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Transcription

1 RVIC HULU St MAX = 37.S MIN =-&&&. PWID= SELECT KH1 CH2 Rt PFI/SFI INJ + Chi dc im Welcome to a new Import Service feature, called "Waveform Of The Month." Each time this feature appears in Import Service, we'll give you the lowdown on a single electronic waveform produced by an automotive electronic component or circuit. We'll tell you why it looks the way it does, what makes it look the way it does, and what it means when it doesn't look the way it should. When you're through reading each "Waveform Of The Month" installment, you'll know more about the waveform than it knows about itself. As much as we know you like to read electronics articles, we also recognize that you've got other things to do with your time, too. This first install ment will be a little longer than the articles that will follow because we need to establish some ground rules to make sure we all understand each other. So without further ado, here are the ground rules: All components used to construct electronic cir cuits have resistance. The effect that resistance has on a circuit is de pendent on the amount of current that flows through the circuit. (Remember Ohm's Law?) A lab scope is nothing more than a very precise voltmeter that is capable of measuring voltage over time. A lab scope reads the voltage drop across the por tion of the circuit that is located between the test leads. The voltage displayed on the lab scope represents the voltage at the red test lead, minus the voltage at the black lead. When examining waveforms for diagnosis, there are two types of information available: November 1996 IM

2 System Information represents the information the system needs to operate. In the case of an injec tor, system information would be the pulse width of the injector signal. Circuit Information can be used to determine the integrity of the circuit. This involves examining the shape of the waveform and is based on our under standing of system strategy and how the circuit components work together to shape the waveform. Peak and Hold Injectors This month's Waveform of the Month is the Peak and Hold injector waveform. I like to use this wave form in my training classes because it illustrates many principles. Figure 1 is a simplified view of what you would expect to see on the wiring dia gram for a peak and hold injector. This diagram uses basic electronic symbols to represent the actual devices in this circuit. Looking at this diagram we can see that the cir cuit is powered all the time and is activated by completing the circuit to ground. Let's examine the key components of the circuit. Power Source The power source in the actual circuit might be delivered via a switch like the ignition switch, a relay, or directly from the battery via a fusible link. Check your diagram to find out how your injector circuit is powered. Coil The coil symbol is used to represent the injector because the circuit uses the coil's ability to store and release energy. So electronically-speaking, that's what an injector is just a glorified coil. Don't mix this symbol up with the resistor symbol. The resistor is represented by a saw-tooth symbol, rath er than the loops that represent a coil. Coils act similar to the flywheel on an engine. The flywheel absorbs and stores the energy of the engine. In return, the engine takes a little bit longer to rev up. But when the engine is turned off, the fly wheel releases the stored energy and causes the engine to continue to spin until the stored energy has been dissipated. Coils absorb the current flow in a circuit, and store it as magnetic energy. As the coil becomes sat urated, the current through the circuit increases. When the current flow is interrupted, the coil releases the built-up energy. Since the circuit is open and there is nowhere for current to flow, it is released in the form of a voltage pressure pulse a spike! Driver The circuit driver inside the computer is shown in Figure 1 as a switch. Most diagrams do not give you much more information then this. As you will see, there is much more than simple switching going on in the case of the peak and hold driver. Peak and Hold Driver Characteristics Integrated circuits and transistors are often used to switch electrical devices on and off. These com ponents can do the job very quickly and cleanly. They also have a very long service life if they are Figure 1 Coil (Injector) Power Source Ground IMPDRT5EF November 1996

