Does using QCSC DAS expand my knowledge base? YES
This htm is dedicated to all the instructors, students and mechanics who recognize knowledge as the best tool for quick and accurate diagnostics. Thanks Henry, Bob, Eric and Chuck.
The 1988 Quad-4 depicted herein, idled rough.
Below we crank and start the engine. Analysis shows that the crank sensor generates from +3v to -3v peak to peak while cranking at 318 RPM. Naturally, because it is mag pulse, voltage increases with RPM. We also see that the bypass goes high and EST is 'ungrounded', just like older GMs, once a given RPM is achieved.
NOTE: Before reviewing these waveforms, channel names are in the upper left corner. Because QCSCVU automatically makes calculations based on T1(transistion 1), T2, and Current X, you can ignore irrelevant data.
Next, with the engine at idle (idling rough), we mark the falling edge of the reference pulse for 4 complete pulses. If the engine was smooth idling, all the reference pulses would be evenly spaced (but they are not evenly spaced).
As a bit of background, the refence pulse is generated electronically by the ignition module in response to crank sensor signals. The seven slots in this four cylinder crankshaft help the ignition module determine RPM at least six times per revolution rather than the normal twice required for a four cylinder. Thusly, as crankshaft RPM fluctuates, the reference fluctuates. The tach pulse is also generated by the ignition module, notice how drastically the tach signal is changing.
We originally thought the tach signal included the equivalent of a cylinder ID pulse, but actually learned from other waveform samples that the tach signal (high) reflects desired dwell.
With both the reference and the electronically generated tach signal bouncing around, the EST signal is doing the same. Are they trying to smooth the idle by controlling ignition timing like a Chrysler?
As a TRICK, we've learned that we can check dwell on IDI as the inverse of the tach signal.
The car had NO codes and throughout our testing the bypass, crank sensor, REF, TACH, and EST signals were always there. We also had revved the engine and found that when engine inertia took over, all the previously fluctuating pulses smoothed out. Injector pulses had also been stable.
Below, we plug into the coil negatives to observe the spark lines. We know that a normal spark line will curve up at the end as the coil can no longer sustain a spark in a turbulent, high compression environment. Can it be the spark is taking another path? YES. Replacing the coil(s) cover/holder and boots fixes the car.
Also on the waveform below, notice the fluctuation in dwell time. Also notice the slight dip in coil- OCV (open circuit voltage) when the other coil is energized. This is the voltage drop occuring the battery power feed wire which the coils share. TRICK: With QCSC DAS you can tap into the battery feed on a GM DIS and determine if the ignition module is pulsing the coils.
For more info, call: Quad Computer Systems Consultants, 973-472-0328, ASK FOR DAN
EMAIL to; qcscdas@aol.com
or; lody@delphi.com
or; danbuckley@worldnet.att.net
Please entitle email inquiries -- QCSC DAS
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QCSC DAS Copyright 1997 -- Developed by; Daniel A. Buckley (NIASE GAM-CMAT-L1)
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