CAN message meaning.

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CAN message meaning.

Postby heady2010 » Thu Feb 08, 2018 11:16 am

Good Morning,

I have a vehicle which I think is sending a brake apply message out on the multiplex without driver (it isn't the brake switch circuit causing the issue) My question is, is it possible to find out what a brake apply signal looks like if I link my scope to the brake switch and the multiplex signals, apply the brake and then look for the changes in the multiplex signal? Or is it possible to determine which computer is supplying these signals as well?

Thanks
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Re: CAN message meaning.

Postby PhilS » Thu Feb 08, 2018 6:07 pm

Hi heady2010

You could use PicoScope on the CAN bus (but these messages may be on an internal CAN bus rather than available at the J1962 diagnostic connector).
If you can access the right bus then you could use PicoScope to decode the CAN messages.

Unfortunately I cannot tell you what particular messages are used for the brake as that is not standard data - maybe one of our other Pico technicians knows (although I expect you would have to provide full vehicle details to even have a chance there).
If nobody knows then you will need to apply some investigation into the messages to check what is consistently sent with different data when you apply the brake.

If you can identify the message that results from deliberate user action in applying the brake then you are part way there.
You can then use PicoScope to monitor the CAN bus and filter on messages, so you can check whether that message is being sent without user action. The serial decode has the search and filter tools you need there, and you can also put a channel to monitor the actual brake switch signal so you can tell if a spurious message is being sent.

I hope this makes sense and is of some help to you.

Hopefully a few others out there have practical experience of this type of investigation.

Best regards
Phil
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Re: CAN message meaning.

Postby heady2010 » Sat Feb 10, 2018 12:17 pm

Hi Phil,

Thank you for the reply, I will carry out some tests soon, I'm hoping that I will be able to identify the brake signal.

My next question is, if I isolate the brake circuit by cutting the brake signal wire before the abs computer for example and apply a direct feed into the computer so no other computer detects the brake switch apart from abs. Once the frame is generated in reaction to this, is there anything in the complete frame that will reveal which computer the signal came from? As you say it wouldn't tell me what that computer is exactly, but does it have a identifier so to speak, which would be the same as if for example if the abs/esp computer sent out a throttle body position change request?

I hope that makes sense.

Thanks
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Re: CAN message meaning.

Postby Steve Smith » Thu Feb 15, 2018 2:40 pm

Hello and thank you for the posts, sorry for the late reply.

It would appear that YouTube has a video for every question and CAN Decoding is no exception.

The following video ironically has the presenter attempting to ID the relevant CAN Node (Control Unit) responsible for transmitting the Brake Pedal switch status (amongst others)

https://youtu.be/Csn2pV3yQu8

Be careful though as whilst each CAN Frame contains an "ID Field", this does not necessarily represent the node responsible for transmitting the message you wish to decode, it represents more of a clue to the identification of the node based on message priority.

For example
E.g. Brakes on most probably is from the ABS node but as you can see in the video 2 Node ID's were indicated in the decode table (F1 and C9) when the brake pedal was applied.

I guess once you have narrowed it down to this level you could then take the nodes off line (disconnect) and review the decode table.

Again be careful here as your ABS node (controller) may contain a CAN termination resister which then introduces other variables and potential error messages.

Here you would then have to simulate the termination resistance value of 120 ohms in order to restore valid communication

I hope this helps, take care.......Steve
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