PicoScope 7 Automotive
Available for Windows, Mac, and Linux, the next evolution of our diagnostic scope software is now available.
Ben, another very interesting write-up. Thankfully, the software team had the good sense in PS7 to retain the original crank maths channel, with its raw data, alongside the new “airbrushed” crank maths channel. As you imply, one should always have access to the raw data, as well as being aware of any processing that might have been applied to a signal.ben.martins wrote: ↑Fri Feb 11, 2022 4:51 pmSometimes direction is all you need when it comes to diagnosis and whilst DTC's can point you in the right direction they can also lead you down a rabbit hole. When faced with the challenges of a crank, no start we must establish some basic essentials for the internal combustion engine to start.
For the diesel engine it's slightly easier than the petrol variety as we don't need to rely on spark igniting our air and fuel mixture but ultimately they both need 3 main things - compression, air and fuel. Making sure you have the basics is always vital. This is where a relative compression check comes in handy as it's quick, easy to perform and non intrusive. Adding in a fuel pressure sensor will let you see if fuel is getting to the right places and if you include a pressure transducer/first look sensor, you can get an idea on whether the engine is pulling air into the cylinder.
What happens though if all these come back OK though? Where to next? This one picture one waveform follows on from the following study - https://www.picoauto.com/library/case-s ... ic-process where we look at just that and how including math channels can help to reveal more about what the engine is doing.
Graphing the crankshaft sensor isn’t anything new and we have a number of forum posts surrounding this technique of observing the crankshaft speed as it increases and decreases during the 4 stroke cycle. These can be seen here - viewtopic.php?p=37861#p37861. Recently we have made some changes to how the crank math function works, in order to make it a little easier on the eye when it comes to analysis. However we do need to be aware of what’s happening to ensure we know when to use this math channel. The old crank math used to leave a gap where the missing teeth were present and we would then have a very long spike as the signal returned and the math continued. With the new crank math channel in PS7A, this spike has been removed and we have joined the points between where the missing teeth are. This makes things a lot easier to view as seen below.
Volvo Crank non start.png
As you can see, the crank math channel in blue clearly shows a slowing down in the crankshaft on four of the 6 injectors. Using the firing order we can also see that cylinders 3 and 6 are the only ones increasing the crankshaft speed. I hope you agree that the new math channel makes it easier to see this slow down in the crankshaft when compared to Freq math which is below in Red. Here we see the dropout in the signal due to the missing teeth.
However, there are some advantages to seeing these dropouts, especially if there was a glitch on the crankshaft sensor. As I’ve mentioned the crank math will now join the gap between the missing teeth which means the crank math channel could mask this glitch. As both the crank and frequency math channels are built into PS7A, when evaluating the crankshaft sensor it may well be beneficial to include both math channels to ensure no stone is left unturned during diagnosis.
The above vehicle was fitted with 4 injectors which cured the crank no start and the lorry was back out on the road again without having to replace all 6. Some may argue that all 6 should have just been done and it solves any problems in the future. However, in a world where prices continue to rise, being able to save replacing parts where not needed urgently will surely be appreciated by the customer.
I hope this helps and thanks to Lee Sharp, L&D Commercials, for supply the captures and ultimately doing the job! Keep an eye out for a case study with crank math back in action but not all is as expected!
If you have any feedback on the crank math channel please could I ask you to post on the PicoScope 7 Automotive Software page - viewtopic.php?p=102798#p102798.
Kind regards
Ben