Can bus Network.

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Can bus Network.

Post by SBautowerks » Wed Sep 21, 2016 6:07 pm

Hello all.

I have this big problem car right now. It is a 2008 BMW 335i the problem is that the Can network keep going down I have tested the wires in the network there are no short ( to ground, power, or the twisted pair) I have also preformed Ohm and resistance tests. All Tests have come back good now my question is has anyone done a voltage drop test on a Can bus and if so how would you set it up with the 4425 pico scope. Thanks for any info.

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Re: Can bus Network.

Post by STC » Thu Sep 22, 2016 7:33 pm

Can signals carry such little current, probably milliamps, I'm not sure Volt drop is going to uncover anything that your wire testing has not. I would guess that the signals could travel through a single strand, interference would be another story.

Volt drop with your scope would be the same as on your Multimeter, it is a measurement of what is on the Red lead minus whatever is on the Black Ground lead. Same with the scope, put the Ground lead on one end of the wire and the Channel 1 Blue lead at the other end. Be sure that current is flowing and look for a close to Zero reading.

The 4425 can do 4 different wires simultaneously.

Have you scoped the 2 can lines when it is good and bad? or better still as it falters.

What is the fault with the car ? There will be fault codes if the network is dropping out and you will suffer No Comms issues with control units if you have done a Global Scan.

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Mark Dalton
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Re: Can bus Network.

Post by Mark Dalton » Mon Sep 26, 2016 1:02 pm

I agree about the current, CAN BUS is minuscule in it's current.
Have you seen the Bus go down while you have the scope on the car?
Is it only the High or the Low side going down. Is it pulled down to earth or High i.e greater than 3.5 Volts?
Do you lose Communication with all modules or just some of them?

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Re: Can bus Network.

Post by docn2mx » Mon Sep 26, 2016 5:49 pm

Michael here, I would like to know if any codes are showing, also you stated you tested the wires. I am not sure if he car has a comb or not but if it does unplug the come and test each module also check the shorting bar it’s self it may be open.

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Re: Can bus Network.

Post by Adam89 » Sat Oct 08, 2016 7:51 am

I guess that's something wrong with the network wiring. You can to have that checked and see if the issues gets solved or technical support is there which would work for sure.

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Re: Can bus Network.

Post by FioranoCars » Fri Oct 21, 2016 12:02 pm

Having jsut spend the last few weeks becoming very familar with CAN-BUS wiring and shorts there in, I can say this sort of problem can be a PITA, but still need diagnosing and fixing.

First thing, Do a capture of the 2-wires at any single point, so you can see the high and low signals.
To do a volt drop, which I'm not so sure is going to help, you need to ideally find the end points (so a Can topology diagram is needed), and if you have the newest of the Pico range 4x25, you can do this on one channel per wire (as the ground can be floated), the older 4xxxx or 3xxx you'll need to use 2 channels for each wire you test and use a maths channel to calculate the drop.

So using the older scopes, using High wire, but same procedure for low wire, put chaA, onto the Pin at one end of the topology (either the external resistor, or the ECU), then ChB will go to the furtherest end of that CAN-BUS, into the same wire, again at the terminator resistor or ECU depending on how the Bus has been terminated.
Math is either A-B or B-A, either and just ignore the sign, the difference is the volt drop.

Repeat for Can low
Maybe do both at the same time to look and see if volt drop on 1 wire is different drastically from the other? Or coincides with the packets?

Really not sure Volt drop will give you anything, but that's how you can do it.

Best to post back the DTC errors in each ECU, the reported fault from the owner (drivability or just a lighton ), a list of the ECU's on the bus, the topology, what resistances you got measuring the Bus, what that changes by when you remove each ECU in turn (to find individual ECU restances - remember ohms law ... the Parralel version :) ), do you know what the resistances should be for each? You can measure the 2 pins on the ECU WHEN OFF the harness to check...

Clean all the Pins and retest? Any water ingress? Pin condition?

Lets see some captured data files (and pics for ease), I'm sure we can give you lots of ideas!


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