OBD2 Diagnostics for Non-Diagnostic CAN Bus

Hello everyone,

In a modern Audi/VW vehicle, in addition to the "Diagnostics" CAN bus, there are other buses such as "Powertrain" or "Comfort." Different control units (each involved in specific functions) are connected to these buses. For example, the "Powertrain" CAN bus connects to the engine control unit, the transmission control unit, as well as systems like the high beam assist and ABS.

Can I theoretically connect to the "drive" bus using VCDS (or other diagnostic tools) and read data such as speed, oil temperature, RPM, etc.? ... or does it use a non-OBD2 protocol that is manufacturer-specific and cannot be interpreted?

Context: I would like to install a "black box" in the vehicle that discreetly records the aforementioned data, especially when I am not driving. A data logger connected to the OBD2 interface would immediately be noticeable and disconnected. Furthermore, it could potentially cause problems with software updates or similar processes if installed parallel to the diagnostic interface.

Sure, here is the translation of the text from German to English:

"Hi,"

Theoretically, you can do what you want, but in my opinion, it's more risky because errors on the diagnostic CAN bus only cause problems with diagnostics or updates, while errors on, for example, the powertrain CAN bus can have direct and serious consequences during driving.
Depending on what you're planning to do, working with the diagnostic CAN bus would be a safer option.

Best regards, Rainer.

You're welcome.

I'm mainly interested in knowing what the workshop is doing during the test drive, so I would need to run it simultaneously and discreetly alongside the diagnostic interface... And if that thing interferes with a software update, the damage could be quite expensive. Or can you even operate two devices at the same time using the OBD2 port? It's probably not designed for that, neither in terms of the bus itself nor by the VAG-specific equipment used in the workshop.

"... and behind the roof module (reading lights), I accidentally came across the 'power' bus, where everything would be easy to wire and conceal."

Okay, then perhaps a simple CAN logger connected to the drive's CAN bus would be the easiest way to record the interesting CAN IDs when the engine speed is greater than 0.

I doubt we'll get much more than "driving it out of the garage and giving it some gas." What could be so special about the test drive that it needs to be recorded, aside from potential issues with turbo boost control? Well, maybe how long the test drive was; it probably wasn't less than 300 meters. However, even then, you can check the speedometer, and whether the technician or the master mechanic actually drove the car, we still wouldn't know.

Best regards, Rainer.

If it's helpful, he can take it to the bakery - as long as he doesn't park next to other cars and, above all, doesn't rev the engine of the cold motor to its maximum RPM. That's just the problem with powerful vehicles; what starts as a test drive for the workshop often turns into a private performance test.

A logger that records instances where the engine speed or accelerator pedal position exceeded certain thresholds within specific time periods would be relatively easy to implement... unfortunately, it would be vehicle-specific.

Is something like that not possible?

Good question - if it's constantly querying the engine control unit via OBD2, then updates or flashing probably won't be possible.

There's a difference between simply observing the messages that pass by on the CAN bus (and recording interesting ones), and actively sending and receiving messages yourself, as is done with OBD2.

Best regards, Rainer.

You're probably right, and I might have to abandon my project. But thank you for the discussion; at least now I'm a little more informed.

The values all come in "analog" form to the MSG (gas pedal position, temperature, engine speed, etc.) and are only converted to "digital" format (via a bus) when they are of interest to other components (when they request the data or are serviced at specific intervals).

... and as you probably correctly wrote, the thing I linked will constantly query... and I'd rather not try that out on the "drive" bus to see if it's a good idea.