For most of you, this story will not hold anything really new. But in my travels, I still meet techs who have no idea on how the use of a DSO can assist in their troubleshooting. The car is a 2000 Hyundai Sonata, with a 2.4 engine and automatic transmission. It was towed in after it suddenly died on our customer while driving and would not restart. It didn't take much to determine that there was no spark and no fuel injection pulse, and a check for codes found one for the Crankshaft Position Sensor. Duh...
Sometimes, a CKP can fail and not set a code. If the engine cuts off before the fault is detected, the ECM doesn't know the CKP failed. It doesn't expect a signal if the engine isn't running. This one, though, must have begun to deteriorate while running, and long enough for the ECM to realize it had a problem. But is it the sensor? Or the wiring? This sensor is buried behind the timing belt cover, and requires belt removal for replacement. Before I charge the customer that kind of labor, I want to know for sure. So, out comes the scope...
This first capture was taken while trying to start the engine. I connected at the CKP harness connector accessible just behind and above the right side of the valve cover. The CMP trace was taken directly at the sensor, and I also monitored the current to the ignition coil to see if anything might happen...even though I didn't expect to see anything.
OK, there was definitely a problem with the CKP signal. I got one, so at this point I am pretty sure that power and ground to this Hall Effect sensor are OK. That made me more comfortable in tearing into the engine to replace the sensor itself. A close look at the old one revealed the damage.
See the crack and discoloration? I know this is probably diagnostic "overkill", but I, for one, like to be SURE before I spend other people's money. Here is the pattern with the new sensor installed, and the engine now able to run...
The relationship between these sensors on many cars can also tell you if a timing belt is incorrectly timed, or if there is damage to the reluctors themselves. Ford, for one, actually lists what these patterns should be on their engines using a camshaft synchronizer to aid their techs in determining whether or not the synchronizer has been installed properly. Even timing chain wear can be diagnosed using these signals. Got a scope collecting dust in your toolbox? Break it out, hook it up, and start using it to assist you in your diagnostics.