Resource - MOST bus fault patterns - Bimmerfest - BMW Forums

Welcome to Bimmerfest -- The #1 Online Community for BMW related information! Please enjoy the discussion forums below and share your experiences with the 200,000 current, new and past BMW owners. The forums are broken out by car model and into other special interest sections such as BMW European Delivery and a special forum to voice your questions to the many BMW dealers on the site to assist our members!

Please follow the links below to help get you started!

Go Back   Bimmerfest - BMW Forums > BMW Model Discussions > 7 Series & 8 Series > 7 Series DIY

7 Series DIY
Do It Yourself forum dedicated for 7 Series BMW Owners

Closed Thread
Thread Tools Display Modes
Old 07-27-2008, 07:42 PM
BMW_tech's Avatar
BMW_tech BMW_tech is offline
Officially Welcomed to the 'Fest
Location: halfway there
Join Date: Sep 2007
Posts: 2,019
Send a message via Yahoo to BMW_tech
Mein Auto: rickshaw
Resource - MOST bus fault patterns

Alright folks. I know I have discussed this before and either we make this a sticky icky in order to stop the MOST noises. If I'm starting to sound like a MOST bus failure, you're wrong. Try another CD or a different AUDIO SOURCE. Take it upon yourself to isolate the problem before you get disappointed. Don't ever get discouraged, it's your vehicle so I ask all of you to be a part of the solution. Equipped with the basic knowledge, you cannot be fooled by an untrained SA or tech. This also helps resolve MOST cases if you could talk to the technician and explain the symptoms. If you are on the same page, what could go wrong? Give your opinion but don't stress that replacing that module will fix your problem. That's NOT how it goes, you're providing inputs to the technician so he can perform all the neccessary diagnostic steps without overlooking other areas like battery voltage or disconnections by the Power Module. Failure of a control module always causes an audio failure in the MOST framework.

The MOST network is a bus for transmitting picture and audio data. In the E65, only audio data and control commands are transmitted. Bus communication functions in one direction. The control modules are connected in series in the bus. The bus has a ring structure. If the temperature exceeds 70 °C in a device in the network, the control display indicates that the corresponding functions are no longer available. The entire MOST bus is powered down if the temperature exceeds 85 °C. Only the control display recognizes the current application and its parameters such as e.g. the volume setting for listening to the radio. The audio-system controller stores information on which control modules are installed. This list of equipment can be called up in the service menu of the control display. The equipment cannot be called up if the ring is interrupted. The audio-system controller manages the audio sources by making available internal channels for outputting and by outputting the individual channel after being prompted by the control display. If the optional extra TOP hi-fi is fitted, the audio low frequency is recalculated prior to output and the improved audio low frequency is output.

Audio system controller (ASK)
---The audio system controller (ASK) is a control module in the MOST network. The ASK is responsible for managing all audio signals.
---The following are part of the ASK: A single drive for audio data media, a combined rotary control/push button, an auto search button and an eject button.
---The ASK generates a number of audible sounds, e.g. the PDC (Park Distance Control) gong.
---At temperatures above 70°C the ASK shuts down. The MOST functions then are no longer available.

Control display (CD): Apart from a few exceptions, the control display incorporates the controls and displays for the entire vehicle. The control display coordinates the function requests from the system and assigns them to the relevant functions.
The Control Display is the only control module that knows which function is currently active on the system. If the volume is to be increased (function request), the control display sends a telegram to that effect to the ASK. That message also contains the information indicating to which audio source the volume adjustment applies. Infotainment audio output is always a certain amount louder than the entertainment source.

LOGIC 7 or Top Hi-Fi : The output stages for the subwoofers and the rear door speakers are integrated in the Top Hi-Fi amplifier. The output stages for the front door speakers and rear parcel shelf are located in the audio system controller. Acoustic control is assumed by the Top Hi-Fi amplifier.
Note : Hi-Fi amplifiers (NOT LOGIC 7) only amplify the audio signals for the left and right subwoofers.

Power module functions
Optimum charging
Reduction of load-side peak consumption
Shutdown of auxiliary consumer units
Quiescent current monitoring
Storage mode
Automatic electrical system isolation
Load deactivation
Electronic circuit breakers
Central battery voltage notification
Rear-window defroster
Interior lights
Trunk lid and fuel filler flap control
Data memory
Fail-safe characteristics
Check-control messages

Optimum charging
The optimum charge voltage is set according to the charge condition and battery temperature. Maximum 16V. The power module knows what the charge level of the battery is by calculating the battery current when the vehicle is being driven and measuring the discharge current. When the vehicle is not in use, the charge level is calculated and updated by measuring the quiescent battery voltage. If the charge level falls below the calculated minimum level for starting, the idling speed is increased to improve the charge balance.

Reduction of load-side peak consumption
If battery discharge is detected while the engine is running (despite increased idling speed), the power supply to electrical consumer units is gradually reduced or completely shut off according to a table of priorities.

These consumer units are :

Rear window defroster
All seat heaters
Heater fan (without defrost function)
Steering wheel heater
Mirror heating
Wiper rest heating

If the battery voltage drops below 10.5 V (for 5 seconds) due to high load levels, the Power Module sends out an instruction to increasing the idling speed and to activate prioritized shutdown of electrical consumer units. At the same time, the power outputs of the power module (interior lights, isolation of consumer units in roof/body zones) are switched off. A Check Control message is output.

