Oscilloscope Patterns and Waveforms
Bosch Frequency Valve:
Background and Application Information:
Bosch CIS (mechanical multiport fuel injection) systems are found on many European vehicles beginning in the late 1970's and are still in production today. The CIS fuel injection system uses the Frequency Valve circuit to maintain proper fuel control under varying driving conditions.
All Bosch CIS frequency valve waveforms look and operate like the one in this example, except Mercedes Benz frequency valves, which have a different driver, and thus waveform, but operate in exactly the same way.
Mercedes Benz mixture control, and its unique frequency valve waveform, is covered in it own section. The frequency valve signal is the most important output signal in the Bosch CIS Feedback Fuel Control System. This circuit pulses about 70 times per second (70 Hz). The frequency of the pulse doesn't change; it stays at 70 Hz regardless of engine RPM or load, but the on-time (pulse width, or dwell) varies, depending on the oxygen sensor's signal input to the PCM.
In the CIS system, this circuit controls how long (per pulse) the fuel pressure in the lower chamber of the fuel distributor is bled off or reduced. When the lower chamber's fuel pressure is reduced, more fuel can flow through the upper chamber, which richens the fuel mixture. Frequency valve commands from the PCM that oscillate around duty cycles greater than 50% mean the system is commanding a rich mixture in an effort to compensate for a long term lean condition (such as a lean CO adjustment or a vacuum leak). And, conversely, frequency valve commands from the PCM that oscillate around duty cycles less than 50% mean the system is commanding a lean mixture in an effort to compensate for a long term rich condition (such as a rich CO adjustment or excessive fuel pressure).
IMPORTANT NOTE:
Before performing the following fuel mixture (CO) adjustment procedure, the oxygen sensor must be tested and confirmed good. Bosch CIS systems require a good O2 sensor to operate properly. This procedure will not be effective if it is performed with a weak or degraded sensor.
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 frequency valve signal from the PCM (the pulsing side of the frequency valve). 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 "SAMPLE" acquire mode. "SPIKE DETECT" mode may also be used on systems that don't have very much noise, and may be advisable to use if a problem with the PCM driver circuit is suspected.
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) position until the waveform is steady and positioned correctly on the display. Set "TRIGGER SLOPE" to negative.
Follow This Procedure:
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. Shut off the A/C and all other accessories. Put vehicle in park or neutral. Adjust the CO (fuel mixture) by using the special 3mm allen hex tool as per recommended service procedures for the CIS system being serviced, and watch the frequency valve waveform on the DSO.
Good Frequency Valve Pattern (Typical Exc. Mercedes):
Expect This Waveform Result
When the CO (fuel mixture) is adjusted properly, the frequency valve signal should oscillate around 50% duty cycle. The waveform example shown is very close to 50% duty cycle. The duty cycle can be checked by using the cursors on the DSO or it can be done visually by analyzing the waveform on the display of the DSO.
This is easy to do using the visual method. If CO adjustments are set correctly, the waveform's tall spike will be centered between the two vertical drops in the waveform that are created by the PCM grounding the circuit. See the pointers on the waveform to find where the PCM turns the circuit on and off. When the CO adjustment is correct, the tall spike will oscillate slightly from right to left and back again but remain very close to the middle of the two vertical drops in the waveform.
The PCM is oscillating the signal right to left, based on input from the oxygen sensor. When the O2 signal is lean, the PCM grounds the frequency valve circuit for a longer time during each cycle (dwell, or pulse width increases) to compensate for the lean condition (tall spike moves to the right). When the O2 signal is rich, the PCM grounds the mixture control circuit for a shorter time during each cycle (dwell decreases) to compensate for the rich condition (tall spike moves to the left). When the frequency valve waveform is oscillating around 50% duty cycle (w/a good O2 sensor), the system is in proper control of fuel mixture and the catalytic converter can perform optimally.
The waveform should oscillate around 50% duty cycle at idle, 2500 RPM and during normal driving conditions (heavy accels and decels excluded). This will insure optimum Feedback Fuel Control System performance and lowest possible emissions. If the waveform oscillates around 50% duty cycle during one operating mode (for instance idle) but not another, look for non-feedback system problems affecting the system, such as vacuum leaks, fuel distributor problems, warm up regulator problems or other system problems that could affect mixture at a different engine speed or load. The object of the procedure is to get the system operating around 50% duty cycle for all normal driving modes, so the system has the maximum amount of mixture compensation (to the lean side and the rich side) available on demand.
IMPORTANT NOTE:
When the tall spike in the waveform moves to the right, the PCM is commanding a rich mixture (based on a low O2 sensor voltage).
For More Information
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix