How to Troubleshoot MOST bus or how to find the faulty MOST unit ON ANY BMW, AUDI, PORSCHE, VW, MERCEDES BENZ and other cars with OPTICAL FIBRE OPTIC MOST network

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In this post we will be describing how to diagnose MOST bus failure in any BMW, AUDI, PORSCHE, ROLLS ROYCE, RANGE ROVER HSE, BENTLEY with MOST fiber optic bus. every car has different configuration, but principle is  still the same!

So in 2000 they introduced newer multimedia bus, called MOST, IT IS VERY NICE AND FAST NETWORK, SO THERE IS ENOUGH SPEED TO CONTROL EVERY MULTIMEDIA DEVICE AND THEY ELIMINATED LOTS OF COPPER WIRES IN THE CAR. NOW SOUND IS FULLY DIGITAL AND CLEANEST (BC NO SIGNAL LOSS ON THE WIRES), SO IT IS A GOOD THING, BUT ITS MORE COMPLEX AND NOT VERY RELIABLE.

So to start you need to understand that all MOST (multimedia) modules in the car work in the closed loop, which means if any device in the loop fails-none will be working.MOST bus primary signal (optical red light signal) is generated inside of the main computer, which is your main screen CID (control display or CID in E65 car only, other BMW have main computer incorporated into head unit or CCC/CIC/NBT etc), then it travels through all MOST network devices (ask unit, amp, navi, fm tuner, phone etc. depending on how many options you have) and comes back into main head unit. Basically all units connected in the loop- one unit fails=broken network! Once you understand this principle, you can easily find broken unit or network interruption, don’t forget dealer will charge up to 500-1000$++ for such a serious job! You  don’t believe me? go and try,  then you will understand how priceless with post is.

Your E65 E66 or Rolls Royce phantom ( yes don’t be surprised it is the same exact car as your 760li btw) car has MOST bus failure if:

-cannot hear any sounds or music

-cannot see tracks playing on the display

-fm stations not available

-navi isn’t functioning

For other BMW like e60/e90/e70/e89/f01 and others MOST BUS FAILURE symptoms are little different because primary head unit has 4-5 modules (SVS, ask unit, MOST GATEWAY, NAVI, Radio tuner) combined and therefore everything will work except for those secondary modules:
-amplifier no sound and you see mute icon on the radio or screen
-TCUM or telephone module
-SAT NAV receiver
Here is the MOST DEVICES order diagram , but this is only valid for  basic order and only for pre 2006 cars, you might have way more devices in the network, so you have to figure it out yourself by looping eac modules and draw a diagram of the modules.
Here is a quick video showing how to find a bad unit in real time:


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FOR 2006-2008 E65 E66 cars basic order will look like this:

 

MMIGT(screen)>>CDC>>AVT(if you have it)>>TEL>>SAT(hd RADIO)>>L7>>SVS>>NAVI>>ASK>>OOPS(GLOVE BOX MOST LOOP)>>MMIGT

FOR E60 E90 E63 E89 E90 E92 E93 and so on:

HEAD UNIT CCC/CIC/NBT>>L7AMP>>SAT>>TCU>>MULF>>HEAD UNIT

 

On these cars in most cases i see sat  and amplifier unit failure  very often, telephone module goes bad too but not that often.CCC navi units have a high rate failure, but the good thing once you replace it, you replace 5 modules at once, so its more reliable and much easier to find a faulty unit because you only have 2 or 3 modules to diagnose and all of them located in the trunk. We also offer CCC or CIC repair service
So to find out the faulty unit you either need to see a dealer (but even dealer makes mistakes),  but here is an easy solution if you don’t have anything, and if you do it right it always works:

You need basically to find out where the most signal gets lost, after what device you don’t see the red light. In other words you will be checking the red light after every device in the network, and the one which is not sending the red light signal is your FAULTY MOST UNIT. OOPS is the last diagnostic connection located in the glove box and it has lollipop attached and it has to be looped there, some people think it is a spare jumper, but its not, without this jumper you will interrupt the network!

 

HOW TO BYPASS NON WORKING MODULE??? EASY! GET THIS P/N FROM ANY BMW DEALER 61136917541, YOU CAN GET IT ONLINE FROM ECS TUNNING ECS TUNNING FIBRE OPTIC JUMPER.  YOU HAVE TO REMOVE INNER PART WITH SOME FLAT SCREWDRIVER OR A KNIFE, YOU CAN ALSO USE ZIP TIE IF YOU PLAN TO KEEP THE JUMPER BYPASSED, IRT WON’T STAY WITHOUT ZIP TIE.

