In would this column I would like to promote the benefits of oscilloscope diagnosis. In the past week I’ve not only been teaching classes for Car Quest Technical Institute (www.carquest.com/carquest/proCTIclassSchedule) on using oscilloscopes in diagnosis, but have also used an oscilloscope twice this week myself in assisting repair shops with diagnostic challenges. I was reminded of the benefits of using an oscilloscope, as well as the challenges of using one in diagnosis, during these activities.
Today’s vehicles virtually require the use of an oscilloscope for the diagnosis of everything from no-start conditions to network DTC’s. Not only do oscilloscopes increase the speed of your diagnosis, but they also ensure a proper repair. Let us take a look at a recent example.
A vehicle comes to the shop on a hook with a no-start complaint and a crankshaft position sensor code (P0335). The technician recognizes there is no spark, or fuel injector pulse present. (See this article for a refresher on the basics of no-start diagnosis: http://modenook.com/auto-repair-in-boston/the-basics-of-no-start-diagnosis). He also knows enough to suspect the crankshaft position sensor (CKP) when he sees the no-spark condition. The next step the technician takes is certainly an appropriate one…a visual inspection. Here is where he begins to get off track however.
While doing a visual inspection, the technician notices a coolant leak from the water pump that is allowing coolant intrusion of the crankshaft sensor. He also notes corrosion on the crankshaft trigger wheel and some play in the crankshaft timing sprocket. Of course, in order to note all of this, the lower timing cover had been removed, a water pump and timing belt had been sold, and a crank sensor subsequently ordered. There must have been an easier and less expensive way, to diagnose the crankshaft position sensor!
With the timing belt and new crankshaft sensor installed, along with a new water pump, the vehicle was started. Or should I say….cranked, but not started! Spark was quickly checked and it was discovered that there was still no spark. The aftermarket crankshaft position sensor was immediately suspected as the culprit. Power and ground to the sensor were inspected and found to be present. The technician also found that the 5 volt reference voltage begin supplied the sensor as present. It had to be a another bad sensor! Or did it?
With all the electrical necessities present, a second crankshaft position sensor was ordered from the supplier. When the next crankshaft position sensor didn’t cure the no-start condition, a third sensor was ordered from the dealership. We now have three new crankshaft position sensors installed and still the vehicle doesn’t start. Not to mention that the customer has been paying for a rental vehicle for three days at this point.
What could have been done differently during the diagnosis? How could an oscilloscope have been used to more quickly diagnose this vehicle? I might have suggested, upon discovery of the no-start condition, that an oscilloscope be used to confirm that the crankshaft position sensor was indeed causing the no-start concern.
The benefit of an oscilloscope is that not only can it be used to determine voltage level in a circuit, but it can give a picture what the voltage is doing over time. Oscilloscopes are especially useful on speed and position sensors. The engine control system uses the pattern of voltage over time to determine how fast the engine is spinning, and at what point the number one cylinder is a top dead center (TDC) so that it knows when to turn on the fuel injectors and fire spark. Without a good signal, no fuel injector operation or spark will occur. An oscilloscope will give an accurate representation of this sensor signal essentially allowing the technician to see what the computer gets to see.
When using an oscilloscope the technician must understand what type of pattern he or she expects to see displayed on the scope. Repair manual data or many internet resources can provide known good oscilloscope patterns for commonly used sensors). In the case of our no-start, the crankshaft position sensor was expected to output a “square wave” signal. The sensor is provided a 5 volt signal that is then switched on and off as a trigger wheel passes through the sensor.
If we’d been present to attach an oscilloscope to the vehicle at the initial diagnosis we’d have noticed that our square signal was not present. Instead we would have had a flat line at zero volts indicating a short to ground. After installing the first crankshaft position sensor, and confirming its operation also using the oscilloscope, we would have found that again there was no signal. Here we would have found that we had another flat line, although this time it would have been found to be at five volts indicating the sensor’s inability to “pull the signal to ground”. We would have recognized that our situation had changed, even though we still had a no-start condition. At this point we would need to confirm (or re-confirm) that the sensor has what it needs to work: battery power, a ground, a five volt reference signal (which we had already seen), and a working reluctor ring. When I was called to look at the vehicle after the third sensor had been installed, this is exactly what I checked for.
Upon inspecting the reluctor ring (or trigger wheel) I discovered that the technician had been putting the sensor on backwards. Each sensor he had installed was unable to “see” the trigger wheel and therefore couldn’t switch the 5 volts to ground and back. Once the sensor was installed properly, we found that not only did the vehicle start, but the oscilloscope pattern was what we had expected.
Certainly oscilloscopes are not magic bullets for every diagnostic situation, but they can help a technician to make a fast, accurate diagnosis, as well as confirm a repair. We all make mistakes, but catching those mistakes sooner rather than later is important! Every technician should own a good quality oscilloscope, and learn how to use it. Practicing with it, taking a course on its operation, and consistently using it to aid in diagnosis will make an oscilloscope your favorite diagnostic tool.
Feel free to email me with questions or concerns. I can be reached through the website www.intelligentmechanic.com. As always, be sure to sign up for email reminders each time an article is posted!