I have had central locking problems for some while (see earlier thread) and believe now that the problem is an open circuit can bus.

 

From studying the subject I read that the resistance (battery disconnected) between CAN H and CAN L should be approx 60 ohms created by 120 ohm termination resistors in parallel at each end of the canbus, please correct me if that is not so for the Octavia.

 

I have 120 ohms measured at either of the front door pillar connectors which indicates a broken wire somewhere, probably at a door or tailgate shut.

 

I suspected the tailgate as that was the first to play up intermittently and I had removed the solenoids etc and generally tugged around the wiring, looking again today I dont see any can bus wiring inside the tailgate unless it uses different colours and is not a twisted pair, the manual indicates no can bus to the talgate components.

 

I have seen can bus wiring or what I thought was (a twisted pair) in the loom after the rear door leading towards the tailgate, possibly it was something for the CD player?

 

So the question is as in the title, where are the 2 resistors located so as to help me with my faultfinding, I really dont want to be cutting apart the door panels and destroying the deadlock mechanism to open the deadlocked passenger doors if the problem is elsewhere.

 

Another strange thing that I cant get my head around, if I disconnect the door cabling at the connector and measure the can bus resistance upstream it is 120 ohms but the corresponding contacts to the door controller are either 5 ohm, drivers side or 20 ohm passengers side yet if one is connected and the measurement taken at the other disconnected one it is still 120 ohm whereas i would expect to see a little less than the 5 or 20 ohm, are they all in parallel? I used to work on RS485 2 wire data networks a long time ago and all the nodes on that were in parallel.

Been reading up some more, seems the resistors are contained within the ECU's at either end of the network, I had misread it as meaning the main engine ECU but the door controllers are CAN BUS ECUs (nodes).

 

So I reckon probably the rear door units but could equally be the front ones, my disconnecting them to measure the resistance might explain the missing 120ohms, will check tomorrow.

 

I measured there because I was getting open circuit between pins 6 and 14 on the OBD socket, I need to do a lot more checking tomorrow.

 

Seems strange that the CAN BUS comms are working for the window motors but not the central locking and even weirder that the fuel filler flap should now not be working as that I'm sure is hardwired, maybe where I have been mucking around with the fusebox.

 

Any info or experience on CAN BUS gratefully recieved.

I'm sorry I can't add anything to your CANBUS learning as yet but I'm very interested to learn along with you as I've recently acquired my first CANBUS car ...

Edited 26 July, 20187 yr by StickyMicky