Welcome back to another edition of “The data doesn’t lie,” a regular feature in which I pose a puzzling case study, followed by the answers to the previous issue’s puzzle.
Just a few months ago, I spoke with my friend Clay Cowell (of CMC Diesel, in Big Timber, Montana) for the first time. He was faced with a 2013 Ford F150, with a 3.5L engine that exhibited a symptom of “Crank/No-start.” Clay diligently pursued the issue but found himself a bit turned around, considering the PCM offered no DTCs as a clue to begin the diagnostic journey. The vehicle was driven flawlessly off-road and crossed a shallow creek before the engine stalled suddenly and wouldn’t restart.
Clay reached out to me and told me of the situation. I suggested we begin determining what does seem to be functioning correctly. With that, Clay informed me that as the vehicle cranked, none of the ignition coils or fuel injectors were firing. I then told him of my tactic to always get to the root cause of either of the symptoms. I do this by asking “why” until I simply can’t find the answer to that question any further.
So, I suggested Clay find out “why” a spark is not present, and what is required to create spark from a coil on this specific vehicle. Clay realized the ignition system utilized a three-wire COP configuration for each of the six cylinders (Figure 1). Using his Snap-on ZEUS 4-trace lab scope, he then verified the COPs had adequate voltage and ground supply present, but the PCM failed to issue a command to fire the coils. Again, the question is, “Why?”
Armed with this knowledge we discussed that the PCM is not a smart device but is one that follows directions quite well. The PCM requires input from the timing signals (Crankshaft and/or camshaft position sensor signals) to decide when to fire the ignition coils and fuel injectors. My suggestion was to capture the signals to ensure they were not only present but reached the appropriate amplitude and perhaps even correlated with one another properly.
Analyzing the captured CKP signal and comparing it to a known-good capture, I was suspicious of what appeared to be a missing pulse. Looking at a trend view of the data revealed that this missing pulse happened once per crankshaft rotation (Figure 2).
The data doesn’t lie
With all the information in front of us, and the desired information not yet obtained, we are faced with deciding how to proceed. Here are some bullet points of what we know to be factual, and I will ask all of you, diligent readers, for your input:
- The PCM will not fire the ignition coils or fuel injectors.
- No DTCs are present or pending.
- The COPs each receive adequate voltage and ground supply, but no triggered command.
- The CKP signal is present, of proper amplitude, but exhibits an anomaly.
Given this information, what would you do next?
- Replace the CKP sensor.
- Inspect CKP reluctor with borescope.
- Replace damaged crankshaft.
- Update PCM software.
