "health" of the wafers themselves. As with most scope applications, look for the anomalies that may indicate an individual problem.

Sidebar 2:

It's Not Just Volts and Amps

The FirstLook sensor is a pressure/vacuum transducer that converts pressure changes to a voltage signal the scope can "see." This opens a whole world of diagnostic possibilities by measuring crankcase vacuum, intake manifold vacuum, exhaust backpressure and much more.

By placing the pickup on the vacuum nipple of the return system's fuel pressure regulator, you can measure the pressure changes caused by the individual opening of the fuel injectors. It's a complicated pattern and will take some practice, but once you understand it, you can identify fuel injectors that aren't flowing as they should.

Sidebar 3:

Don't Forget Current

While I focus on the voltage pattern in the main text, don't neglect current. Current is what actually makes a circuit work; too little or too much, and there's a problem. The advantage to checking current is that it can be tested nearly anywhere in the circuit. This makes hook-up as easy as locating the fuse that supplies the power to the injectors.

Because the injectors typically share the power supply, you can read all the injectors at once. Missing current ramps, or current ramps that don't match the rest, indicate a problem that you can then zero in on. Zooming in on an individual current pattern will reveal the same little spike caused by the pintle opening as I discussed in the main text. Often, you will see it here when you don't see it on your voltage trace. Put the two together, and you can measure actual on time for comparison.

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