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.
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!
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.
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).
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)!
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.
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?
