Oscilloscope Patterns and Waveforms

Fuel Injector:

BACKGROUND AND APPLICATION INFORMATION:
Saturated switch injector drivers are used primarily on multiport fuel injection (MFI, PFI, SFI) systems on domestic and imported vehicles alike. This type of injector driver is used on systems where the injectors are fired in groups and on systems that fire the injectors sequentially. It is not used very much, if at all, on Throttle Body Injection (TBI) systems. Determining, or "reading" injector on-time from the waveform of a saturated switch type injector is fairly easy. The injector on-time begins where the Powertrain Control Module (PCM) grounds the circuit to turn it on (see the left pointer box on the waveform). The injector on-time ends where the PCM opens the control circuit. Since the injector is a coil, when its electric field collapses from the PCM turning it off, it creates a spike (see the right pointer box on the waveform).

How to "read" injector on-time: First, count the number of divisions between where the PCM turned the injector on and where it turned the injector off. In the database waveform example, the injector is on for just less than two divisions, 1.96 divisions to be exact. Since the time base is set at 2 ms/div on the sample waveform, multiply 1.96 divisions times 2 ms/div; the injector was turned on by the PCM for 3.92 ms. Therefore, 3.92 ms is the injector's on-time. This figure can be used to see if the Feedback Fuel Control System is doing its job. The mixture can be manually enriched by adding propane or leaned out by creating a vacuum leak, while looking for the corresponding injector on-time change.

NOTE: Some DSO's display the injector on-time numerically on the display with the waveform, which makes manual calculations a thing of the past.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the negative battery post or engine block.
2. Connect the "CH1" probe to the injector control signal from the PCM (the pulsing side of the fuel injector). Use a wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 10 V/div.
4. Set the time base to 2 ms/div.
5. Select "GND" coupling and position the trace on the second division line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" acquire mode so the peak heights will be accurate and bad injector drivers will show up better on the display.
8. Set "TRIGGER" mode to "AUTO" to begin with, but as soon as a waveform appears on the screen, change to "NORMAL" mode and use the up/down buttons to raise or lower the trigger level until the waveform is steady and positioned correctly on the display. Set "TRIGGER SLOPE" to negative and position the trigger level where the PCM turns the injector on.

FOLLOW THIS PROCEDURE -- RICH/LEAN COMPENSATION TEST:
1. Start the engine and hold throttle at 2500 RPM for 2 - 3 minutes, until the engine is fully warmed up and the Feedback Fuel System enters closed loop. Verify this by viewing the oxygen sensor signal on the DSO.
2. Shut off A/C and all other accessories. Put vehicle in park or neutral. Rev the engine slightly and watch for the corresponding injector on-time increase on acceleration.
3. Induce propane into the intake and drive the mixture rich. If the system is working properly, the injector on-time will diminish, or get smaller, to try to compensate for the rich mixture (high O2 voltage).
4. Create a vacuum leak and drive the mixture lean. If the system is working properly, the injector on-time will increase, or get larger, to try to compensate for the lean mixture (low O2 voltage).
5. Raise the engine to 2500 RPM and hold it steady. On many systems the injector's on-time can be seen modulating (changing) from slightly larger to slightly smaller as the system controls the mixture. Generally, the injector on-time only has to change from 1/4 (0.25) to 1/2 (0.50) of a millisecond to drive the system through its normal full rich to full lean range.

IMPORTANT NOTE: If propane is added or a vacuum leak created while looking for the corresponding injector on-time change, but the on-time isn't changing, one of the two following things may be happening:

^ The system is operating in an "open loop" idle mode. Some newer (mostly 1988 and newer) vehicle systems temporarily or completely ignore the O2 sensor signal at idle. There are various system design reasons for doing this, but it may provide a surprise the first time it's encountered. Raise the engine to about 1800 RPM and try adding propane or create the vacuum leak again. Most systems will go back into closed loop before this RPM so the test should work.
^ The oxygen sensor may be bad. If the O2 sensor can't "see" the mixture change, the PCM can't either, so the injector on-time will not change. Time spent performing the propane and vacuum leak procedure with a bad oxygen sensor is wasted time.

Good Fuel Injector Pattern (Typical Saturated Switch Type):

EXPECT THIS WAVEFORM RESULT
When the Feedback Fuel Control System can control fuel mixture properly, the injector's on-time will modulate (increase or decrease) according to driving conditions and oxygen sensor input. Generally, the range of injector on-time run from about 1 - 6 ms at idle to about 6-35 ms under cold cranking or Wide Open Throttle (WOT) operation.

The influence of oxygen sensor voltage input to injector on-time is relatively small when compared to driving condition demands. The oxygen sensor voltage input to the PCM functions more as a "fuel trim fine tuning" instrument than the leader of the band. Most of the injector on-time is calculated using the MAF or MAP, RPM, and other inputs to the PCM. The oxygen sensor voltage input to the PCM just polishes the effort to increase catalyst efficiency. Although the O2 sensor is responsible for only a relatively small change in the injector pulse width, that small change may make the difference between good driveability or poor driveability; an emissions test pass or an emission test failure.

IMPORTANT NOTE: Shorted injector windings will usually make the coil release spike(s) shorter or may even make them disappear. Coil release spike height varies with vehicle make and engine family. A reference waveform is the best source of comparison. Normal ranges are from about 30 volts to 100 volts. Some injector spike heights are "chopped" to about 30 - 60 volts by clamping diodes. These are usually identified by the flat top on their spike(s) instead of a sharper point. In those cases a shorted injector may not reduce the spike height, unless it is severely shorted.