1997 BMW 740 audio question/help

Factual Questions
1

As usual, the BMW specific boards have failed me completely. They are so compartmentalized that unless someone has the exact model, color, year, tires/wheels, shoe size and problem as me, the will never even see my post. So, once again, I resort to The Dope:

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(Long story short: What is downstream of the Head Unit between the speakers, if anything? Is there a speaker distribution processor or something? The beast has 16 speakers and there sure as heck doesn’t seem to have that many wires coming out of the head unit.)**

97 740 with all stock BMW audio stuff. 230k miles on the car.

For a while, the audio system would cut out for a few seconds, then come back on. No presets or eq settings were changed. Everything would work as normal.

Then, suddenly, no sound at all.

I first swapped the head unit (had one laying around, but don’t know if it is good or not). Nothing. I then swapped the amp. Nothing. Tested both my amps in another car. Both good. (anybody need an amp cheap? )

All the computer stuff works, radio info, clock, tuning (what do they call that little skinny unit? Business center or something?) and DSP all seem to work fine. The tape in the head unit works, however, the bass/treble and loudness aren’t working.

Do I have two bad head units or is there something downstream from the head unit to the speakers. The car I used to check my amps had a different head unit with the GPS stuff, so I was unable to use that to check my system further.

What do you all think?

      • Updated - - -

Messed with it some more just now. All the functions on the head unit seem to work, its just that the display on both units is very dark and hard to read. The display on the original unit was working fine right up till and beyond the no sound issue. Now, it seems to have gone toes-up.

Could it be something on the steering wheel? Is there some kind of master “reset” button somewhere?

Special shout-out to Rick: Can you get me a wire diagram for the back of the head unit in the dash? I can do some more probing if I can isolate the power in and speaker outs.

2

Better question is what is between the head unit and the amp? Surely the head/dash unit controls the amp in the trunk and the amp powers the speakers.

But the amp is good. It works in another car. Same model, specific multi-pin plugs.

So where am I going wrong? Is there some kind of loop, where signal goes from the head to the amp, back to the head, then out to the speakers? Bah! Need more Beer!

On a side note, driving around at 75 mph with no music and sheer silence makes you appreciate how quite this car really is at speed. I heard a tiny little squeak up under the passenger side dash I’ve never heard before. And don’t want to hear anymore! :smiley: