Cruise Control Failure When Lights On Audi A6 C5

Yesterday night, I discovered that on my Audi A6 C5 (year 1999), when the cruise control is activated and I then increase the brightness of the dashboard lights, the cruise control immediately deactivates.
I can turn it back on when the high beam is activated, and then it stays on until I turn the high beam off and on again.
I also made sure that I couldn't reach any switches.
Does anyone else experience this problem and have a solution for it?
Greetings.

Have you ever just swapped the gear lever and seen what happens?

No, I only realized yesterday night that it was like that.
There might be problems like that more often.

Hi Klaui!

I'm familiar with a similar problem from my A4 B5, manufactured in 1998:
Occasionally (very rarely), the GRA cannot be activated, or it may switch off intermittently due to an accidental touch of the lever.

The problem here is corrosion on the contacts of the GRA switch (a slide switch, not the buttons!).
When examining the GRA bits in the engine control unit (ECU) using VCDS, you can clearly see the instances where the system has failed.

Try to see if your automatic brightness adjustment (ABA) still turns off when you quickly and repeatedly switch the ABA slider back and forth between its two extreme positions (essentially cleaning the contacts).

Best regards from Munich!

Thank you for the tip.
I also had to find out today that it turns off when the turn signal is activated and can no longer be turned on if the high beam is on.

You're very welcome!

The question remains whether you have a wiring interruption in the steering column (lever), or – as in my case – a corrosion problem.

Best regards from Munich!

Will be tried tomorrow .

Okay, so I tried it and found that if I do this, I can at least get a very faint flashing light on the Autobahn without the engine immediately shutting off.
However, if I pull the turn signal lever down all the way, it turns off again.
I'd like to try cleaning the contacts; do you have any tips for me on that?
How do I remove the steering column lever? Or what's the procedure for doing so?
Greetings.

Hi Klaui!

No, you misunderstood me when I mentioned "cleaning contacts."
I didn't disassemble anything or perform any visual inspections.
My A4 had been sitting unused in a damp garage for over a year due to [reason omitted]. Afterwards, I discovered that the cruise control (actually GRA, not cruise control!) would intermittently fail, and also that the sunroof (the rotary potentiometer on the headliner) would sometimes automatically go into tilt mode and/or not fully close.
Throughout the entire year, the steering column switch for GRA was set to "ON", and the sunroof control knob was in the "CLOSED" position.
Consequently, corrosion has formed specifically at these contact points.
If you repeatedly move the slider switch of the GRA (roof control module) or the sunroof dial past this temporary stop position, it will work again (for a while). That's what I meant by "cleaning the contacts."

Did you approach the problem in that way? As mentioned above, it could be a broken cable rather than a corrosion issue with the sensing surface(s), which is something you might be able to identify with that method.

In reality, only replacing the part will help, because when dealing with such delicate connections, contact spray often causes more damage than good.

Best regards from Munich!

Yes, I misunderstood you there.
Okay, then let's call it a speed control system, or in common parlance, cruise control .
The vehicle had also been standing for a good half-year, and it was equipped with GRA ("Gravity Recovery and Acceleration") enabled.
Okay, then I'll start looking for a used steering column lever.
Thank you.