Chapter 34 Advanced Diagnostics 679 Copyright by Goodheart-Willcox Co., Inc. As the magnetic fi eld continues to collapse, voltage drops to the level supplied to the injector (point A) and the pintle returns to the closed position, stopping fuel fl ow. Notice that injector on time is measured from point B where the ECU grounds the circuit to point D where the ECU opens the circuit. Typical saturated switch injector circuit on times vary from around 2 to 9 milliseconds. Compare actual on time with specifi cation on time, or compare the questionable on time with that of other good injectors in the engine. Injector, circuit, and ECU problems will show up when the trace has major variations from normal. If a trace is supposed to be square at one point, it must be perfectly square. If a part of the trace is supposed to have a spike (injector closing inductive kick), it should have a spike of suffi cient voltage. Th e PNP fuel injector waveform is similar to the saturated switch waveform, except that because the ECU completes the injector circuit by making a connection to the positive side of the voltage source, the waveform is inverted. See Figure 34-36. Peak-and-Hold Fuel Injector Waveform A peak-and-hold fuel injector circuit uses a high amount of current to open the injector and a low amount of current, slightly less than one amp, to hold the injector open. Th is type of circuit has less than 12 ohms of resistance to provide the high amount of current to pull the pintle from its seat and open the injector. Th e ECU provides the current limiting resistance needed to hold the injector open. A peak-and-hold fuel injector waveform will, there- fore, have two induced voltage spikes: one when the ECU lowers current by switching to the limiting resistor circuit and another when the ECU shuts off current fl ow to the injector. Refer to Figure 34-37 as the waveform for this type of injector is discussed. At point A, voltage (about 13.5 volts) is applied to the injector. Th e circuit is open, and the injector is off . Th is part of the trace should be fairly smooth. At point B, the ECU grounds the injector circuit, allowing current to flow through the injector coil wind- ings. The voltage drops to almost zero (due to the small resistance in injector circuit). The injector is on and the pintle is pulled up. If a sharp drop toward zero volts is not produced, the power transistors or drivers are weak and failing. Point C shows the voltage drop across the injector windings and circuit. Th is trace should be almost at zero volts. If this trace reaches zero volts or ground, the injector windings or wiring may be shorted to ground. If the trace is located several volts above ground, the injector circuit or scope test lead has a bad ground. At point D the ECU switches to a current limiting cir- cuit to prevent the fi ne wire in the injector windings from overheating and burning up. Th e magnetic fi eld in the injec- tor windings responds to the change in current fl ow with an inductive voltage spike, (point E). Aft er the inductive volt- age spike, the injector is held open (point F) at a lower volt- age level with about 0.6 amps fl owing through the inductor coil windings. As the injector is held open, fuel continues to spray into the engine. At point G, the ECU opens the circuit, inducing another inductive voltage spike (point H). As the pintle snaps into the closed position, a small ripple or hump occurs in the waveform. Voltage returns to charging voltage, and the injector is turned off . Figure 34-36. PNP fuel injector waveform. Time ECU Closes Positive Side of Circuit Injector On Time Inverted Inductive Voltage ECU Opens Circuit Voltage Supplied to the Injector Figure 34-37. This is a waveform for a peak-and-hold injector circuit. Note how voltage drops to almost zero when the ECU grounds the circuit (B). Then, when current reaches maximum for the injector windings, the ECU switches the injector to a current-limited circuit (D). The change in current flow causes an inductive voltage spike (E). A inductive spike also occurs when the ECU opens the circuit (H). Time Inductive Voltage Due to Switch to Current Limiting Circuit E Injector On Time H Inductive Voltage Due to ECU Opening the Circuit I Pintle Snaps to Closed Position F Injector Held Open G ECU Opens Circuit D ECU Switches to Current Limiting Circuit C Injector Opens B ECU Ground Circuit Voltage Supplied to Injector A Voltage Voltage