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Weird CAN issue I've now seen a few times (different makes and models) - ideas?
In the last few months I've had a couple of different cars which had communications issues which would come and go and I really don't understand what was going-on.
When the problem was NOT present, the signals were a normal 1-2.5-4v pattern but when 'disrupted' the values of the signal were WEIRD - as follows.
On a Ford - I had what looked like a 'normal' CAN signal (reflected clean pattern) but high was 0v to1.5v and the low was 0v to -1.5v (and yes, my scope was grounded correctly) - almost as it something had lost it's ground/reference?
On a Skoda (VW) - I had high from 0-3.5-6v (varied slowly over time) with low being 0-1.5v or less - pattern still OK, just obviously not the right values...
Both of these faults would come-and-go even when no-one was anywhere near the car (not moving wires around or interacting with the car) - the Ford was resolved with an ECU replacement but the Skoda is still awaiting a fix as I cannot isolate the module causing the issue (VWs are a pain to break-up the CAN on)
Note: The VW starts and runs with or without the fault - it just won't engage gears when the problem is present - being a VW it's possible to diagnose the ECU/DSG via KLine even when the CAN is down but the Ford was a nocomm with the problem active...
Anyone got any idea what would cause this - I've seen plenty of CAN networks which were shorted to ground or 5/12v as well as missing a terminator but this is something else entirely??
