BMW MOST Bus: A Troubleshooting Guide to MOST problems

This article will help you to:

• troubleshoot MOST fiber optics bus failures on BMW.

The information we provide on the functioning of the MOST bus is also relevant for Audi, Porsche, Rolls Royce, Range Rover HSE, Bentley, Mercedes or any other car model which uses the MOST fiber optics network technology.

A quick overview of how the MOST bus was developed

In the late 1930s, the AM radio became standard in cars. Modern for that time, it introduced entertainment in the auto-mobile industry. As technology grew, the entertainment grew to infotainment, offering both information and entertainment.

Come 1986 and Buick Riviera introduced the very first touch screen infotainment system, definitely preceding its time. With the sudden uproar in infotainment technology, car manufacturers needed something to keep up. So, in the 1990s, CAN-Bus was introduced to the market.

Widely used in production vehicles of that time, CAN-Bus was sufficient to handle most of the infotainment system. However, its bandwidth was limiting it to control signal only and it could not be used for audio / video streams. This drawback of the CAN-Bus made it literally impractical for use in infotainment equipment as a sole method of communication and, by 1995, the world needed something, well, more revolutionary.

Here is where the MOST network technology comes into play, a technology that was made possible by collaboration of car manufacturers and suppliers.

The Media Oriented Systems Transport or MOST has become the new communication system of 21st century. Able to transmit synchronous audio signals, it has a function-oriented design . It can easily handle the transmission of streaming and packet data along with control information, fulfilling all multimedia needs.

In fact, it is so practical that it is used by almost every car brand including BMW, Mercedes, Audi, Honda, Hyundai, Jaguar, Land-Rover, Porsche, Toyota, Volvo, SEAT, Skoda and many more.

MOST: how it works and where it fails

So far, we talked about how great MOST is. Well, now it’s time to understand why. What makes the MOST fiber optics network so special?

How does the MOST bus work?

To understand how the MOST bus works, imagine a subway network with only one set of tracks. This network is in a closed loop and your train (i.e. your signal) can only travel in one direction. And since we want our network to be extremely fast, we use optical fiber to send the signal and our signal is basically a red light.

To generate a signal, we first need to provide an input into our infotainment system. For example, starting the car or trying to change the radio station from the control button would count as an input. As the main-computer registers the input, it generates a signal and launches it on the MOST bus. The signal then travels through the network passing through each and every device attached to this bus (a bluetooth module or a sound amplifier, to name just a few), just like a subway train passing through each station.

The number of stations depends upon the number of options or features your infotainment system offers, which varies from car to car. These stations include the bluetooth module, the AM/FM tuner,,the sound amplifier etcetc. Once the signal travels through all the stations it goes back to the head unit that incorporates the main computer hence completing one full loop.

Since the whole MOST Bus is one big loop that is transited by light signals, it’s extremely fast and it is not sensitive to electromagnetic interference. These are its two main advantages However, the MOST bus also comes with a large drawback.

Problems with the MOST Bus

As the MOST Bus has a ring type topology, the signal has one path to travel and it can only reach its destination if it passes successfully through each node. And each of these nodes is another electronic module. So, if something goes wrong with any of the nodes (if any MOST network device malfunctions), the ring becomes broken and the communication path becomes unusable . Obviously, this is not something that happens daily, but once it does, it’s not pretty.

The worst part is that unless you know how to diagnose a broken MOST network, you can’t spot the problem yourself and you might end up paying hundreds of Euros in diagnostic fees at the dealer’s. So, how do we diagnose MOST bus malfunctions and how do we solve them ?

Symptoms of a BMW MOST Bus Failure

The easiest way to spot whether you have a MOST failure or not is by interpreting certain symptoms. Do note that these symptoms vary between car models and they also depend on the particular engine they use.

To give an example, if you have a BMW E65 or BMW E66 or you own a Roll Royce Phantom, a faulty MOST can cause any of the following symptoms:

• Your audio system doesn’t work i.e. no sound or music
• You lack the navigation function
• Your AM/FM is disabled
• You cannot see tracks playing on your screen

In case your BMW is an E60/E70/E89/E90/F01, it has multiple modules (SVS, ASK, GPS, MOST Gateway, Radio Tuner) incorporated into your head unit. This means that many of the common functions will continue to work, even when a faulty module is preventing the communication on the MOST bus.However, secondary modules or additional features would not work due to the MOST network being blocked by a faulty node.

In such a case, one would experience any of the symptoms:

• The sound amplifier won’t work and you see a“mute” icon on the display
• The SAT NAV receiver not work
• The TCM or Telephone modules fails

So, what causes MOST bus failures ? The modules which most commonly fail and lead to a blocked MOST bus are the navigation units, the sound amplifier or the bluetooth module. And in some unfortunate cases, you would see two of them failing at the same time

Now that we know about which symptoms we would see due to faulty modules, it’s time to learn to troubleshoot and find out which one is causing it.

How to identify a faulty MOST module?

Finding the faulty module on a MOST bus is quite easy once you understand the logic behind it. Since the MOST bus is one single loop, all you have to do is to find the module after which your optical signal stops travelling. To do so, you first need to understand how the modules are connected i.e. what order they are connected in.

• For the E65 BMW and BMW E66 7 series (2006-2008) the order is MMIGT Screen – CDC – AVT (optional) – TEL – SAT (HD Radio) – Logic 7 Amplifier – SVS – Navigation GPS – ASK – OOPS (MOST loop in Glove box) – MMIGT.
• On BMW E60/E63/E89/E90/E92/E93,you will have a combined head unit with multiple modules, so their order is a bit different. HEAD UNIT CCC/CIC/NBT – Logic 7 Amplifier – SAT- TCU – MULF – HEAD UNIT.

In case you are not sure about the model of your BMW iDrive head unit, here is an article that will help you find out what iDrive head unit you have on your BMW ?

Now that you know the order in which your MOST devices are connected to the network all you have to do is to start with the first unit. In our first example, it would be your MMIGT screen and for the second one,it would be your iDrive head unit, – most likely a M-ASK CCC, CIC or a NBT navigation. These four iDrive head units are only the most popular models. Check this article for a complete list of iDrive navigation systems on BMW.

Now simply check each module from the MOST network, one at a time, in the order we mentioned You just have to unplug the fiber optics connector from one module at a time. Now check to see if there is a red light coming out from the fiber optics cable that you just unplugged. If you see the red light, reconnect the cable and move on to the next module until you find a break (no red light).

Once you find a break (you remove the fiber optics from one module and see no red light coming through), it means that this unit is not receiving any fiber optics signal. And what does this indicate ? As you surely guessed, this means that the module preceding this one is not allowing the fiber optics to go through. So, the module you checked right before this one is the faulty node on your MOST network.

Usually, it is the iDrive head unit, the bluetooth module or the sound amplifier the faulty module that is causing the interruption in the communication on the MOST fiber optics bus.

Bypassing BMW MOST

In order to further confirm/diagnose if a module is faulty or not, we use what is called the MOST bypass female connector. Once you found a break in the MOST loop (once you identified the suspect module), you can use such a fiber optic loop to bypass the faulty module. What this does is basically to remove the faulty module from the network and to replace it with a connector that closes the loop back so that the light signal can continue its path.

Doing this not only can save a lot of time and money but it also tells you whether you have other faulty modules on the network or not.

All you have to do is to find the module after which no red light is passing through (see the previous section). Now unplug the fiber optics connector from this unit and fit a MOST bypass connector into the cable you just unplugged. If this fixes all your issues, then that is your only faulty module. Otherwise, you need to continue to check the next modules attached to the MOST network

Now that we have finally found the problem, let’s see how we can solve it.

BMW Bluetooth Module Diagnostic

The BMW Bluetooth modules are one of the most common reasons for MOST failure. The MULF, MULF2 and MULF 2 High are the three common Bluetooth modules for BMW. The bluetooth unit is usually located in the trunk of your car. The most common symptoms caused by a faulty MULF are:

• Bluetooth menu is disabled on your navigation’s menu
• Phone cannot connect to it
• Disabled phone connection icon on the head unit display
• Disrupts the functioning of the iDrive head unit.

If you are experiencing such symptoms or if you used our troubleshooting steps and identified the MULF as a possible cause, all you have to do is to use a MOST bypass connector to confirm the module is faulty. The MULF modules are located in the trunk, close to the boot trim around the left wheel arch of BMW 5 series (E60). For BMW 1 and 3 series with E87 and E90, remove the lower boot lining and look for the MULF module.

If you still want to enjoy the functions of a bluetooth module, you can use a MULF bluetooth repair service, which would be the most cost effective solution.

BMW Sound Amplifier Diagnostic

The Logic 7 Amplifier is another module that commonly leads to a dysfunctional MOST bus. This can happen due a lot of reasons, however you can spot it easily. Since it is basically responsible for your audio system, the following symptoms are most likely caused by a faulty amplifier

• lack of sound on one, multiple or all sound channels (basically, no sound is comming out from some or all speaker)
• unusual noise coming out from the speakers (crackling, loud noises, etc)
• Or it interferes with the functioning of iDrive Navigation (the iDrive responds slowly to commands, there is a lag until it executes the commands)

If you used the steps we mentioned above and saw that no red light is coming out through the fiber optics cable after the Logic 7 sound amplifier, you can have your Logic 7 repaired, you can remove it and recode your iDrive head unit to work without an external sound amplifier or you can purchase another unit (the most expensive option).

But what if you have a faulty HEAD UNIT? What do I do then?

• Get a M-ASK iDrive repair
• Get a CCC iDrive repair
• Get a CIC iDrive repair
• Get a NBT iDrive repair
• Get a NBT EVO iDrive repair
• Get a HU ENTRY iDrive repair
• Get a HU ENTRY NAV iDrive repair

Finally, we’d like to close with reassuring you that even if having a faulty MOST bus sounds like a big headache, it doesn’t have to be a big dent on your pocket. With simple tools and a little knowledge, you can identify the faulty module from your MOST network and you can have it repaired by professionals for a fraction of the price the dealer would charge you.

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