Hi,

I have a 2010 GLK350 that has the no audio issue problem. Command still works fine, but randomly reboots when door is opened or closed. I’ve spent hours in these forums leaning about this system and I believe the rebooting is because the MOST ring isn’t closed. ie a failed component. I’ve also read that you can do a bit of troubleshooting using the initial blinking red light that occurs when you first start the command system.

so I pulled the amp and turned on command. I get the red blinking lights so I know there isn’t a break in the fibre. I then located the satellite radio and I’m also getting the red light there. So I think that actually means my amplifier is good since it’s sending the signal onto the Satellite radio module. I also confirmed this by unplugging the fibre at the amp and the red light no longer is present at the satellite radio module.

I’ve purchased a bypass loop for the satellite radio module as I don’t really care if that works. I just want audio again. It hasn’t arrived yet so I’m not sure if that’s the issue and I’m also not 100% that’s the broken module.

I’ve poked around a bit in the trunk and found the subwoofer amp, although that is all copper connections fed from the main amp. Does anybody know what other modules are on the MOST ring in a GLK350 and where they may be located.

Could somebody also confirm if my troubleshooting was even valid? I assume if the amplifier was dead it wouldn’t be able to pass on a fibre signal to the satellite radio module.

I just don’t want to buy a used amp on eBay if that isn’t even the issue or if I could just bypass the other failed module with the bypass loop.

TIA

 

Here are the engineering menus for my MOST system. So I’m assuming I have one more module, the UCI, if I’m am understanding the information correctly. Meaning the Rated MOST configuration is what should be there and Current Configuration is what it actually sees.

 

eBay has used units cheap ! Worth a try .

 

Which unit though? I don’t want to just buy the amp or command unit without determining which one it could be. The rebooting is pretty suspect, but then I read in one of these forums over the passed few days that happens when the MOST right is open (I mean I am not an expert, so no idea if that’s true or not)

im pretty annoyed with the fact they use a fibre optic ring for all this. There is a reason why token ring topology in the Ethernet world is long gone. This is essentially that. One item in the ring can kill it all.

 

The unit in the dashboard has outputs in back you could plug in a something like a boom box that takes input. Headphone might work . I’m guessing here. Have you pulled the unit out of the dash to check the connections? an electronics repair shop should be able to diagnose it. My stereo installer shop would know how to test it , that’s their full time job. A good stereo installer shop might be the easiest option . My shop upgraded my stereo and had everything out very easily and has fixed issues along the way over the years. You are guessing, a good tech will know what to look for.

 

Okay, I may take it in. I’m trying to avoid dumping too much money into this car. We have had it for 10 years now and it’s almost time to just cut losses and move on lol.


just as more information. I went into the engineering menus again and completely turned off MOST 7. HW Setting > 7.5 TGW Parameter > MOST : OFF and rebooted the command unit.

No longer getting the reboot when the door is opened or closed so I truly believe it’s a MOST issue with one of the components in the loop. Once I have that loop bypass I’m pretty sure I can narrow it down to which component in the loop isn’t working.

If I put the loop bypass on my amp and restart the system I’d expect to open and close my door and not have it reboot if the MOST loop is now closed. Assuming it’s the amp that’s dead (hopefully it’s not)

Again I am just guessing right now and mainly putting this all here as a compilation of all of the other 20ish forum posts I’ve gather information from. I am a network engineer by trade so perhaps I am a bit more technology inclined inclined then others, but now this is almost like a challenge for me and I don’t want to let the car win lol.

 

Is feed to amp fiber optic or line output with cable ? If cable plug in something to see if power - sound . Maybe a meter, or headphones? Or a portable stereo . I don’t know how to test fiber optic. Or a bad connection maybe just loose. Again I’m just guessing.

