I'm at my wits end here and without being a software engineer, I can't figure it out.
Engine is a 2002 ALH TDI with a 038 906 012 FF (or FD, I have two ECUs to work with). IMMO3 disabled via full version VAGdashCAN on both ECUs. Vehicle is 1986 VW syncro westfalia
VCDS (v11.11.4) USB-HEX-CAN communicates just fine key on, engine off. I scoped the k-line with our picoscope/breakout box and it has a normal waveform.
When I try to communicate with the engine running, VCDS comes up with too many communication errors. It usually manages to pull the ECU part number off and once pulled the full information and lit up the buttons. When I scoped that, the k-line voltage rises up to alternator voltage ~14v but instead of the waveform dropping slightly to almost ground while communicating, it falls on it's face, full ground signal for a significant amount of time. Only once while VCDS is trying to open the ECU does it give me a normal waveform (very short amount of time). So it's obvious I have an issue with the k-line.
CAN high/low lines communicate pretty much the same with engine running or not.
I have tried adding in another ground to ECU connections, unplugging the wire to the front DLC, different ECUs, multiple tools (VCDS, superVAG, Launch, I'll try our 5052 once the battery charges) and everything says the same thing.
Anyone have any insight into this? I scan cars all day long at work with my VCDS cable, never any issues so my hardware on my PC is correct.