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Learning the Digital Storage Oscilloscope

Wednesday, November 1, 2017 - 07:00
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The engine is started and the vehicle’s MIL comes on almost immediately. A quick scan of the PCM for codes reveals a P0016 CKP-CMP correlation code stored. PCM codes were cleared and the engine started again with same results. This code can be a tough diagnose without time consuming engine disassembly unless you harness the power of a labscope. My logical approach is to scope the CKP and CMP sensors and verify their signals and most importantly their relationship or correlation to one another. This case study also illustrates the need for a “known good” waveform for comparison. Some OEMs provide waveforms in their service information (SI), others do not. There are pay online sites like iATN that have a great database of waveforms. In this particular case there are a lot of 3.6 CKP-CMP waveforms but this 3.4 F motor is a rare option and at the time I tested it there was no known good available online. I used all my “phone a friend” options to no avail. I suspect this is a mechanical issue but have to prove it. There was no TSB related to this issue at the time I looked at this vehicle. To complicate matters the engine runs smoothly, has great compression, and fuel trims are decent….the other issue is the MIL. Furthermore the SI at the time doesn’t list how much differential between CKP-CMP sync will set this code; it simply stated something along the lines “exceeds a predetermined value” in the enable criteria for P0016.

So my POA is simply to scope to CKP and CMP signals and figure out a way to find one to compare it to.

I often say “I would rather be lucky than good” and lucky I was — not 2 days later I got a call to program a replacement PCM in a 2008 Equinox with 3.4 F engine. Post programming, the shop was kind enough to let me get a baseline of the CMP-CKP signal off this engine with 90K on the odometer with no MIL on or P0016 code present. A quick comparison confirmed my suspicion that there was a mechanical issue with the replacement remanufactured engine. In this case it isn’t timing chain stretch but rather the pressed on reluctor for the CKP was “misclocked” or indexed incorrectly. The engine was replaced and the issue was resolved. It wasn’t but a few months later that GM published a TSB for this condition of the shifting or incorrectly indexed reluctors with pictures of how to recognize but nothing about labscope testing. I can’t think of a better tool that shows just what exactly the PCM is seeing to make the determination if CKP-CMP is in sync and the correlation is correct. Moreover, this case study illustrates the need of a good database and the importance of archiving waveforms for use at a later date. 

The real tool?

Here are some final thoughts regarding using the DSO to help diagnose vehicles. Scope skills are experiential; you have to use it to learn it. Practice on known good vehicles. Create a database. Do the extra test and use a logical plan of attack and design the experiment to test your theories. Leverage the technology of the modern DSO to your advantage and remember that you are the most powerful diagnostic tool in your arsenal!

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