Left Coast Geek

as mentioned before on this thread, MOST is an optical bus that connects the head units in the dashboard with the amplifier(s) in the trunk which drive the speakers.

way I see it, there's at least 3 failure modes. 1) bad MOST optical transmitter inside the head unit; 2) bad MOST fiber optic 'cable'; 3) bad amplifier


I dunno how you would diagnose MOST without a known working MOST decoder box and a spare pair of MOST fibers so you could verify the head unit has output.

Ceric

I FOUND AND FIXED THE PROBLEM:


I spent some time doing research in to MOST and how it works, and I bought myself an Autel MaxiAP200 scanner that has a VERY GOOD compatibility with Mercedes (I had an iCarsoft MB V2.0 and its junk, don't buy that).


I took the scanner and looked at any of the modules related to the audio/entertainment and that's where I found the MOST diagnosis menu, it let me know that I had an error code:
D019 MOST communication is faulty (ring break at position 2)





So, what is position 2 - what component is that and where is it? Because obviously that's where the problem seems to be. Most people think that it's the audio amplifier and I think I know why, but that is WRONG. We now look at the MOST component list and it looks like this:





It shows us a list of all possible MOST components in the Mercedes W212, whether they are programmed in to the car, and weather the MOST system sees them. Here it looks like the Sound System is in position 2, but that is wrong. This is just a list in a random order and does not represent the actual order of communication that happens in the MOST system on the Mercedes W212 (the so called MOST ring). If we dig a little deeper trough the menus on the scanner we can find the actual sequence of the MOST components in the MOST ring, that's what we need !!!





You absolutely need this sequence to know what component is in what position of the MOST ring on your car. I don't know if all the cars follow this sequence, but in my car this sequence aligns PERFECTLY with the physical distance of the components from the head unit.

MOST position 1 ---- MOST master ---- Audio/COMAND
MOST position 2 ---- Component 1 ---- Media Interface
MOST position 3 ---- Component 2 ---- Sound System
MOST position 4 ---- Component 3 ---- Satellite Digital Audio Radio (SDAR) tuner
MOST position 5 ---- Component 4 ---- TV tuner

Now we can actually intelligently come to the conclusions that the earlier mentioned error code D019 MOST communication is faulty (ring break at position 2) means that MOST position 2 � Component 1 � Media Interface is having some kind of problem and is not communicating with the rest of the MOST ring, and since the MOST ring is fiber-optic and not copper/electrical it requires that every component have an INPUT and an OUTPUT so that it can take in the fiber-optic line do what it needs to do with that date, and then pass it along to the next component down the line, but if the component in question is bad/burned out/fuse blown/or just simply dead -- it is going to take input data and never pass it down the line to the other components and this is now our so called "ring break".

SO, ring break at position 2 is the Media Interface module, it's that module that allows you to connect an old school iPod or an Aux cable and connect it to your car. It's now 2024 and this is officially ancient technology, so instead of looking at repairing this module, or finding out what caused it to fail and spending countless sleepless nights doing even more research on how to buy a new one and program it to my car, I am simply going to completely remove it from my car and bypass it, so that the fiber-optic signal can just get passed on to the next module down the line. All I lose by bypassing and removing the Media Interface module is the iPod and Aux cable function. I personally don't care about either one. I still have Bluetooth, I still have the USB input, I still have the memory card input, I still have the onboard hard drive, the radio, the satellite radio, and the CD/DVD player.

Bypassing a component in the MOST loop is very simple, all you have to do is to disconnect the MOST connection to the component in question and connect a MOST Fiber Optic Loop to the end of the MOST connection, it looks like this and you can buy it on Amazon for dirt cheap and have it shipped to you the same day:




I had to remove the entire Glove Box assembly on the Mercedes W212 in order to get to the Media Interface module as it is buried deep inside the dashboard, I then disconnected the MOST input to the module, took the MOST Fiber Optic Loop and connected it to the end and the entire Audio system in my car immediately came back to life !!! The audio now works, the satellite radio menu is back, and everything just works!! I took the Media Interface module completely out of the car and tossed it out, I wrapped the electrical connections that went in to the module in cloth tape so that they are protected from any accidental short circuits and protected from creating any rattling inside the dashboard. Problem Solved !!!

I will attach a few pictures to show exactly where the damned Media Interface module is:

Left Coast Geek

so what function does that 'media interface module' perform ?

Ceric

It allows you to connect an old school iPod to the car, it allows you to connect a USB to the car, and it allows you to have an AUX IN cable.

However, our W212 cars also have a seperate USB connection in the arm rest storage compartment that connects directly to the head unit, sonyounstill have USB in to the car even without the media interface module.

Therefore, by disconnecting and bypassing the Media Interface Module; the only thing you lose is the ancient iPod function and the AUX IN cable function. Both of those are no longer used today, anyways.