This 1994 Econoline van with a 5.8L V8 had been to several shops to have a very intermittent and slight misfire diagnosed. This engine uses a distributor type ignition system with a remote mounted Ignition control module (grey ?Push Start? version).
I could feel a brief hiccup type misfire occasional at idle.
I knew the defect was probably not fuel delivery nor control as I could see no disturbance in the O2 sensor scanned graph. See Fig 1
I caught this lack of spark event while watching the ignition secondary pattern drop out/disappear on the ignition scope. See Fig 2.
I then scoped the Ignition module (ICM) to see what was happening.
See Fig 3
CH1-Power feed to ICM
CH2-Spout
CH3-Ignition primary control
CH4-PIP
When SPOUT rises the ICM is signaled by the PCM to fire the ignition primary. The Spout signal would not rise completely in this capture at about 38mS. It only displayed a runted attempt to rise. SPOUT finally did rise at about 41mS, a bit too late. Being the SPOUT pulse from the PCM did not reach sufficient amplitude the ICM did not fire the primary on time.
The SPOUT signal is not rising signaling the ICM to fire the ignition primary as it should. As a result there is a missing firing event. This missed fire causes the RPM to slow down and the PIP signal to runt coincidentally at 38mS. Simply viewing the waveforms without a good understanding of these circuits might leave you wondering if the PIP signal from the distributor caused the problem or not. A further question might be is the defective SPOUT signal being caused by the PCM or the ICM? A quick way to tell is to remove the inline SPOUT shorting bar connector. This puts the system into based timing or ICM SPOUT only controlled timing. Now only the ICM controls dwell and firing point not the PCM.
As you can see in the next capture.
Fig 4:
CH1-SPOUT from the PCM side of the shorting bar
CH2-Spout from ICM side of the shorting bar
CH3-Ignition Primary control CH4-PIP
Now the ICM generated SPOUT, Ignition primary and PIP are all interacting normally with no misfiring occurring. The SPOUT signal from the PCM is still erratic and dropping out. Proof positive that the PCM was the cause of the erratic SPOUT behavior while the PCM was connected to the circuit through the SPOUT shorting bar connector. A new PCM fixed this one up.
I could feel a brief hiccup type misfire occasional at idle.
I knew the defect was probably not fuel delivery nor control as I could see no disturbance in the O2 sensor scanned graph. See Fig 1
I caught this lack of spark event while watching the ignition secondary pattern drop out/disappear on the ignition scope. See Fig 2.
I then scoped the Ignition module (ICM) to see what was happening.
See Fig 3
CH1-Power feed to ICM
CH2-Spout
CH3-Ignition primary control
CH4-PIP
When SPOUT rises the ICM is signaled by the PCM to fire the ignition primary. The Spout signal would not rise completely in this capture at about 38mS. It only displayed a runted attempt to rise. SPOUT finally did rise at about 41mS, a bit too late. Being the SPOUT pulse from the PCM did not reach sufficient amplitude the ICM did not fire the primary on time.
The SPOUT signal is not rising signaling the ICM to fire the ignition primary as it should. As a result there is a missing firing event. This missed fire causes the RPM to slow down and the PIP signal to runt coincidentally at 38mS. Simply viewing the waveforms without a good understanding of these circuits might leave you wondering if the PIP signal from the distributor caused the problem or not. A further question might be is the defective SPOUT signal being caused by the PCM or the ICM? A quick way to tell is to remove the inline SPOUT shorting bar connector. This puts the system into based timing or ICM SPOUT only controlled timing. Now only the ICM controls dwell and firing point not the PCM.
As you can see in the next capture.
Fig 4:
CH1-SPOUT from the PCM side of the shorting bar
CH2-Spout from ICM side of the shorting bar
CH3-Ignition Primary control CH4-PIP
Now the ICM generated SPOUT, Ignition primary and PIP are all interacting normally with no misfiring occurring. The SPOUT signal from the PCM is still erratic and dropping out. Proof positive that the PCM was the cause of the erratic SPOUT behavior while the PCM was connected to the circuit through the SPOUT shorting bar connector. A new PCM fixed this one up.
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