P0340 Camshaft Crank Synchronisation Incoherence - What good and bad oscilloscope traces look like

[Rhothgar]

Afternoon all,

Been promising to do this for some time now.

Here is an oscilloscope trace for both camshaft and crankshaft on my Xantia 2.0HDi.

Technically, I believe this is what you should see with any Hall effect Camshaft Position Sensor and any inductive Crankshaft Position Sensor when a car is idling. The blue traces are the camshaft. Red being the crankshaft trace.

The trace above is from 13th August when the car was running right. Idling at 800rpm. The vertical dashed rulers show one complete revolution of the crankshaft. You can see that the camshaft completes a full rotation over two crankshaft rotations.

The car since stopped running between then and now and Lexia reported P0340 Cam Crank Signal Incoherence.

You can see from the trace below the crankshaft sensor is now faulty and not producing the correct signal. The car will not start and you can see it is turning over at 210rpm which is sufficient if a little slow.

After removing it, I checked the resistance and it was 1.2 mega-ohms!!! It should be around 370 Ohms when new. Another useful check if you do not have access to an oscilloscope. Of course, just checking resistance will not highlight any intermittent faults.

I hope this helps others in the future. Any questions, please ask and I will do my best to answer them accurately.