SOS malfunction, no audio issue SOLVED

[warranty]

Hi guys,

I'm posting this information to share with others who may have the same issue.

I have an '08 M3 with CCC iDrive, with the Individual Audio System option (S752), aka enhanced premium sound (EPS). Two days ago I lost audio (no sound whatsoever), lost bluetooth connectivity, and the "sat" and "aux" inputs of my system is shown as grayed out in iDrive. I also got an "SOS malfunction" alert in the system. This was somewhat "intermittent" in the sense that for a few minutes the next day, I did get full audio & bluetooth functionality back for a few minutes, and then it malfunctioned again.

The audio system has a "MOST" fiberoptic daisy chain that, in my car, goes from head unit -> amplifier (on the left side of the trunk, under a plastic cover) -> TCU/MULF box (the Continental-made silver box on the right side, under the trunk floor cover) -> satellite tuner (the silver box on the left side, under the trunk floor cover, with a cooling fan next to it) -> diagnostic connector under the driver side knee panel -> head unit.

I followed the MOST diagnostic instruction shared by others, and concluded that:
1. My headunit is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the amplifier (when unplugged, of course)
2. My amplifier is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the TCU/MULF box (with amplifier plugged in, but TCU MOST connector unplugged)
3. My TCU/MULF box is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the satellite tuner module (with amplifier and TCU plugged in, but satellite tuner MOST connector unplugged)

Having eliminated headunit, amplifier, and TCU as the likely culprits, I concluded the satellite tuner module (the only electronic unit left in the MOST daisychain) is mostly likely flaking out and dying. I was able to quickly test this out by plugging in one of those fiberoptic bypass loops I bought online into the satellite tuner MOST connector. Indeed, my audio system worked just fine, with bluetooth working, when I bypassed the satellite tuner module.

If you have the same issue (a dead satellite module), you'll have to replace the satellite module if you care about listening to satellite radio, or else you can just bypass it in the MOST daisy chain.

One final tip: since I never listen to satellite radio anyway, I decided to have a more "elegant" solution than leaving a fiberoptic bypass loop plugged into the satellite MOST connector. This involves swapping the fiberoptic cables to bypass the satellite tuner in the MOST junction box located behind the driver-side rear seat bolster.

To do so, pull the bolster out and find the MOST junction box that's clipped into a metal tab. The junction box has green fiberoptic cables going into it. You'll want to (carefully) pry open the plastic covers of the black plastic boxes that can each accommodate 5 pairs of fiberoptic connections. You can pull out the fiberoptic cables by using a very small flat screwdriver to gently lift the plastic retainers in the box. This involves a little trial and error: I would pull out the cables one at a time and use a flashlight to shine into it and confirming which is connected to the TCU MOST output ad which is the satellite module MOST output. Then, basically, make re-insert the fiberoptic cables so that the TCU MOST output goes directly to the satellite module MOST output, thus bypassing the satellite tuner module completely.

I was worried that I would have to replace something expensive, but as it turned out I just had to bypass the useless (to me) satellite tuner to solve the problem. I didn't take any pictures, but it's all pretty straightforward. Hopefully this helps somebody in the future.

[Nickco43]

Originally Posted by warranty

Hi guys,

I'm posting this information to share with others who may have the same issue.

I have an '08 M3 with CCC iDrive, with the Individual Audio System option (S752), aka enhanced premium sound (EPS). Two days ago I lost audio (no sound whatsoever), lost bluetooth connectivity, and the "sat" and "aux" inputs of my system is shown as grayed out in iDrive. I also got an "SOS malfunction" alert in the system. This was somewhat "intermittent" in the sense that for a few minutes the next day, I did get full audio & bluetooth functionality back for a few minutes, and then it malfunctioned again.

The audio system has a "MOST" fiberoptic daisy chain that, in my car, goes from head unit -> amplifier (on the left side of the trunk, under a plastic cover) -> TCU/MULF box (the Continental-made silver box on the right side, under the trunk floor cover) -> satellite tuner (the silver box on the left side, under the trunk floor cover, with a cooling fan next to it) -> diagnostic connector under the driver side knee panel -> head unit.

I followed the MOST diagnostic instruction shared by others, and concluded that:
1. My headunit is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the amplifier (when unplugged, of course)
2. My amplifier is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the TCU/MULF box (with amplifier plugged in, but TCU MOST connector unplugged)
3. My TCU/MULF box is likely fine as I can see the blinking red fiberoptic signal going INTO the MOST connector at the satellite tuner module (with amplifier and TCU plugged in, but satellite tuner MOST connector unplugged)

Having eliminated headunit, amplifier, and TCU as the likely culprits, I concluded the satellite tuner module (the only electronic unit left in the MOST daisychain) is mostly likely flaking out and dying. I was able to quickly test this out by plugging in one of those fiberoptic bypass loops I bought online into the satellite tuner MOST connector. Indeed, my audio system worked just fine, with bluetooth working, when I bypassed the satellite tuner module.

If you have the same issue (a dead satellite module), you'll have to replace the satellite module if you care about listening to satellite radio, or else you can just bypass it in the MOST daisy chain.

One final tip: since I never listen to satellite radio anyway, I decided to have a more "elegant" solution than leaving a fiberoptic bypass loop plugged into the satellite MOST connector. This involves swapping the fiberoptic cables to bypass the satellite tuner in the MOST junction box located behind the driver-side rear seat bolster.

To do so, pull the bolster out and find the MOST junction box that's clipped into a metal tab. The junction box has green fiberoptic cables going into it. You'll want to (carefully) pry open the plastic covers of the black plastic boxes that can each accommodate 5 pairs of fiberoptic connections. You can pull out the fiberoptic cables by using a very small flat screwdriver to gently lift the plastic retainers in the box. This involves a little trial and error: I would pull out the cables one at a time and use a flashlight to shine into it and confirming which is connected to the TCU MOST output ad which is the satellite module MOST output. Then, basically, make re-insert the fiberoptic cables so that the TCU MOST output goes directly to the satellite module MOST output, thus bypassing the satellite tuner module completely.

I was worried that I would have to replace something expensive, but as it turned out I just had to bypass the useless (to me) satellite tuner to solve the problem. I didn't take any pictures, but it's all pretty straightforward. Hopefully this helps somebody in the future.

When I was doing CIC kits professionally for five years whenever there was a most bus issue and I did not find water damage in the trunk it was almost always the external sat module. Every once in a while I would see a faulty TCU.

Thankfully everything connected to the most bus (besides the EPS amp) can be found for less than $200 used now.

[warranty]

Originally Posted by Nickco43

When I was doing CIC kits professionally for five years whenever there was a most bus issue and I did not find water damage in the trunk it was almost always the external sat module. Every once in a while I would see a faulty TCU.

Thankfully everything connected to the most bus (besides the EPS amp) can be found for less than $200 used now.

Is anything special needed in case the EPS amp or the TCU needs to be replaced? Like, would one need to find an exact same type of amp, and does the TCU need special coding to work with the headunit?

[Nickco43]

Originally Posted by warranty

Is anything special needed in case the EPS amp or the TCU needs to be replaced? Like, would one need to find an exact same type of amp, and does the TCU need special coding to work with the headunit?

9 times out of 10 you can get away without coding any of the trunk most modules. The amps have coding depending on the body style and the rest of the modules are different depending on if the car had nav or not.

Regardless any of these can be coded using a phone app like Pro tools or Bimmercode with just a cheap license.