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

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Rhothgar
Donor 2023
Posts: 1758
Joined: 22 Nov 2004, 00:21
Location: Nottingham - UK
My Cars: 2013 Peugeot 3008 Allure 1.6HDi - FD63 FWA VF3**************[VIN obfuscated, can be read by forum staff]
1995 Citroen Xantia 1.9TD S1 - M728 GDL VF7**************[VIN obfuscated, can be read by forum staff]
1996 Citroen Xantia 1.9TD SX S1 - N707 MGP (Currrently laid up)
2000 Citroen Xantia 2.0 HDi S2 - X435 JGJ VF7**************[VIN obfuscated, can be read by forum staff] (Clutch died Dec 2017 - Resurrected Easter Sunday 2021)
1997 Citroen ZX SX TD - P788 AJL
1959 Landrover Defender S2 - Two owners from new
1968 Triumph Vitesse Convertible 2.0
1980 Ford Escort RS2000 Customer - 2nd Owner
1988 Saab 900 T16S - A 1980's exercise in understated Hooliganism...
Oh! and two Harley Davidsons - A 1990 Sportster and a 2003 Fatboy 100th Anniversary (the only vehicle I have owned from new)
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P0340 Camshaft Crank Synchronisation Incoherence - What good and bad oscilloscope traces look like

Post by 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.
Cam-Crank Synchronisation.JPG
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.
P0340 Cam Crank Incoherence.JPG
I hope this helps others in the future. Any questions, please ask and I will do my best to answer them accurately.
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