Fault Tolerant CAN (J1939)

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Interrupt
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Fault Tolerant CAN (J1939)

Post by Interrupt »

Recently I captured some BUS activity of a Cummins QSX15 (KOEO), the point of the exercise was basically CAN sniffing as I'm playing with a project to make myself I very basic scan tool for J1939(so during inspections I can hook up a hand held device to check for codes instead of getting out and setting up a laptop based scan tool....and just to play :D ). I wanted to get a capture to reverse engineer the SPN conversion method.

What I captured amazed me! The wave form was so terrible after checking my equipment and setup serval times I didn't believe the engine would run or defiantly not communicate, to the extend that I even said to my boss I thought/think there was/is a fault, well I defiantly don't think this is normal??(don't have a similar setup to to compare to.)

this what I captured https://1drv.ms/u/s!Akah1Kac30UAr0cVHWR ... p?e=IO1tOS
The next day I took my tool down to the pump and hooked it up to my amazement it read data and when I created a fault(un plugged the ECT coolant sensor) it read that to https://1drv.ms/v/s!Akah1Kac30UAr0qltfc ... D?e=FmLIJ3 and https://1drv.ms/v/s!Akah1Kac30UAr0uSdsM ... I?e=Gi3ngs this is obviously what the ECM sees in red https://1drv.ms/u/s!Akah1Kac30UAr0gavL7 ... W?e=MmiKao not sure if I should try chase it down or just monitor?
Last edited by Interrupt on Sun Nov 21, 2021 7:49 am, edited 2 times in total.

liviu2004
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Re: Fault Tolerant CAN (J1939)

Post by liviu2004 »

Can you upload to this forum? I find links posting very doubtful.

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Interrupt
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Re: Fault Tolerant CAN (J1939)

Post by Interrupt »

as requested liviu2004
Attachments
CumminsQSX15_QSX15_Diesel_20211121-FaultTolerantCAN.psdata
(17.27 MiB) Downloaded 15 times
VID20211115113846.mp4
(1.47 MiB) Downloaded 15 times
VID20211115113253.mp4
(2.98 MiB) Downloaded 13 times
This is what ECM_Sees_Fault_Tolerant_CAN.jpg
Fault_Tolerant_CAN.jpg

Steve Smith
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Re: Fault Tolerant CAN (J1939)

Post by Steve Smith »

Hello and thank you for the posts, fault tolerant CAN indeed

Based on your capture it would appear:

1 We are communicating via a single wire of our twisted pair (CAN Hi Only)

2 Our idle bus voltage of 2.5 V (CAN Hi) has also dropped to around 2 V

We therefore have CAN Lo at near zero volts with minimal activity and CAN Hi dropping below typical idle voltage of 2.5 V

I cannot help but think "measurement error" at this stage and it was great to read you have checked and checked again (Awesome)

The structure of CAN Hi peaks is also sporadic along with the nature of the 2.5 V idle voltage.

I would like to check CAN bus resistance and bus lines to resistance values

Could we recheck the CAN bus at another location and determine if this is specific to a "branch" or the entire bus?

Could we connect to the CAN network with high impedance probes too?
https://www.picoauto.com/products/test- ... 00-mhz-bnc

This would eliminate the sporadic nature we see at CAN Hi from our intrusion with the Scope
i.e. we must ensure our connection to the network is not responsible for the structure we see on CAN Hi

I hope this helps and I look forward to the outcome

Take care......Steve

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