Please note that this is an identical thread to one I just posted in the MB Roadster -> SL-Class (R129) section. Just trying to get the widest possible review of this problem.

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My '96 SL600 Bose system has always been a bit 'bassy'. There is pair of 'baby' speakers (tweeters?) installed in the top of the dashboard, but no sound comes from them. The entire setup is factory-installed, standard Bose.

I've asked in previous threads and the general consensus is these two speakers should have sound coming from them. The test suggested was to crank the bass all the way down, treble all the way up and listen. Logical. On my system, I get sound from all speakers except these two.

The car had a factory telephone installed that had been partially removed by the time I got the car. So, now I'm wondering if these two speakers are tied to or controlled by some of the MB-factory telephone hardware. For example, in a *complete* Bose radio/factory telephone setup, when the phone is in use, does the radio 'cut out'? If so, does the telephone use just the two dashboard speakers? See where I'm going with this. I'm wondering if some of the missing factory-installed telephone hardware actually is needed to pass audio to the dashboard speakers.

Does anyone have an electrical schematic for a R129 model, around the mid-90's? Preferably, for a '96 SL600 - but at this stage, I'll look at anything.

 

I'm not familiar with your car's electronics - but it seems unlikely (possible, but unlikely) the speakers would be tied only to the phone. Most OE car systems mute the audio for all stereo sources, which would include the other speakers as well.

Rather than needing all of the phone system, it is possible that the muting wire for the partially removed system is activated - but I believe that if that happened you would get no audio at all. That's not likely to be the cause.

I suggest trying to physically trace the speaker wires and see if they are cut or disconnected. My bet is, they are. If so, it is likely the wires or connections that went to the speakers are near the cut or point of disconnection.

At my age and with my back pain, if it were my car I'd take it to an audio shop and let a young guy do the twisting and crawling under the dashboard.

 

Thanks Skylaw for your thoughts. It is plausible and certainly feasible, even though I have not come across any sort of cut or damaged wires. But ya know... it never hurts to check out each possibility.

As for tracing wires ( ), I agree that there are far younger and more nimble folks who might enjoy a good wire-chasing challenge.

 

In a nutshell, the little dashboard speakers are *not* tied directly to the amplifier. After accessing the amp (directly behind the driver's seat in an R129), I disconnected the one cable harness that interfaces to the amplifier. Using a small telephone tone generator, I shot signals down each logical pairs of wires, listening for the tone out of each speaker. Of the wires remaining, I used a very small speaker to "listen" to the audio output from the factory head-unit. Only three wires remained, and they were very logical. Here is the result:
Left-Front Speaker Output from the Amplifier - pins 13 & 21
Right-Front Speaker Output from the Amplifier - pins 14 & 22
Bose Bass-Box Speaker Output from the Amplifier - pins 15 & 23
Left-Rear Speaker Output from the Amplifier - pins 16 & 24
Right-Rear Speaker Output from the Amplifier - pins 17 & 25
Left Audio INPUT to Amplifier (from Head-Unit) - pins 10 & 18
Right Audio INPUT to Amplifier (from Head-Unit) - pins 1 & 2
Power-ON signal to Amplifier (same as Power-On Control for Antenna) - pin 8
Main +12vDC Power - pin 26 (Red w/White Stripe)
Ground - pin 9 (Brown w/Green Stripe)

No other wires are connected to the amplifier. Thus, the baby dashboard speakers are not connected directly (if at all) to the amplifier. This suggests either the speakers are used exclusively by the factory-installed telephone unit, or there is another connector/junction box that controls the baby dashboard speakers.

The wires leaving the back of the Head-Unit look to be about 22-gauge. These are similar to what is feeding the amplifier. The speaker wires leaving the amplifier are about 20-gauge. But the wires feeding the baby dashboard speakers look to be about 18-gauge (!). Note: this is relative insulation size, not necesarily true wire size.

If I go to the trunk, where the factory-installed telephone stuff is located (bolted next to the CD-Changer), there are *no* wires of the large size - coming or going. Same is true for the wires that feed the dashboard telephone control module.

Also, after looking closely at the baby dashboard speakers I find it very hard to believe they could take the power generated by the amplifier - the speaker coil/magnet are fairly small; about the size of an old transistor radio speaker. I know these are MB factory parts as there is an MB part number on each.

Conclusion: Unless someone is willing to share a schematic, I (or someone more nimble than I) would have to do some *serious* interior tear-down to trace the wires. So, I plan to live with quirk (if it is a quirk). I suspect however, there is another control box - somewhere - that turns off the amp and uses the baby dashboard speakers for 'hands-free' telephone operation. Lacking additional information, this is the only logical conclusion I can come up with. Any other ideas?

 

you are correct... the front two speakers are for the telephone on your model. there is a small box that connects the speakers back to the the motorola phone module though its been so long i cant remember where it is. also keep in mind that on many year SL's those speakers were connected to the audio system: the radio had two separate little 1/4" round 2 pin connectors that were the speaker outputs. those went to the goofy fader control on the center console and then back up front behind the radio to hook into the dash speaker connectors. an additional harness feeds the audio back to the amplifier where (as you traced) it feeds sound back to the door and rear speakers. the front speakers did *not* as i remember get power form the amplifier but rather directly from the radio.

i would do a quick check behind the radio and look for a set of harnesses with the round connectors on them... there should be the return from the fader knob (you do have the fader knob right?) and then also a loop through from the telephone so that it can interrupt the sound and send the phone output through those speakers. it may be the case that the phone system was disconnected and they never patched those speakers back into the audio system. the SL had alot of revisions in addition to the upgrade audio systems. after a while they stopped using those front speakers as tweeters and put seperate mid/tweeters in the doors. thats when i believe they switched to using those speakers only for the phone. like i said check the harnesses behind the radio including the ones not directly attached to the radio.

finally you might want to pull one of the speakers and make sure that they are not blown which was *very* common with those speakers.

 

Thanks rob13572468. My model ('96 SL600) does not have the center console fader control (I do remember that silly thing on an early year E-class). And I do not believe my Becker Head-Unit has individual "speaker" outputs, but will check as it is possible that I missed something. Finally, I know the dashboard speakers are functional as I used one for testing during my identification of the amp wiring (see text). However, your suggestion of looking behind the Head-Unit, within the center console is valid, and I'll give it a shot.

 

take a look at the back of the radio... since you dont have the fader, you should have the becker unit with the two 8 pin and one 10 pin connectors. the middle 8 pin connector has the speaker outputs and the middle four should have the front speakers connected to it (if they are connected). i have schematics for the 97 S600 somewhere but ill have to dig them up... if you really need them let me know and ill start looking...

 

one more thing: if you go in the trunk and disconnect the 25 pin connector from the motorola transciever box (its a DB 25 like a printer cable) you can test for the speaker output there. external speaker uses pins 19, 20. you can then try measuring impedance or hooking up a toner to see if the wires go direct to the speakers.