Intermittent CANbus short to ground?

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Cliff Roberts
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Intermittent CANbus short to ground?

Post by Cliff Roberts »

Had a tractor with intermittent CANbus communication code being set by ECU and dropping engine RPM to idle. I checked CAN high and low and found what appears an intermittent short? Fault happens when hitting a bump. I'm mostly looking for a little insight/advise as to if I'm interpreting the waveform correctly.
Attachments
John Deere_5090GN_Intermittent failed CANbus 5_20181105-0001.psdata
(5.06 MiB) Downloaded 929 times

Steve Smith
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Re: Intermittent CANbus short to ground?

Post by Steve Smith »

Hello Cliff and thank you for the capture, sorry for the late reply.

This is a great capture thank you

I will look at this a little deeper tomorrow and feedback ASAP but for now buffer 2 reveals similar characteristics to VAG Convenience CAN as I believe the CAN lines shorted together.

The following forum post will help here:
topic20761.html?&p=94181&hilit=can+hi+s ... +lo#p94181

Note how CAN Hi and Lo have identical voltage levels in the highlighted area below:
CAN Hi Lo S/C Together
CAN Hi Lo S/C Together
You will notice some additional channels called "Math Channels" that will help with evaluating the behavior of the CAN network at a glance rather than trying to view each data packet.
There is more information here:

CAN DECODING IN COLOUR
topic12871.html

Triggering on CAN using MASKS
topic12871.html
topic14691-10.html

I hope this helps for now and I will follow up ASAP

Take care......Steve

Cliff Roberts
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Re: Intermittent CANbus short to ground?

Post by Cliff Roberts »

Hi Steve,

Thank you for the links and insight. I've attached another waveform that I found interesting. The CAN high appears to be getting pulled down and if I remember correctly, there was no CAN fault active at this time. Is this due to the wires being partially shorted?
Attachments
John Deere_5090GN_Intermittent failed CANbus 6_20181105-0001.psdata
(1.28 MiB) Downloaded 627 times

Steve Smith
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Re: Intermittent CANbus short to ground?

Post by Steve Smith »

Hello again Cliff and thank you for your patience.

In your first post you mentioned "Codes being set by ECU" do you have these codes and if so were there multiple CAN Communication codes?

Do you have similar codes in other ECU's? I ask this because we often see a number of ECU's "pointing the finger" at one ECU. For example ABS,ENG,and Power Steering all claim they have lost communication with the Transmission ECU.

If you have this scenario we have a pointer in the form of multiple ECU's all claiming they have lost communication with the same device (ECU).

Here we can concentrate our efforts on the Transmission ECU, power, ground, CAN lines & harness routing to this ECU

What was interesting with this second capture was commutation was still ongoing with no errors even with the glitch on CAN HI (See image below) This supports your feedback "No CAN fault active at this time"
Loss of CAN Hi
Loss of CAN Hi
In both of your captures we are not looking for a short to ground as "transmissions" are always present (with or without) errors. A short to ground would pull down the voltage level to Zero Volts

What we are seeing is identical transmissions of data at identical voltage levels (your first capture) that suggest CAN Hi and CAN Lo are joined together. (Not necessarily at the harness, this could be within an ECU)

The behavior of CAN under these conditions (CAN Hi and Lo shorted together) differs from manufacturer to manufacturer. Some networks will function in Single Wire Mode (VAG Convenience CAN) others will halt communication where the bus returns to a voltage level of 2.5 V BMW PT CAN and in your case it would appear the opposite occurs where transmissions continue but communication stops because CAN Hi - CAN Lo when shorted together = Zero Volts.(See image CAN Hi Lo S/C Together above)

I think a valid test here Cliff would be to simulate this condition. With CAN Hi and Lo performing normally, short them both together and capture this event with the scope. Can you reproduce the activity we saw in your first capture?

In your second capture we are seeing a loss of transmissions on CAN Hi with reasonably stable CAN Lo at the correct bus level voltages (2.5 to 1.5 V) suggesting the CAN circuit to be intact and a possible CAN node error

If possible Cliff can we measure and confirm the CAN network resistance (Just to put this to bed)
Can we locate the termination resisters on the network (these will most probably reside inside an ECU)

Could we then disconnect the ECU's from the CAN network that we can live without? Ensure the ECU you disconnect is not used to link 2 halves of the CAN network together as we then open the BUS

Road test and monitor CAN activity for the symptoms you have described/captured above.

I know this is long winded but it will help to narrow down the area of concern.

I hope this helps and please feedback when you can, take care......Steve

Cliff Roberts
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Re: Intermittent CANbus short to ground?

