Using a Digital Storage Oscilloscope to test fuel injectors adds depth to your fuel system diagnosis.
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.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.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.Sidebar 3: