DIY: Troubleshooting BMW MOST Bus Fiber Optic System Sound Failure
How to Troubleshoot MOST BUS
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!
In 2000 newer multimedia bus was introduced, called MOST (Media Oriented Systems Transport), which is a very nice and fast network designed to allow enough speed within the system and to control every multimedia device. It also cut production costs a lot because it eliminated the need for expensive and heavy copper wiring throughout the car. With fiber optic cables the sound is highly quality and more pure (no signal loss through the metal wires), so, overall a good upgrade and much more reliable.
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's 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 (M-ASK unit, Amplifier, Navigation GPS, AM/FM Tuner, Car Phone, etc. depending on how many options you have) and then comes back into main head unit. Basically, all units are connected in loop and if one device fails, they all stop working! Once you understand this principle, you can easily find broken unit or network interruption, don't forget dealer will charge up to hundreds of dollars in diagnostic fees to locate the failed device!
Your E65 E66 or Rolls Royce Phantom (which is basically the same audio system as the BMW 760Li) commonly have MOST bus failure if:
- Cannot hear any sounds or music
- Cannot see tracks playing on the display
- AM/FM stations are not available
- Navigation System is not 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, Navigation GPS, Radio Tuner) combined and therefore everything will work except for those secondary modules:
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:
The 2006, 2007 & 2008 E65 E66 7-Series cars basic order will look like this:
MMIGT(screen) >> CDC >> AVT(if you have it) >> TEL >> SAT (HD RADIO) >> Logic7 Amplifier >> SVS >> Navigation GPS >> ASK >> OOPS (GLOVE BOX MOST LOOP) >> MMIGT
The E60 E90 E63 E89 E90 E92 E93 and so on will look like this:
HEAD UNIT CCC/CIC/NBT >> Logic7 Amplifier >> 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!
Want to know how to loop the Fiber Optic BUS line??? EASY! You can buy the Jumper Loop HERE.
- First Remove the inner loop with a flat blade of knife.
- Second, use a zip tie if you choose to keep the Fiber Optic Loop Bypassed (It will not stay without the zip-tie)
Most common MOST BUS device 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 Amplifier
- M-ASK CD unit (ASK)
- Navigation GPS
- AM/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.
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.
If you've located the bad module using the procedure outlined above, it saved you hundreds of dollars in dealer diagnostic charges. Remember, most BMW dealerships are not savvy on these systems and often misdiagnose the problem. They will commonly point to the ASK unit or Amplifier, "BECAUSE THERE IS NO SOUND" which is not always true. Remember, this is a first generation fiber optic system and with any new technologies there are always going to be small issues that need to be worked through for future generation models.