MSA12 Connection Interruption, 1Z Renovation

Good evening.

After a motor conversion in a commercial vehicle, I'm encountering a peculiar problem.
Boundary conditions:
1.9 TDI engine, code 1Z, from a 1996 Audi A4 (B5).
Control unit MSA 12 with WFS disabled.
Engine wiring harness modified and reduced to the essentials (no EGR, no glow plug heating, no GRA).
The brake pedal switch and the clutch pedal switch were both implemented using a relay circuit and a pressure sensor in the brake line.
The brake signal relay is controlled by a "positive" voltage, while the clutch is switched to ground. The control unit receives a 12V positive signal from a normally open contact.

VCDS runs smoothly on a MacBook with a virtual machine and Windows XP, and works without issues on all vehicles.

When I press the clutch pedal, my relay activates, the normally open contact disconnects, causing the 12V to drop, and at that moment, my MacBook loses connection to the diagnostic dongle with the familiar sound of a USB stick being unplugged. Afterwards, I can easily reactivate the dongle, and the process starts all over again.
In MWB 6, I can still see the signal switching, and then it stops.
In MWB 7, there is no voltage drop at this moment (suspected short circuit).
The engine is also running smoothly without any issues.

What could be the reason for this error? It's not a problem for me, but it's still interesting to observe.

Best regards,
Thilo.

You mention that the error occurs when a relay is activated and cuts off the 12V power. Is the diagnostic interface also connected to this 12V circuit?

Alternatively: Is another relay failing at the same time? The reason I ask is that if the relays don't have flyback diodes connected in parallel with their coils, it could simply be a voltage spike from the relay coil when it switches off.

Regards,
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

Hello Guste and thank you for the reply.
"Yes, you're right. The relays are connected to the same terminal as the OBD interface. If I remove the normally open relay instead of energizing it, this effect doesn't occur. This confirms your theory about a voltage spike."
I will temporarily connect the OBD connector directly to the battery for testing purposes; that should be sufficient to provide a stable power supply.

Brief feedback.

After I moved the OBD port to a different location for power supply, the problem was resolved.
Thank you to Guste for the correct idea .


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

"Best regards,
Thilo"