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The importance of asking 'why?' during automotive diagnostics

Monday, December 3, 2018 - 08:00
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Remember, current is the output generated by the PCM’s reaction (or processing) of an input. This vehicle uses a sequential fuel injection strategy. It uses a CKP 18x signal and sync signal referenced from the crankshaft balancer’s reluctor. It also monitors a CMP pulse referenced from the nose of the camshaft.  These same inputs effect ignition timing as well. The Ignition events were also being affected but I’m just chasing the symptom though. Regarding injector control only, these signals are processed by the PCM to determine injector timing and TDC of the number one cylinder, so that it may synchronize the correct injector to #1 cylinder. A PCM just does what it is programmed to do. In this case, drive an injector when it sees the CKP 18x, sync pulse and CMP correlate in a certain manner. Because the failure was reflecting a fault pertaining to injector control, if the PCM receives a bad input, it’s going to create a bad output (unless the bad input is recognized as such). Shortly, you can see exactly what my next plan of attack is. The strategy was to capture the fault occurring and use that as my point of reference for the other correlating signals. Here, I monitored the fault (injector current) and I will correlate that to the responsible inputs that the PCM relies upon for fuel injection calculation (CKP 18x, sync, CMP).  I will simply see if, when the fault occurred, did my inputs to the PCM show any kind of deficiencies?

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Figure 6

Keeping in mind, each test I perform is justified by the previous test. No time is being wasted. The beauty in approaching drivability faults from this angle is that logic prevents a step from being missed. Every test performed will almost always yield a diagnostic clue. Referring to Figure 6, it displays the fault quite clearly as my blue injector current ramp is once again, deficient. More importantly, there is an anomaly visible within the CMP pattern and the 18x pattern as well (only CMP signal visible for better clarity). So, let’s take a moment to ask why again. Why is the voltage CMP signal dropping low? There could be a few possible causes for a failure of this kind:

  • faulty CMP sensor
  • damaged CMP reluctor
  • poor connection or voltage drop within the signal circuit or the reference voltage circuit
  • shorted/ loaded sensor signal circuit or the reference voltage circuit

This simply requires another test. In this next step, I studied the wiring diagram and saw that the CKP 18x, Sync and CMP signals all shared the same reference voltage source. I will monitor the fault as carried out in the previous step but add some new data to the acquisition. We must now view sensor reference voltage feed, the common feed to all three suspect inputs. Viewing this piece of data will explain whether the reference voltage has a fault. It allows us the ability to divide the circuit up and determine on which side of the input the fault lays. Now, I want to mention something that I feel is a valuable point to make. I’m asked regularly, if it is necessary to own an 8-trace lab scope like the one I’m using in this case study. It certainly isn’t a necessity, but you will see how having one allows me to save a ton of time. As John Anello (the Auto Tech on Wheels) says, “It’s like fishing with a net instead of a hook.” Having the capturing capability of an 8-trace lab scope allows me to see relationships between multiple inputs, the ECUs response and the actions carried out, all simultaneously. You will see how this characteristic works to my advantage in this next step. There is one more tool that I will utilize in tandem with the scope, to further nail down the fault to a pinpoint. I will implement the use of a microamp clamp. The microamp clamp is a very sensitive device designed to accurately measure very miniscule amounts of current flow.

The final showdown
The final test will be to monitor the CMP sensor signal under the fault conditions (all inputs reflected the fault so, I just chose to monitor the CMP only). At the same time, I will be monitoring the reference voltage feeding the sensor. The third piece of the puzzle is to monitor current flow through that sensor reference voltage circuit. My thought process is simple.

When the sensor signal is deficient, I will immediately be able to see whether it is due to a deficiency in the reference voltage circuit feeding the sensor. At the same time, the current flow will tell me a story too.  If a poor reference voltage feed is due to a voltage drop, the current flow through the sensor will diminish. On the other hand, if the reference voltage circuit is being loaded/partially shorted to ground, the current flow through the reference voltage circuit will INCREASE!

Figure 7

Figure 7 tells the whole story. When the vehicle was operated under fault-conditions, the engine began to “ping” hard. This occurred while the injector current ramps showed a deficiency. The inputs responsible for the injector commands were deficient as well. They were fed a reference voltage that was common among all three inputs (CKP 18x, Sync and CMP). With the microamp probe surrounding the reference voltage feed wire, it was quite clear to see the amperage increasing as the fault presented in the CMP, CKP 18x and sync signals. This tells me that I must pursue a short circuit.  So now, the hunt is on for a rubbed-through harness. After a quick visual inspection, a suspect area was located. Just below the power steering pulley, but above the crankshaft balancer, the CMP sensor harness was unsecured and intermittently touching the crankshaft balancer (Figure 8). This exposed some copper and the wire suffering the damage was the sensor reference voltage circuit, common to all the sensors discussed above.

Figure 8

If you take the time to ask yourself “WHY” at least five times, you typically find yourself face to face with the root-cause of the fault and co-workers looking at you like you are a wizard. So, to sum it all up, lets revisit the chain of events through the questions I asked, to lead me down the path to beat the system:

Why is the engine “pinging”? (Lean condition)

Why is the engine running in a lean state? (Not a fuel delivery issue but a fuel injector control issue)

Why is the PCM failing to drive the injectors correctly? (The PCM operating with skewed inputs)

Why are the CKP 18x, CMP and Sync signals skewed? (A Loaded common reference voltage circuit)

Why is the reference voltage circuit loaded? (Ref. voltage wire feeding CMP is shorting to ground)

As mentioned before, the steps taken were not achieved in record-time but not a step was missed, and this led to an accurate and efficient diagnosis without any parts replaced unnecessarily. Taking the time to interrogate the vehicle will yield you some valuable diagnostic clues that will save you time in the long run. A great side-effect is the developing the understanding of PCM strategy and how different inputs are used in different applications.

So, in the end, being a “WHYs GUY” can really make you a Wise Guy.

Author's note: I’d like to say that this was a fast and simple find but that wouldn’t be correct. True, it was not difficult but did require some thought. Logic, studying of circuit topology, system strategy, and lots of practice with the tools I have were a huge part of drawing an accurate diagnosis, but my process would’ve been random without inquiring “why.”

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