 

a lit cigarette lighter near one end of the fiber and your eyeball looking into the other end of the fiber cable :-)

 

It is fibre optics fed which is how the amp receives the audio signals. IMO it was a terrible design, I would hope that the new Mercedes aren’t using this system because like I said from all the research I have done if one component dies then you lose audio even though the amp may not be dead.

that is the idea behind the bypass loops. It is essentially just a piece of multi mode fibre that loops back on itself so you can bypass a component in the fibre ring.

once I receive that I can bypass the amp, see if things are at least working, no rebooting on open door, MOST diagnostics in engineering menu showing the other components, etc. I’ll then move to the Sirius Module and do the same thing if bypassing the amp doesn’t at least close the fibre ring.

I am hopeful that because right now I get the red light pulse on the fibre line on command boot at the Sirius module the amp isn’t actually broken and it is either the Sirius module or the UCI module that is broken, both of which I don’t care if it’s bypassed in the loop. I suspect that the amp passing on the red light pulses (Head Unit trying to communicate with the items in his fibre ring) means it is powering on and working otherwise I wouldn’t see the communication passing through the amp.

I’ve also determined what the UCI component is in the loop. It’s is the media interface in the glove box where you would plug in a iPod for example. I haven’t ripped the dash apart yet to get to it because I am very hopeful it is the Sirius module which is much easier to get to.

so I guess to put more information out there for anybody else searching this in the future. The 2010 GLK350 MOST fibre loop is as follows

Head Unit

External Amp ( located in left rear compartment)

Sirius Module ( located in spare tire area of trunk, left of the center fuse panel. You’ll see it if you open the door to the fuse panel and look left. The trim can be pulled apart enough without fully disassembling everything if you need to troubleshoot and bypass it.)

UCI Module ( located on left side of glove box, haven’t determined yet how to access the connections, but I suspect pulling the Head Unit may be the solution to get access to it.)

it then connects back into the head unit to complete the loop.

 

You can also purchase a Visual Fault indicator for cheap ( also know as a ruby red in slang terms ). I have them at work, but didn’t really feel like I needed it since like I said the command unit sends the communication pulses on startup which you can see. If it was single mode fibre though where the laser used isn’t in the visual spectrum I would never suggest you look in a fibre. You could damage your eye sight.

So as a Network Engineer I guess my disclaimer is never ever look in the end of an active fibre if you don’t know if it’s single mode or multi mode.

Hint Multimode has Orange, Teal or magenta Jackets (Typically safe to view the laser light being emitted, though I didn’t tell you that). Single Mode typically Yellow or Black Jacket which is high powered laser light in the ~1200-1600nm spectrum and never safe to look at. You’d also instantly know if you goofed and took a laser to the eye. Pretty instant headache from what I hear.

I also have actual light meters at work which would give me the dBm receive levels on the loop, but again I don’t really think it’s necessary to go that deep as it’s short runs of multi mode fibre where I can physically see the laser pulses.

 

Yeah, you would want both ends of the fiber disconnected before looking into one end. We used the cigarette lighter now and then for a quick check. The light from the cigarette lighter flame will travel hundreds of feet or more down the fiber. Won't tell you attenuation or about unwanted reflections but... then again, as you mention, you don't want to be looking into the end of a fiber if the person at the distant end didn't disconnect the right one :-(

 

I did it! It was the UCI media controller. The box that the little cables plug into in the glove box. Holy **** they make it super hard to get to though. I had to drop the entire glove box. What an absolute pain in the ***.

 

Wow great 👍 was it just a bad connection?

 

that orange loop is the fibre optics bypass loop. So really I just unplugged it from the UCI module and bypassed it permanently. Wrapped it all in electrical tape to the side of the UCI module so it’s not moving around and rubbing on anything sharp. I don’t need it so not gonna bother replacing it, but you can get used ones on eBay if someone had same issue and really wanted that feature. I did recheck a bunch of the fuses so no idea what’s wrong the UCI.

I’ll also say dropping the glove box is not easy. Almost a full dash disassembly required to get access to three screws under the trim piece in front of passenger seat.

 

You can see the loop sticking out here too before I taped it all up and put it all back together.