After reading through the past year's worth of threads on the ubiquitious H/K failures, I feel my problem is unique enough to warrant a new thread.

This is my system: 2004 R53 with factory H/K system, OEM iPod and aux inputs, and MFSW. I do not have a CD changer, nav system, bluetooth, 3rd-party add-ons, or any other potentially relevant nodes in the system. Up to this point, I have never noticed any issues with the sound quality or operation of the audio system.

This is my problem: I started up the car on a cold night (approx. 40F) after it had been sitting for about 3 hours. The stereo was off at that point (as a pilot, I have a habit of turning off all electrical components prior to shutdown). I then turned on the system and found that no sound (music, static, humming, or otherwise) was coming from the speakers. The head unit and MFSW controls appeared to work normally: cycling through various inputs, changing CD tracks and radio stations, going through audio menus... EXCEPT the H/K menu setting no longer existed. Sounds familiar so far?

Here's the rub: After about 5 minutes of fiddling with the head unit to attempt to get a response, I heard a series of high-pitched tones emanating from the right rear panel where I believe the H/K unit is mounted. This was NOT coming through the speakers; it was coming from behind the interior paneling in the boot. I have a rear seat delete, so it was very easy to determine the source of the sound. The tone would last for about 5 seconds, then would not recur for a couple of minutes. I heard about 4 instances of these solid tones, with the third instance being a lower pitch than the first, second, and fourth. The only thing I can liken this to is the sound that some motherboards produce to give POST error codes, although longer in duration (dashes rather than dots, for those familiar with Morse).

And the plot thickens: I turned the car off and on a couple of times to try to reset the system, but to no avail. The tones never occurred again. BUT...after I decided to continue with my trip, I discovered that the cruise control no longer worked! Pressing the on/off button on the MFSW would illuminate the green light on the speedo for about 2 seconds, then it would go out on its own. I tried to "capture" my speed by quickly turning on the cruise control and setting the speed hold in quick succession, before it could turn itself off, but no joy. I eventully gave up on that as well.

Has anyone experienced these unique problems? Any advice on where to start?

 

Since your seats are out it should be easy for you to just disconnect the amp. I would see if this fixes your cruise control issue. From there you will know that something is broken on the LIN circuit of the amp. Sounds like it could simply be the input or output transistor on this signal or the LIN decoder module on that circuit board. With the high pitch sound you heard it could be the switch mode power supply died thus making the LIN decoder hold the signal down.

 

My guess - your H/K died, and like gangrene, the death is spreading.
Seriously, never heard of this failure mode. The first could definitely the amp taking a (noisy) dirt nap, but the cruise is a new one. Have you checked the cruise fuse?

 

UPDATE:

After a couple days of not driving the car, the cruise control worked normally the next time I drove it. So now we have a more "normal" looking H/K failure, except for the mysterious tones. I hope someone can offer a theory on the origin of those tones.