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Do I really need a lab scope?

Thursday, November 29, 2018 - 09:00
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Figure 3
Figure 4

I tapped into the wires at the PCM for CKP A and B, as well as the CMP sensor (located on the front cylinder head) (Figure 3). Now that that was done, I started the vehicle and gathered a few screens of data on the Pico and shut the vehicle off. I knew the amount of time it ran was long enough to set the code so if the problem is present, we should be able to see it. At first glance it did not jump off the screen to me (Figure 4). I could see the crank timing was slightly more retarded compared to my known good but when I zoomed in a little closer the problem stood right off the page. Aha! I found it (Figure 5)!

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Figure 5

What's wrong with this picture?
I could see there was an anomaly in the CMP waveform and it was very consistent. One of the cam pulses was noticeably wider than the rest. It seemed to be triggering off at the correct time but for some reason it was turning on sooner. Why? Heck if I knew but I knew it was likely causing the problem and triggering this code.

Now that the data was gathered, it was time to do some investigative work so I removed the timing covers to have a look. I knew if the air gap on the cam sensor or trigger was changing it could cause an issue like this. I inspected the cam gear for runout on the engine because it retains the raised bosses that trigger the cam sensor. I figured if it had gotten bent somehow it could cause this issue. Maybe someone used a puller on it servicing the cam seals? I knew it was a long shot and needless to say it ran true as could be.

Figure 6

Scratching my head for a moment I knew I had to go with the data. It never lies. I removed the timing belt and front cam pulley to have a look at the raised portion of the cam gear thinking.... well heck I really wasn’t sure what I would see. Was it physically damaged? Was it magnetized? Was there something stuck to it causing this to trigger early? Come to find out It looked perfect (Figure 6)! I even dug out the digital calipers and could find no difference between any of the three raised sections. I went with my gut (and the data) and made the call on the cam pulley. After getting approval from the customer I replaced the front cam pulley and installed an OEM timing belt to correct the correlation being slightly off. Needless to say, I was 100% correct. After installing the new parts, relearning the CKP pattern in the PCM using a scan tool the light was finally off! I also grabbed the waveform from the vehicle to stash in my library of know goods. A full drive cycle was performed and the vehicle returned to the customer after months of being out of service.

Could this problem have been found without guessing and without a scope? I honestly do not believe that it could have. The scope is a powerful tool and I believe it is a necessity in any shop today. What better way to “see electricity,” gather data and make the right call the first time?

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