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Do I really need a lab scope?

Thursday, November 29, 2018 - 08:00
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I think we can agree on the fact that we like to buy new tools that can make our job more efficient or even easier. Sometimes we have to buy them out of necessity to be able to handle the tasks at hand or to keep up with technology. However, it often happens that some of these tools end up in the bottom of drawers or tucked away on shelves collecting dust years later, only to be dragged out when we try to stump our friends with a game of name that tool.

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One tool I have never regretted getting my hands on and learning how to use is a lab scope (better known as the digital storage oscilloscope aka DSO, Figure 1). Anyone who has been in the shop long enough knows this is not a new invention and variations of them have been around since I was a kid growing up in my dad's shop. I remember the day my father bought the Snap-On Counselor II and the hours he spent learning to look at secondary ignition waveforms, figuring out what’s good, what’s bad, practicing on known good vehicles to get the feel for it. As a young boy in the shop I was hooked watching the waveforms parade across the flickering green screen. For once I could see the electricity, as opposed to feeling it rush through your body as you learned not to touch old plug wires!

Figure 1

Just as the scope was an important part of a growing shop in the 70s and 80s, I feel it’s just as important today to make the call.  After all, test — don’t guess!

Here's why!

I recently had a 2006 Honda Odyssey with a 3.5 Liter engine and 188,000 on the odometer in the shop that had a complaint of a Check Engine light on with a DTC P0389: Crankshaft Position (CKP) Sensor B Intermittent Interruption. Now this vehicle had been dragged around to several other shops before landing in my lap. It has had the timing belt replaced several times including all the components that go with it. Water pump, timing belt idlers, tensioner, several crank sensors and a cam sensor. It was determined that the vehicle needed a new PCM because that was the only item left that hadn't been replaced. Before installing and programming the customers used PCM I had I asked him if he would like me to have a look at it first. He was hesitant but I assured him if I determined it was the PCM I would not charge him for the diagnostic time. He agreed, so I proceeded with the diag.

My practice is always to look at service information to get the code setting criteria and any related TSBs. I know what the code number is and I also knew it is a hard fault that sets immediately when cleared. So, what is the PCM looking at? Why is it unhappy?

According to Honda service info (located on Identifix), this is the criteria to generate the P0389:

Crankshaft position (CKP) sensor B consists of a rotor and a semiconductor that detects rotor position. When the engine starts, the rotor turns and the magnetic flux in the semiconductor changes. The changes of magnetic flux are converted into pulsing signals to the powertrain control module (PCM). CKP sensor B detects injection/ignition timing for each cylinder and engine speed. If an abnormal amount of pulsing signals are detected from CKP sensor B, a malfunction is detected and a DTC is stored.

Malfunction threshold

Other than 22 pulses are detected during intervals between reference pulses for each crankshaft revolution. This condition has been detected at least 30 times.

Figure 2

Now that I was armed with the information and the insight as to what the PCM was looking for, I decided to grab my Pico scope and have a look myself. In my opinion, this is the only way to see what the ECM is seeing. If I determine the signal is good, then the other shop was on the right track and the PCM moves higher up the list as a possible suspect. However, before I can make that determination, I have to know what good or bad is and the only way to do that is to refer to a "known good" example, specific to the car I'm testing today. Fortunately, I had a known good CMP/CKP correlation waveform saved from one of these engines (Figure 2).

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