3 Waveform of the Month INJECTOR Peak Figure 2 & Hold4.4amp not abused. Mechanical contacts can easily become corroded, and also tend to bounce every time they are switched on or off. The driver in a peak and hold circuit is more advanced than most. This driver incorporates current sensing and control, as well as the basic switching functions. It is designed to open the injector very quickly by allowing maximum current flow. When the current reaches a predetermined level (PEAK), the current is reduced by about a 4:1 ratio. This allows just enough current flow to keep the injector open (HOLD). The waveforms for this article were captured on a standard TBI injector with a resistance of 1.2 ohm and an inductance of about 2.0 mh. In this case the PEAK current is 4.4 amps and the HOLD current flow is 1.6 amps. 60 v 44 v Figure 3 INJECTOR Peak & Hold4.4amp 3.8mS Shaping a Waveform Whenever the voltage level changes on a lab scope, the voltage, current, or resistance must have changed in the cir cuit. This also means that some action has occurred in the cir cuit to change one of these ele ments. Refer to Figure 2 as we describe the factors that shape the peak and hold waveform: Driver Open 14.4v Ov : 1 1 : * : 12.8v 1.6v The waveform starts by dis playing source voltage. At this point the circuit is OPEN be cause the driver is OPEN. The driver is located between the test leads, so the resistance be tween them is at the maximum (infinity). The circuit voltage is the same as the power supply voltage. We have maximum re sistance between the test leads, no current flow, and the signal is at source voltage. 1mS/Div Driver Closed Next, the driver completes the 12 November 1996 IMF^OFHT! ERVICZS

4 circuit to ground and opens the injector. The resistance between the scope test leads is at its low est point because the circuit is complete. The voltage displayed on the waveform drops, and cur rent begins to flow through the circuit. Current Flow Builds What causes the ramp in the waveform during the PEAK peri od? Earlier we mentioned that all components in automotive circuits have resistance. The ef fect of this resistance depends on the amount of current flow. Okay, so how does the coil effect the current flow? It causes it to build slowly in the circuit. As the current flow builds, so does the voltage buildup across the driver which causes the ramp in the waveform. At this point, the circuit volt age is the same as the supply voltage. The resistance between the scope test leads is at its low est point. Current flow is in creasing to the peak level of 4.4 amps, due to the effects of the injector coil. current flow, also because the circuit is open. Spike Suppression Phase The top of the spike in our peak and hold waveform is flat. This indicates the presence of a Zener diode in the circuit to suppress the spike and protect the driver from high voltage surges. When the spike voltage reaches the switching voltage of the Zener diode, the diode com pletes a circuit to ground. This allows the voltage buildup to be released as current flow to ground (through the diode). The Zener diode also affects the time it takes for the coil field to dissipate. If there were no suppression diode in the peak and hold injector circuit, the spike would reach a higher volt Repair Information age and would dissipate more quickly. Conversely, using a Zen er diode with a lower switching voltage will prolong the collaps ing field dissipation process. At this point in the signal, we still have the high voltage caused by the coil field collapse. But there is a very minimal amount of current flow through the Zener. The driver remains open, so there is no current flowing through the rest of the circuit at this time. Injector Hold Phase After the initial voltage spike caused by the driver temporarily opening the circuit, our wave form enters the HOLD phase. Close inspection of this section of the waveform reveals that it is 1.6 volts less then the supply Driver Opens when you have 51 of the best ASE Certified We know that a coil produces a spike when its magnetic field collapses, and that's the next thing we see on the peak and hold injector waveform. The field collapses when the driver opens the circuit. The driver "goes open" when a current peak of 4.4 amps is reached. The driver will stay open long enough to release some of the built-up energy in the injector coil, but not long enough to al low the injector to close. The period of this spike is approxi mately us in our sample waveform. As the coil field collapses, it creates a voltage surge in the cir cuit. Resistance between the scope test leads is high because the circuit is open. There is no technicians constantly tuning it up, how do you a thinks An,,.» The new Mitchell Repair Manuals. Call your local Mitchell Representative and put them through your own paces in your shop. Or call us toll-free at (in the 619 area, call , extension 6313). Mitchell InternationalMitchell Repair Information Division Circle No. 106 on Reader Service Card IMPORT SERVICE November 1996