Post by Cliff Roberts »

Codes are ECU 60717.31 Incorrect CANbus message (Defect on ABS / ASR / ESP CAN controller, CAN cable problem) and ECU 37323.31 Incorrect CANbus message (CAN Bus Message is intermittent or non-existent. This indicates an open circuit in the CAN Bus or a short circuit). ECU in this case is the Engine controller. This system only has 3 controllers, Engine, Transmission, and VCU (instrument panel). The ECU is the only controller logging CANbus errors.

I'm somewhat confident this is a harness issue. The fault would only occur when hitting a bump. I was able to recreate it when performing a wiggle test on the engine harness. We found some suspect areas of the harness that were rubbing but did not see any wires rubbed through (not to say they could be shorted within the bundle). After reinstalling panels that were removed to access harness for inspection, the fault did not reoccur when I drove it. Customer I believe has been operating it with no faults. We may be replacing the harness as this unit is still under warranty and areas are extremely difficult to access for inspection.

I've also attached a waveform of the CANbus on a known good tractor that I shorted the CAN high/low.
Attachments
John Deere_5100GN_2017_Intentional CAN short together 2_20181107-0001.psdata
(15.56 MiB) Downloaded 583 times

Steve Smith
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Re: Intermittent CANbus short to ground?

Post by Steve Smith »

Hello Cliff, thank you for the feedback and sorry for the late reply

Thank you for the insight into CAN in your application as this is a learning curve for me.

Given the only ECU logging CAN bus errors is the Engine Controller, we have a view of the CAN bus condition from purely the Engine Controller.

The Engine Controller reports an incorrect CAN bus message and intermittent or non-existent message on ABS/ASR/ESP controller (I assume this is one controller?)
This does sound very "harness/wiring" like in nature and certainly must be pursued between Engine & ABS

Do we have a time/date stamp against the fault where we may be able to link the codes to temperature/cold start operating condition etc.?

I ask this because at the point of start up CAN nodes will check who is online and announce their presence, whilst also ensuring their internal configuration is applicable for the vehicle in which it is fitted. (One ECU can fit multiple vehicles but configured accordingly)

In scenarios where the power up supply wire to the ABS ECU is different (originates from another source/connector) to other ECU’s we could have an event where momentarily, the ABS is offline due to delayed power up whilst the other ECU’s are present and “looking” to determine who is online.

Most certainly Engine, ABS and Transmission will have high priority and if ABS is not online at Start up, the fail-safe will be ABS off and warning lights on

I think it would be worth checking the power up (supply) timing between Engine and ABS, perhaps include a Wiggle test too.

Whilst thinking about a Wiggle test, you mentioned you were able to recreate the fault which is a piece of gold for sure.I know how frustrating it can be when the wiggle test fails once only, never to return!

During the wiggle test, could it also have been possible that the multi-plug to each (or one) ECU been subjected to a "pull or tug"?

I ask this as good CAN terminal pin contact at each ECU is paramount, as is the terminal at the ECU to the internal PCB

Would it be possible to wiggle test the harness with the power shutdown whilst measuring the CAN bus resistance (60 Ohm) ?

Here we could view any fluctuations in CAN Network resistance that may not immediately bring about fault codes but certainly deviate from 60 Ohms

I agree with you Cliff at this stage that this is looking like harness (not shorted to ground) but lets keep an open mind.

Thank you for the psdata file of your CAN Hi shorted to CAN Lo which behaves like the BMW PT CAN in that both CAN Hi and Lo revert to the bus level voltage of 2.5 V (whilst still attempting to "message" on the bus)
Deliberate CAN Hi S/C to CAN Lo
Deliberate CAN Hi S/C to CAN Lo
This appears very different to our previous 2 captures where both CAN Hi and Lo are identical where the signals still have "structure"
CAN Hi Lo S/C Together
CAN Hi Lo S/C Together
And very different again where we appear to have lost just CAN Hi
Loss of CAN Hi
Loss of CAN Hi
The plot thickens Cliff but its all great work thank you, and please let us know how this develops

Take care.......Steve

mennovf
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Re: Intermittent CANbus short to ground?

Post by mennovf »

Has this ever been resolved? I seem to be in the well-known situation of facing a similar issue and finding a relevant thread without a conclusion.

muttnjeff
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Re: Intermittent CANbus short to ground?

Post by muttnjeff »

Have ran into this before, different brand of machinery but same type issue, the fault shows up to the operator as a soft failure of can c or the engine can.

Typical finds are harnessing issues near the hot side of the engine, routing issues, etc… other finds are normally a engine or exhaust sensor that talks on the can c network, another thing that comes to mind is depending on the circuit design the sub networks can be powererd and grounded through its own fuse(s) & relay(s).

Good luck.

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