672 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Alternator Diode Tests Most analyzers are capable of checking alternator diode condition. Th e scope will display the alternator’s voltage output. If the alternator diodes are good, the pattern should be almost fl at with little ripples. Th is was discussed in Chapter 26. Electronic Fuel Injector Tests Oscilloscopes can also be used to check injector operation in an electronic fuel injection system. Refer to equipment and service manual instructions for details. Figure 34-24 shows a typical waveform for a conventional fuel injector. If your scan tool displays a misfi re code for a specifi c cylinder and other conditions point to a fuel emissions-related problem, inspect the fuel injector pulse waveform for the specifi ed cylinder number. A sticking or clogged injector or an ECU circuit fault could be causing the misfi re. Use a scope to make sure the questionable injector is opening and closing normally by studying the trace on the oscilloscope of the injec- tor winding. Closely compare the actual injector voltage waveform to a known good pattern. Some advanced scopes have speci- fi cation injector waveforms in memory for easy comparison. Analyze fuel injector operation by noting the length in mil- liseconds, slope, dip, and other characteristics of the injector waveform. Detailed information on injector waveforms are covered later in this chapter. Oxygen Sensor Tests An oscilloscope can be used to check the signal pro- duced by the oxygen sensors. See Figure 34-25. By compar- ing specifi ed voltage (zirconia sensor) or resistance (titania sensor) values to scan tool values, you can determine whether the sensor, the wiring, or the ECU is at fault. If you are using a dual-trace scope or a scan tool with a scope function, you can compare the voltage signals from the primary oxygen sensor (located before the catalytic converter) and the secondary oxygen sensor (located aft er the catalytic converter). If the signals from the sensors are similar, the cata- lytic converter may not be working properly, Figure 34-26. In Figure 34-27, oxygen sensor voltage and MAP sensor voltage waveforms are compared. By disabling an injector (injector one in this example), you can monitor how the oxygen and MAP sensors react to the leaned out fuel mixture. When the fuel injector is shut off , the oxygen sensor voltage normally drops and the MAP sensor voltage increases. If this does not happen, there might be a problem with the fuel injector or one of the sensors. Figure 34-28 shows two oxygen sensor voltage signal patterns. When the engine throttle is snapped opened and Figure 34-24. Conventional fuel injector waveform. Time in Milliseconds 50 40 30 20 10 0 1 2 3 4 5 6 Figure 34-23. Knock sensor signal can also be analyzed with a scope. A—Connect the scope lead to the knock sensor and the other lead to the ground. Tap the engine with a small hammer or wrench to produce an output signal. B—Knock sensor should produce a normal frequency and amplitude signal when the engine is tapped. (Fluke) Tap Lightly Next to Sensor Ground Wire Disconnected Scope A Knock Sensor Amplitude Changes B Frequency Change Voltage