5 Waveform of the Month voltage to the circuit. In this case, circuit voltage is 14.4 volts and the HOLD voltage is 12.8 volts. The current level should be amps. There's just enough energy flowing in the circuit to maintain the magnetic field inside the injector coil. This keeps the injector open and allows fuel to continue flowing. Something has caused a rise in circuit voltage, compared to the PEAK section of the waveform. Did the voltage or current increase? We know this didn't happen because the current level actually decreases during the HOLD section of the signal. The circuit voltage has not changed, current is flow ing, the coil is not shorted, and we know our scope is referencing a good ground. So what's happened? The resistance between the test leads has in creased. Resistance at this point is greater than dur ing the PEAK section, but less than the DRIVER OPEN section. Injector Close The last spike marks the INJECTOR CLOSE. Often there is a small hump at the base of the falling edge. This hump marks the actual moment the injector pintle reached the closed position. As the injector closes, the coil field collapses for a second time, which causes a second voltage spike that is greater than the supply voltage. Resistance between the scope test leads is high because the cir cuit is open once again. There is no current flow through the circuit, also because the circuit is open. Injector Circuit Abnormalities circuit to reach a current flow of 4.4 amps (the cir cuit's designed maximum current flow) can be an excellent indicator of circuit integrity. What kinds of problems would you expect to find if the time required to reach a current flow of 4.4 amps were to decrease? Raising the supply voltage to the circuit would cause the current to build more quickly, which would shorten the time needed to reach 4.4 amps of current flow. Lowering the circuit resistance would have the same effect. A short in the injector coil winding would be the only way the circuit resistance could decrease. If the injector coil were shorted, this would decrease the coil's overall resistance and reduce the "coil effect." Current would build faster and the spike would be weaker when the injector driver was opened. What are some of the things that could increase the time it takes to get to 4.4 amps? A decrease in the supply voltage level would in crease the time needed for the injector current flow to build to 4.4 amps. Adding resistance to the circuit would also in crease the time needed to reach 4.4 amps. Ad ditional resistance in the circuit could be caused by a corroded connection or perhaps a malfunction in the coil windings. Injector Driver Problems We've considered several injector circuit prob lems that are not directly related to the injector dri- The total ON time for the sample peak and hold injector waveform in Fig ure 3 is 5 ms. The ON time is governed by the PCM, and is the result of the sensor information the PCM receives, along with its internal programming logic. However, the time it takes for the injector to go into HOLD mode is a func tion of the circuit driver and the current flow through the circuit. We can draw many conclusions about the integrity of the circuit, just by looking at its waveform. The integrity of the cir cuit and its components determine the current flow, so the length of time that it takes for the injector 60v INJ 89 GM C L 0v_: 500uSec/Div 10 Volls/Div Figure 4 14 November 1996 IMFH3FTT SSRVKZe

6 Waveform of the Month Figure 5 ver. However, many things can also go wrong with the driver itself. For example, a bad driver might be unable to trigger the current control mode, eliminat ing the first spike in the waveform and extending the injector on-time beyond the initial 1.2 ms shown in Figure 3. A partially failed driver could also cause the HOLD mode voltage signal to be erratic instead of steady. Another common problem are oscillations that occur during the HOLD mode (Figure 4). Some technicians report driveability problems associated with these symptoms, while others do not. A major driver manufacturer states that the HOLD mode should be oscillation-free. If oscillations are pre sent, they recommend the installation of a 0.01 uf capacitor connected between the driver and ground. Other sources recommend a 0.1 uf capacitor piggy backed across the injector (Figure 5). A word of caution: A frequency-modulated cur rent control injector waveform may look a lot like an oscillating peak and hold injector waveform. This is an easy mistake to make, especially if your scope is not capable or is not adjusted correctly to display the high frequency of the modulating por tion of the signal between the spikes. You certainly don't want to install a capacitor on a frequency-modulated current control system. It doesn't need one. So if you are not sure which sys tem you are dealing with, consult a reliable manual. Another thing you can do is to set your scope up for a fast enough sweep speed to determine if the signal between the spikes are actually digital instead of a sine wave. Differences and Similarities The basic logic that operates peak and hold cir cuits applies to most systems. However, the values listed in our examples do not apply to all systems. Some systems switch to the HOLD mode at 2.4 amps, for example. The time it takes to reach the current peak will also vary. In fact, I have seen it vary as much as +/- 0.3 ms. Just remember to keep your wits about you, use your information resources, expect the unexpected, and always relate what you see to the problem you are looking for! By Jorge Menchu 16 November 1996 IMPOFTT SSRVICZS