TSB : Bosch Type Peak and Hold Injector/Driver Test

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Bosch Fuel Injector: BACKGROUND AND APPLICATION INFORMATION: Bosch type Peak-and-Hold fuel injector drivers are found on a few European models with MFI systems and some early to mid-1980s Asian vehicles with MFI systems. Bosch type Peak-and-Hold injector drivers (inside the Powertrain Control Module (PCM)) are designed to allow approximately 4 amps to flow through the injector winding, then reduce the flow to a maximum of about 1 amp by pulsing the circuit on and off at a high frequency. This differs from the other Peak-and-Hold variety because the other current limiting method uses a "switch-in" resistor to achieve the same result. One type lowers the current with a resistor and the other pulses the circuit on and off. Generally, a solenoid requires 4 times the current to initiate (or start) mechanical movement than it does to hold the component in place. Hence, the name "Peak-and-Hold." Peak-and-Hold injector drivers get their name from the fact that the PCM uses 4 amps to raise the injector pintle, but only 1 amp to hold it open. The example waveform is from a good Bosch type Peak-and-Hold injector driver and injector. From left to right, the trace starts at battery voltage, indicating the injector is off. When the PCM opens the injector, it provides a ground to complete the circuit. The PCM continues to ground the circuit (hold it at zero volts) until it detects approximately 4 amps flowing through the injector. When the 4 amp "Peak" is reached, the PCM reduces current flow by pulsing the circuit at high speed. The PCM continues the "Hold" operation for the desired injector on-time, then it shuts the injector off by stopping the pulsing and opening the ground circuit completely. This creates the spike at the right of the waveform (see right pointer box). Determining, or "reading" injector on-time from the waveform of a Bosch type Peak-and-Hold injector is as follows: The injector on-time begins where the PCM grounds the circuit to turn it on (see the left pointer box on the example waveform ). The injector on-time ends where the PCM opens the control circuit completely (the release spike to the right). To "read" injector on-time, count the number of divisions between where the PCM turned the injector on and where it turned the injector off. In the Nissan example, the injector is on for just over one division (1.1 divisions to be exact) . Since the time base is set at 2 ms/div , multiply 1.1 divisions times 2 ms/div ; the injector was turned on for about 2 ms , or to be exact, 2.23 ms . Therefore, 2.23 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 enriched manually by adding propane or leaned out by creating a vacuum leak, while looking for the corresponding injector on-time change. 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 (20 V/div may work better on some models with taller spike heights). 4. Set the time base to 2ms/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 the 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 the 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 mixture. Generally, the injector on-time only has to change from 1/4 (0.25) to 1/2 (0.50) of a millisecond in order to drive the system through its normal full rich to full lean range. IMPORTANT NOTES: ^ When adding propane or creating a vacuum leak 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. 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 be surprising the first time this is encountered. Raise the engine speed 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 modulate (change). Time spent performing the propane/vacuum leak procedure with a bad oxygen sensor is wasted time. Good Fuel Injector Pattern (Typical Peak-And-Hold): 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 runs 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 inputs ( MAP , MAF, RPM, etc..). 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 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 emission test pass or an emission test failure. IMPORTANT NOTE: Shorted injector windings will usually make the coil release spike shorter or may even make it disappear. Coil release spike height varies with vehicle make and engine family. Normal ranges are from about 30 volts to 100 volts . A reference waveform is the best source of comparison. Some injector spike heights are "chopped" to between 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. IMPORTANT NOTE: On some Asian vehicles there may be two release spikes, similar to normal Peak-and-Hold type injector drivers. FOR MORE INFORMATION Description of Automotive Signals Labscope Quick Reference Appendix