Focus intermittant no start CAN BUS

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Re: Focus intermittant no start CAN BUS

Postby STC » Fri Mar 24, 2017 10:06 pm

I have had a network connected to a scope and removed the 120 resistors (one then both) to see what would happen. You could try yourself to be certain and gain education. I do not see those symptoms on your waveform.

I have added a couple Math Channels to your capture to analyse its integrity.

If you add Hi to Lo then you will see that it is consistent at 5v throughout the capture, does not faulter.
If you subtract Lo from High you will see that the differential in the voltages ranges from 1.9v to 2.5v

Faced with those numbers, CAN Voltages would not be in the line of fire if I was looking at that car.
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Sun Mar 26, 2017 12:31 pm

I have a waveform from a zafira that had a faulty under bonnet fuse box that every now and then you could pick out a "spikes message". Trouble is I never did re-scope the CAN network after the fix to see if it had disappeared. I will try and upload it when I am back at work.
Last edited by RYM6746 on Sun Mar 26, 2017 11:46 pm, edited 1 time in total.
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Sun Mar 26, 2017 12:49 pm

I have had a network connected to a scope and removed the 120 resistors (one then both) to see what would happen. You could try yourself to be certain and gain education. I do not see those symptoms on your waveform.


Thanks. I will definitely try removing one of the resistors and then both to see what the outcome is. I do wonder if it will have a different outcome to a resistor changing in resistance due to heat or vibration? Maybe I could set up a decade resistance box at the right resistance to begin with and change it to see the results? Would this damage anything?

Thanks for all your input it has been a learning curve :D
I will try not to accidently over sample and upload on to here in the future :wink:
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Wed Mar 29, 2017 2:38 pm

Here is the low speed CAN waveform of the Zafira that had the faulty under hood fuse box notice the spikes on capture 15. This car would intermittently loose comms with the fuse box.
In an earlier post you mentioned an LC component possibly causing the spikes, What is this may I ask?
In the same post you uploaded two .rar files what program are using to open these as I am struggling? :D
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Re: Focus intermittant no start CAN BUS

Postby STC » Mon Apr 03, 2017 10:07 pm

At Pin one of the DLC on that car you are looking at the Vehicle Area Network not Low Speed Can. I would say that capture, including 15 look ok.
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Wed Apr 05, 2017 3:04 pm

All data list it as low speed CAN see attached file. Anybody have any ideas what component would CAUSE the spikes to be present on a lone message as seen on capture 15 ??????????????
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Re: Focus intermittant no start CAN BUS

Postby Fat Freddy » Wed Apr 05, 2017 4:40 pm

An interesting question and something I had pondered before. Your question got me doing some research/googlenostics (which is always a good way to go)

The answer really is to do the googlenostics yourself as there is no one simple answer. Ringing, overshoot and undershoot will all give results.
It appears it is not ideal but may or may not be a problem or may not even exist. I would guess it would be more apparent on a Sangyong than a BMW.

Would I call a fault based on those spikes alone? No.

HTH
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Wed Apr 05, 2017 8:55 pm

Googlenostics, I like that. Googlenostics is what I shall do. I neither would call it a "fault". I certainly wouldn't condem a component on it. I may however use it as a "clue" to see if I was in the right area and to stay in that area for a little longer especially if the intermittent fault reared it's ugly head. At this point I am really kicking myself that I didn't re-scope the CAN network to see if the "spiky" messages had completely gone after changing the faulty under hood module. The only reason for this is due to the battle we all face, time.
That particular fault would intermittently take out that network and stop the engine from starting. Even when it hadn't taken the network out (when the waveform had been taken) the under hood module wouldn't operate the horn,lights etc at all. All the messages being sent from steering column module and instrument cluster wernt be processed. I Replaced the under hood module and programmed it in and it was a complete fix.
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Re: Focus intermittant no start CAN BUS

Postby STC » Wed Apr 05, 2017 9:40 pm

All data list it as low speed CAN see attached file.

Not wishing to split hairs or be pedantic but I have always known CAN to be a 2 wire, twisted together and a mirror image of one another. I have seen wiring diagrams where the describe any type of network or communication line as CAN hence not strictly accurate and a pain for those of us that are anal with a hint of OCD.
Its all Morse Code really :)

Anybody have any ideas what component would CAUSE the spikes to be present on a lone message as seen on capture 15


Problem here is catching it the act, you would have to systematically scope everything on the car and look for a correlation or cause. Being intermittent, you would need lots of time and patience or a 200 channel scope. I have always found, in certain cases, that 4 channels is not enough - 8 would be good and 12 being ideal.

The one thing I have found is that these networks are Robust and take more than a spike to render them bad. Next time you have an hour to spare put a small resistance to Earth on a Can line whilst scoping it. Then slowly increase that resistance. You will be shocked at how much you can distort that signal before the network falls over. Then do the same to Positive. That will be an eye opener and may help you not to over analyse spikes etc.

Some of the "ideal images" we find whilst googling are probably recorded in a Lab and not on an old car with issues parked under a 50 year old electricity pylon with rusty connections

I look at lots of "CAN" issues it always physically (water/ impact) damage wiring or control units. Rarely straight forward component failure for no apparent reason.

Faced with your Capture 15 I would swiftly move on keeping that on the back burner for the foreseeable.
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Re: Focus intermittant no start CAN BUS

Postby RYM6746 » Sat Apr 08, 2017 3:00 pm

Not wishing to split hairs or be pedantic but I have always known CAN to be a 2 wire, twisted together and a mirror image of one another. I have seen wiring diagrams where the describe any type of network or communication line as CAN hence not strictly accurate and a pain for those of us that are anal with a hint of OCD.
Its all Morse Code really :)


Yes I know what you are saying regarding high speed CAN as a double wire twisted together, with a mirror image message of one another. I'm guessing due to its reliability and that it carrys safety critical information. The VAN network on VW's is also a double wire with a mirror image message. As far as my knowledge goes there is such a thing as a single wire CAN network that I would imagine doesn't need to be as fast and as well protected (2 wires twisted with a double message) due to the information on it e.g lights, horn etc.
It's the whole tomayto tomarto scenario.
Some great information being shared by pico users :D
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