Where to splice into the CAN bus?

[u/Smoksul]

I'm looking for advice. This is more or less a continuation of my previous post:

https://www.reddit.com/r/CarHacking/comments/1ep2rwv/can_is_silent_esp32_via_obd2_port/

I'm doing some custom ECU data handling and for this purpose I need a performant way of accessing data from the engine. I made a PoC using the OBD2 port but this won't do in any serious case since it's limited by its protocol to 1 message per ~200ms.

Therefore I want to hook into the CAN somewhere in the car and sniff the frames. Unfortunately, I'm not a mechanic nor an electrician so reading the schematics doesn't give me the best idea how to do it.

You can find schematics here (SWIFT RS413): https://jdmfsm.info/Auto/Japan/Suzuki/Swift/2004-2007%20Service%20Manual/

From what I understood from the manual there's no easily accessible place where I could hook into the CAN. I analyzed some subsystems which have the access to the CAN and I think the reasonable shot there would be accessing it right before the BCM (Body electrical Control Module) unit which actually handles OBD2 communication. All the sweet data should be there. Still, I don't know if it's easily accessible at all.

I have no idea however how safe it is for the car, even if I'd use a CAN shield etc. My car isn't worth a lot but I'm kinda attached to it and don't want to fry something etc.

Can anyone with experience with these matters hint me towards the best approach here? Maybe I missed something obvious in the diagrams or maybe there's some pretty generic, easy way to hook into any car's CAN bus?

ECM Input / Output Circuit Diagram. 21 is OBD2 port, RED/WHT is CAN low/high

H is the BCM, no idea how to look for it though. Manual says: included in junction block assembly.

[u/Smoksul → u/[deleted]]

What do you mean there's no canbus lines? The diagram I screenshot is supplemented with an explanation (similar for RED):

[u/Lee2026]

Can wiring is always twisted pair.

Look for the twisted pair wiring in your harnesses.

Also some modules are “limited” by gateways. So while you may tap one section of the bus, it may not allow you see everything if there is gateway between the devices you want to query.

[u/Smoksul → u/Lee2026]

If I found them, I guess I'd need to disconnect them from the thing they're connected to and add some splitter in between. That's a bit scary for me.

[u/Scallyswags]

It’s pretty hard to screw up a CAN, they are super robust. And is the red and white twisted pair the only CAN wires in the ecu? If so that would be a fine place to splice. I’d personally use some Deutsch connectors. Cut the red and white wire, find the can power and cut that too, hook one end to a male connector, the other to female and then build a y-jumper using a separate set of connectors. That way if you ever need to undo something, just plug in the 2 connectors and the car will be back to stock.

[u/Scallyswags → u/Scallyswags]

Looks like you can even buy a Y connector. I’d prefer it to be a DTM size but you could easily make the DT size work.

[u/Scallyswags → u/Scallyswags]

This is my setup at the moment when I was sniffing the CAN from my AEM ecu.