 

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most common MOST devices failure (descending from most common to less):

 

-SAT SIRIUS unit always goes bad, we will offer repair of this unit soon

TCU TELEPHONE BLUETOOTH MODULE RECENTLY WE SEE TONS OF THEM FAILING!

-Logic7 amp AMP

-ask cd unit ASK

-navigation NAVI

-fm tuner

-CD itself main screen CID

-cd changer CDC

-SVS (gets flooded with hydraulic fluid from trunk opener cylinder, if you see sticky fluid on the NAVI, SVS- replace asap)

other modules. depending what you have in your car

Always check if you have red light in the glove box, disconnect that jumper and check for the light, if you found and bypassed bad unit correctly you should see the red light in the glovebox and then you can install the jumper and everything should start working. I know it sound complicated but once you understood how it works it is kind of very easy.

BUT PLEASE DO NOT BLINDLY BYPASS EVERY MODULE, LIKE LOTS OF PEOPLE DO, YOU MIGHT FIND THE PROBLEM, BUT ITS NOT EFFICIENT AND SOMETIMES YOU CANNOT DIAGNOSE IT RIGHT. ALSO MAKE SURE EVERY MODULE HAS 12v POWER. BASICALLY EVERY MODULE NEEDS FIBER OPTIC SIGNAL AND 12V POWER ATTACHED TO FUNCTION.TO WAKE UP MOST NETWORK IT IS ENOUGH TO OPEN/CLOSE THE DOOR, YOU DON’T HAVE TO START THE CAR, THE THING IS MOST GATEWAY WILL GO TO SLEEP AFTER 10 MINUTES IF NETWORK IS BROKEN AND TO WAKE IT UP YOU NEED TO OPEN/CLOSE ANY DOOR OR TURN ON THE RADIO. 

THAT’S about it, IF YOU FOUND THE BAD MODULE YOU JUST SAVED 300-400$ AND TIME. REMEMBER LOTS BMW SHOPS STILL CANNOT DIAGNOSE IT RIGHT AND ALWAYS POINT TO ASK UNIT OR AMPLIFIER, “BECAUSE THERE IS NO SOUND”, WHICH IS NOT ALWAYS TRUE, I WOULD SAY IN 50% ITS NOT CORRECT EVEN FROM OFFICIAL DEALERS. WHY THEY MADE IT SO COMPLICATED AND NOT RELIABLE?? WELL THIS A FUTURE OF CARS AND TECHNOLOGY, YOU ARE THE ONE WHO ASKED TO HAVE ALL THESE TOYS IN THE CAR, SO YOU EITHER LEARN IT OR HAVE YOUR WALLET READY TO GET SKINNIER, OR BUY HONDA/TOYOTA….

 

86 thoughts on “How to Troubleshoot MOST bus or how to find the faulty MOST unit ON ANY BMW, AUDI, PORSCHE, VW, MERCEDES BENZ and other cars with OPTICAL FIBRE OPTIC MOST network

  1. Alan Alvarado

    I have a 2007 bmw 335i with the Idrive display my radio sound quit working out of no where I’ve replaced the amp twice due to the fact tht I had a leak in the trunk and it had got wet ,not we fixed the leak and put a new amp(used) and still no sound and also the fiber optic wire on the drivers dose not turn red ,the only ones tht turn read are the one from the amp (left rear trunk ) and the other 2 modules in the truck I belive the tire monitor and Bluetooth module ,wgats the next step to take ?

    Reply
  2. Bob Schweitzer

    A bit of a long story. I bought the 2010 528 used and the BT wouldn’t work ( phone never saw signal) and I got the failed emergency phone message (SOS) from day one. I started doing some web surfing and decided to investigate the MULF2 in the spare well. The casing is pretty oxidized and the board inside shows some signs of moisture. I blew it off with air and looked for any obvious failed components and re installed it. Before I started car decided to check all fuses that seemed to impact the MASK unit ,tcu or MULF2 ( I don’t have navigation) . Found no bad fuses but while checking inadvertently dropped one in the battery well. I disconnected battery and removed , fished the fuse out and reinstalled. Started the car and now to the BT and SOS failures I added failure to the I drive ( nothing on the screen) , no warning noises for seatbelt and no radio. I tried registering the battery but this did not help.

