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, BENTELY 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), then it travels through all MOST network devices (ask unit, amp, navi, fm tuner, phone etc. depending on how many options you have).

Your E65 E66 or Rolls Royce phantom ( yes don’t be suprised 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 avalaible

-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 cars basic order will look like this:

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

 

MOST BUS ORDER: FOR e60/e90/e70/e92/e91/e93

Head unit (MASK or CCC/CIC>>>SATtuner(if equipped)>>>L7DSPAMP>>TCUM(telephone module)

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 the located in the trunk



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, 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

-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.

LIKED THIS SITE AND THIS POST?


THAT IS 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?? YOU KNOW THE ANSWER-PROFIT$$!

 

43 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. Juan a Burgos

    Ok replaced ask unit bit on getting sound mids& highs on passenger side only oh & both subs work! Any idea?

    Reply
    1. logic7repair@gmail.com Post author

      most likely your new ask unit is no good. did you install new one or used??
      if ask unit is ok, you should get front doors and rear deck working at least. speakers almost never go bad

      Reply
  2. durobinet

    hello,
    both my front passenger speakers are crackling, is there any possibility that those speakers are defect (is it possible)? all other speakers are working fine on my logic7 .

    Reply
    1. logic7repair@gmail.com Post author

      speakers are fine, your amp is bad causing it.
      check on the bottom of the page, so we can help you out

      Reply
  3. jack chipindi

    My subs re not working but when I tested the two cables powering the woofer the be power both of them now what cud be the problem

    Reply
  4. jack chipindi

    My subs on my BMW 745i 2002 re not working but when I tested the two cables powering the woofer the be power both of them now what cud be the problem

    Reply
  5. Aaron

    What device sends the red signal to the navigation unit? When i unplugged the harness that’s where i don’t see a red light. The other harnesses have red light

    Reply
    1. logic7repair@gmail.com Post author

      Did you read the post with the diagram?
      Logic7>> Svs >>sends signal to navi, so check before and after logic 7. If car is before 2006 it is most likely the amp

      Reply
      1. andy joo

        Hi
        I have a more complicated l7 idrive issue, maybe this is one of the spin offs of common MOST BUS device failure or other equipment in the Multimedia network.

        After a minor collision i started having lights come on weird messages. I lost the beep for pdc, initial key beep, and the voice command works but no sound from anything.

        the whole system will keep on cycling on and off consistently every few seconds whether car is movingor stationary.

        The car was sitting in the body shop for 40 days, but i installed the correct AGM battery 7/14 and registered it witht the software and usb obd cable. I have looked through the inpa software or ncs expert but couldnt find anything logical that would help out.
        I also have driver and pass airbag faults and light is permanently on. The airbag light wasnt reset by the shop to see if it would come back or a mere jolt from the impact.

        The car was drivable after just popped tire and damadged rim. no fluids leaked, no check engine.

        If i try to turn the audio system on (all options are still available if i can carry out the functions i need (like setting the3 clock) before the whole system resets and turns off and screen flickers and turn back on, then ill have music for a few seconds (cd works, changer works, nav hasnt worked in over 6 years without effecting anything else. I have adjustment in the l7 levels,radio stati8ons, everything. My TPMS hasnt worked in 4 years.

        I also had an aftermarket amp and 10″ sub installed or added to the system (some piggybacking on the wiring and that was working without a single hitch for over 6 months. Now i noticed the aftermarket amp is not getting a remote turn on signal from where it was tapped into the factory wires right above the battery and on/off switch for power.

        I just figured out why my previous battery didnt last, my nav cd reader in the trunk has the ON and CD IN light always on, but i could never eject the cd, and nav screen says insert cd.

        Am i dealing with multimple possible failures? I do not have the fiber optic jumper eliminator pieces to eliminate a failing module. I have no desire to have navigation im fine without that old archcaiac setup.

        One last thing the idrive knob sometimes loses its feedback tension and it lags like on a computer when you type and it appears a second or two later sometimes too. when the things stays on for the few seconds i have full sound except my aftermarket sub connected to aftermarket amp. nothing is greyed out in the idrive.

        If you have a few minutes and can give me some tips with the symptoms above.

        It is a 1/2003 build 03 745i, 95k, bought as a CPO with 20k miles on it.

        Which bmw software is best to test modules and reset them? INPA? ive read through your threads btw. thank you for your efforts!

        Not having music daily is now starting to effect me wanting to drive the thing. I had no idea i was addicted to audio!

        is it possible to just run an aftermarket independent sound system without touching any existing pieces (l7 amp, speakrs, sub, ask, changer etc). Im sure it will labor intensive running new to the doors for some MBquart 6.5″ seperates with an existing 4 channel amp and just have 2 channels for the satelittes in the front doors and the other 2 channels bridged for the 10″ sub i l aready have in the trunk?
        only pro lem where do you mount an aftermarket deck. I know on this car there no real usable glovebox. Is that just foolish and crazy? The only wires i need to use from the factory is power ground, switched lead.

        Thanks,
        JSPEC E65
        jspec1@gmail.com
        510-444-4669

        Reply
        1. logic7repair@gmail.com Post author

          nothing serious… you need Logic7 amp fixed or replaced. send me your units and ill diagnose them and rebuild as well

          Reply
  6. Cosme Romero Jr.

    I troubleshoot my system and the red light is missing in the ask unit, communication module and the GPS. What could be the problem?

    2004 745Li

    Reply
    1. logic7repair@gmail.com Post author

      cannot be 3 modules, you didnt do it right.
      you need to find out which unt causes failure and why no red light in the last endpoint in the glovrebox

      Reply
      1. Sergio R

        I have the same problem with my car 2003 745i.
        All lights on the modules are working except the one at the glove compartment, What do you think the problem is?

        Thanks!

        Reply
        1. logic7repair@gmail.com Post author

          GLOVE BOX IS THE LAST POINT, SO IF YOU SEE THE LIGHT BEFORE ASK UNIT, IT MEANS ASK UNIT IS BAD

          Reply
  7. Cyprian Oldakowski

    Okay so I have an e60 545i 2004. There is no sound at all coming through my speakers. How this happened was that my mechanic replaced my alternator and did not tighten up the clamps on the battery and while driving hit a pot hole and the clamp came off which made my car shut off. I figured out that the clamp was off so put it back on and turned the car back on. The sound started going in and out immediately. Did this for a few minutes and then just cut off completely. I bought a used amp. Plugged it in and no difference. What do you think is the issue??

    Reply
    1. logic7repair@gmail.com Post author

      sounds like amplifier problem, but could be other unts a swell, you need to torubleshhot with my article

      Reply
  8. Jeff

    I have a 650i that has no sounds at all. I see the changer, nav radio stations, music on harddrive(?) and can connect bluetooth. My dealer said it was the tcu but I bypassed it with the loop. I see the red light going to the tcu.
    In the MOST diagram are you speculating that it is the ASK since that is the only thing after the TCU (if TCU=Tel).

    Reply
    1. logic7repair@gmail.com Post author

      most likely the amp is the problem, you didnt read it right, E65 system differs from e60/e63/e64

      Reply
  9. Shaun

    My stereo starts sounding a little staticky when volume is turned up. Also my driver side woofer under seat isn’t working at all. My driver door speaker doesn’t work either. Sometimes it may appear out of the blue then go off. Per your description of faults, it sounds like it is the ASK? Do you agree?

    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
  11. 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
  12. 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
  13. 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
  14. 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
  15. 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
  16. 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
  17. 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
  18. 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

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