CAN Bus Passat 3C: Brief Interruption

Hello Diesel Mechanic Community,

I'm new here and I hope someone can help me. My name is Jörg, I'm just over 50 years old, and about 25 years ago I worked for a few years in an independent workshop (mostly bodywork). I trained as a toolmaker. Until now, I've always been able to get my cars repaired without outside help. However, I'm now facing a problem that I can't solve myself, and I'm hoping you can help me:
On December 25th, 2017, the alarm system (DWA) of my Passat 3C (2006) went off in the afternoon. I unlocked and locked the doors once, and then it stopped. On December 27th, the battery was so drained that even the central locking system didn't work. Since then, the battery has always been completely discharged by the next morning (after a 40-minute drive in the morning and evening). After the weekend, the battery is so low again that the car barely starts, or only with jump starting. I measured the quiescent current using a clamp meter. The current drops to about 300mA, then rises again to approximately 1.2 / 1.4 A. This repeats roughly every 10 seconds.
"I can also see this using VCDS. If I don't lock the doors, the control modules remain inactive. In the error memory, both door control units show the "thermal protection active" error. I can clear the error, but it reappears immediately after the next "ignition on." The window lifters were definitely not activated. When I check the control unit using the measurement block 001, it shows "thermal protection inactive." The second error message that I can't explain is in module 69 (trailer). It indicates an implausible signal at terminal 15. Measurement block 001 says "status of terminal 15: off," while measurement block 006 says "status: on."
I'm also hearing a clicking sound from the comfort control unit. After each click, the bus state changes.

"I'm sorry for the long text..."

My question is now: Who is responsible for activating the control units? Are both door control units defective, or are the door locks the problem? Is the trailer control unit malfunctioning and causing this disturbance?
I am hoping for help or assistance.
Regards,
Jörg.

I would, as it is the simplest and simultaneously produces a conspicuous error message, temporarily remove the trailer control unit for testing purposes and observe the situation.

Hello dieselmechanic. Thank you for the quick response. What do you mean by "removing" it? Do you mean disconnecting the power supply to the control unit (STG), pulling a fuse, or removing it from the parts list using VCDS?

Jörg.

Disconnect the plug and remove it from the parts list.

Best regards, Rainer.

Thank you very much. I will test it tomorrow.

Regards,
Jörg.

In the "09" section of the extended measurement data, the last 4 alarms should also be included, along with their triggering reasons .

Hello,
I have now disconnected the control unit for trailer operation and removed it from the parts list. The current state is that the bus system goes into a quiet mode, which lasts until the quiescent current drops to approximately 35mA (sometimes several minutes). Then, there's a click in the central locking system (ZKS), and the quiescent current increases to approximately 800mA. After about 5 seconds, there's another click, and the quiescent current decreases again. The only change now is that the quiet state lasts longer. Could it be the door control units? I'm starting to despair. I've been working on this problem for about 3 weeks (measuring, removing fuses, measuring, etc.). The only advantage of all this is that I now have VCDS. Previously, I used CarPort to solve my problems. My wife got tired of my complaining and surprised me with a Hex-V2 . (Unfortunately, not from you). Nevertheless, I hope for further help in the forum.
Best regards,
Jörg

Hello Roger,
I checked the logs from 2009. I don't find any errors in the extended measurement blocks. Did you mean the error paths (I can't recall the exact name)? There are many entries there that all start with "DTC" (?).
Regards,
Jörg.

Do not look for errors, but rather for the source of the alarm, the trigger cause, or similar.

Okay, I'll search the manual and the internet to find some information about how to proceed. Thanks for the tip.
Jörg.

I just saw that he also has a separate size 46. I'll take a look at that too.

I will do it. Thank you .

Hello,
I've looked at the alarm sources and trigger conditions. The trigger conditions are understandable because they are described in plain text. However, the alarm sources don't make sense to me. For number 19, I might guess it refers to the control system (STG), but what about number 14? Can you help me understand this?
Thank you and best regards.
Jörg.

Address 46: Comfort System (3C0 959 433 M)

17:29:53

1. Alarm trigger, alarm horn.
2. Alarm trigger, alarm horn.
3. Alarm trigger, alarm horn.
4. Alarm trigger, alarm horn.
1. Alarm source 14.
2. Alarm source: 0.
3. Alarm source 19.
4. Alarm source: 0.

It looks like the backup horn is defective. This has happened in similar cases before. For example, it has its own battery inside.

The numbers mean nothing to me.

I'm assuming that the backup horn is the alarm horn, right? It's been broken for years. So I don't need to look any further in that area. Or am I mistaken about something?

Jörg.

According to Speicher, the horn is definitely the trigger. This is monitored for manipulation. Eventually, the defective battery will run out and cause the circuit board with the control logic to corrode. From that point on, it malfunctions and can naturally also wake up everything else on the bus.

However, that doesn't explain why the central ventilation system ended up being used for thermal insulation...