    I am a little flummoxed because. I know when I leave the MULF2 power connector on I get no red fiber optic signal from the incoming fiber cable or the unit itself. I tried to see if I got power from the connector and concluded no power if the wires I judged as power supply are accurate ( not positive if accurate) .

    Any thoughts? Are you aware of any diagnostic fault trees on this system? I would assume the MASK unit is the central controller of the other ECU’s and this would imply that it failed but it is hard to rationalized that disconnecting and reconnecting the battery killed it ?

    Reply
      1. Bob Schweitzer

        Ultimately I found a fuse had failed that caused the MASK unit to not function. Now radio works as well as I-drive function. I still have the problem with no Bluetooth and the SOS failure message. It seems that the MULF2 module is the last in line. I can confirm I have power to the unit and there is the red light coming into it from the MOST system. When I plug the MULF2 unit in ( power) I do not get the red light on the unit. If the unit is powered up and functioning properly should this light be on ?

        Reply
  3. BIGCANNONBALLZ

    I have a 2007 750 li, I washed my car, then went inside to clean inside. Heard the disc changer changing disc and humming/ ringing from speakers, (key not inserted). Put in key didn’t hear normal chim, bmw logo stuck on screen, no sound from aux or radio, seat belt indicator doesn’t make noise either. Read a few posts which directed me to trunk where modules are, and I hear the navigation DVD unit making noises like it’s on, even without key inserted. I did a 3 button reset which worked once but it only got screen back to normal, but no radio stations no sound no time or date. Turned car off and back to nothing and idrive spinning loosely. Before this happened the rear speakers sounded busted, I thought they were until I read up on these other issues that are connected

    Reply
  4. sam

    Hi there,

    i really need your help.
    I have 2009 e90 and currently my nav, audio, idrive, sos are not working.
    I’ve verified that the fiberoptic signal goes into TCU, but no signal to SDARs unit beside it.
    What could have gone bad? any input will be much appreciated.

    Reply
  5. Mark

    Hello. I have a 2007 650i. Found out that from a rainstorm my drain holes for vert top was plugged dumping water in the trunk area on the units. The first warning indicator to appear was sos failure. Then no sound for a short time then I had sound. Then in a second rainstorm the sos failure remained and a loss of sound from audio and my parking distance indicator stopped beeping also. I just read your article about the checks to do in the power flow with the lollie pop. Will have to purchase one. Thanks in advance.

    Reply
  6. BC

    I’m my friend BMW when u look at his antenna and separate it you see a light but there’s not no light on mines is there a fuse that could be causing this?

    Reply
  7. BC

    I have a 2008 750i in the glove compartment where the antenna is located I have no light at all no red light no light is present what could be causing this?

    Reply
  8. Beth

    Based on your info my Logic 7 Amp and ASK need to be serviced/replaced but will the failure of either one of these cause my AC to start kicking on by its self and my screen to sometimes go blank?

    Reply
    1. logic7repair@gmail.com Post author

      NO AC UNIT IS SEPARATE PROBLEM, YOU NEED TO REPLACE IT. SCREEEN FLICKERING COULD BE BC OF BAD LOGIC 7 OR ASK UNIT BUT COULD BE SCREEN ITSELF

      Reply
  9. Rudy

    Where is the multi-media for the most bus, I tried bypassing everything but my display is still blank. I get light from l7 but not to ASK,I’ve looped everything and still nothing.
    Please help

    03 BMW 745li

    Reply
    1. logic7repair@gmail.com Post author

      you should not blindly loop everything. idrivee is fully blank??? it should not be blank in any case, if it is it means actual scren is bad.

      Reply
  10. Omar

    2004 745 Li. It started with crackling. Then sometimes it wouldn’t come on. Now I get zero audio whatsoever. No door open sound, no radio, no navi, no anything. Sometimes my screen goes white. I can’t even select anything on I drive. If I go to settings and try to select something, I can’t. I’ve tried getting a lollipop and hooking it up to each module but I don’t get anywhere. What can it be??
    What’s the proper way to test with lollipop? Any help would be appreciated.

    Reply
    1. logic7repair@gmail.com Post author

      read the article on how to troubleshoot MOST failure, use diagramm ad you will find a faulty unit right away

      Reply

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