Shutdown of auxiliary consumer units
In order to ensure that the car is capable of starting, the charge level of the battery is also monitored when the vehicle is not in use.
Dependent on :
outside temperature measured over last few days.
engine model and capacitance of the fitted battery.

a minimum battery charge level required to ensure that the car can be started again is calculated. If the charge level of the battery gets close to that minimum level as a result of running an auxiliary consumer unit, the Power Module instructs that unit to switch off.

In the case of auxiliary consumer unit shutdown, two operating states must be distinguished:

1) Auxiliary consumer units at terminal R
2) Auxiliary consumer unit at terminal 0 (ignition off)

Auxiliary consumer units at terminal R
With the change from terminal 15 to terminal R, the information priority 1 is transmitted. The reduction of the power consumption is reduced according to the corresponding table.
The charge of the battery is calculated. If the value of starting capability is still not reached, a Check Control message is displayed.
If the charge condition does not improve, the power module sends the message ”Shutdowncounter” after 5 minutes.
After a further 90 seconds, the vehicle's electrical system is shut down.
The condition here is that no legally prescribed consumer units are switched on.
The message Shutdowncounter reports to all control modules that the battery will be shut down in 90 seconds. This gives all control modules with memory function the possibility to write their data in the EEPROM.
If the switched on consumer units include legally prescribed consumer units, the battery is not shut down. Complete discharge of the battery is permitted.

Auxiliary consumer units at terminal 0
If the driver switches on an auxiliary consumer unit, this logs onto the power module with the message ”Powermanagement auxiliary consumer units”. This log on enables the power module to calculate the charge more precisely.
Following correct log on, the auxiliary consumer unit goes into operation and the charge level of the battery is monitored. If the limit for start capability is not reached, a Check Control message is issued.
After another 5 minutes, the power module switches over to quiescent current monitoring. Precondition: No legally prescribed consumer units are activated.

Quiescent current monitoring
When Terminal 0 is active, the power module switches over to closed-circuit current monitoring after 60 minutes. If an operation is performed on the car before 60 minutes has elapsed (e.g. operation of the interior lights, central locking system), the timer for closed-circuit current monitoring restarts.
Once that period has elapsed, the quiescent current should not exceed 80 mA. If the closed-circuit current exceeds 80 mA, after 5 minutes the message ”Shutdowncounter” is transmitted by the power module. After a further 90 seconds, the vehicle's electrical system is shut down for 5 seconds. This shutdown for 5 seconds resets the hardware of the control modules. This places the control modules in a defined initial state. A control module that may be defective can be returned to proper functioning by this operation.
If the closed-circuit current still exceeds 80 mA when the system is switched on again, the sequence described above is repeated.
If the closed-circuit current is then still over 80 mA, the system is permanently shut down via the battery master switch.
The reason for the shutdown is stored in the power module.
At terminal 15 On, the electronic battery main switch is closed and a Check Control message is displayed.

After disconnecting terminal R, an additional immediately current monitoring run is carried out. If the measured current exceeds 120 A, the interior lights, consumer units in the roof and body zones are switched off immediately.

The power module calculates and stores the charge condition and the time of the battery with the relevant charge.

The charge ranges of the battery are displayed in the following increments:

The time the battery spends in each charge range is displayed in hours.
During normal driving, the battery should spend the longest period in the charge range above 60%.

A higher proportion of dwelling time in low charge ranges around 10% - 40% can have the following causes:

1) Poor driving cycle with greater discharge than charge
2) Frequent operation of auxiliary consumer units: telephone, independent heating, etc.
3) Frequent operation of sidelights, parking lights, hazard warning lights. In the case of legally prescribed consumer units, the power module does not shut down the vehicle electrical system. Complete discharge of the battery is possible.
4) Increased standby current (fault memory entry present)
5) Defective alternator

Unfavorable driving profile
The battery can be depleted due to unfavorable driving characteristics :
- In the case of extreme short-distance driving, it is possible that starting the engine takes more energy from the battery than is charged in the subsequent trip. The charge balance deteriorates if a great many current consumers are switched on during the subsequent trip.
- Very few trips: If the vehicle is moved very infrequently and the individual trips are not sufficiently long, it is possible that during the immobilization period more energy is taken from the battery by normal closed-circuit current than is charged while the vehicle is being driven.

All audio signals are sent as digital signals across the fiber-optic cable (MOST bus). If the light flow is interrupted, not only the synchronization in the MOST ring but also all data transfer is interrupted. The interruption in the light flow frequently originates from a control module and is rarely of mechanical origin. Every MOST bus node receives the light signal from its predecessor and passes it on with its own diode to the next.

Audio symptoms

Sustained noises in the CD Changer
---The MOST ring is permanently interrupted. Ring break or MOST bus diagnosis should be carried out.

Brief audio failures
A) When the failure occurs followed by a magazine check from the CD changer and the sound returns......
MOST likely cause :
Stationary vehicle: CD changer or Power Module disconnection.
On engine start: CD changer or a poor state of charge of the battery.
As of terminal 15: CD changer.

B) If the stored radio station lists are lost here.....
---The MOST likely cause is the antenna tuner.

C) If the failure occur in the radio mode and do not occur in other audio sources. The communication systems work normally and not affected.......
---The MOST likely cause is probably the antenna tuner.

Sustained noise from the loudspeakers

A) If the noise only come from the front door loudspeakers or from the back shelf :
---The ASK supplies the front door loudspeakers and the back shelf. Noises from these loudspeaker can also originate from other audio control modules (telephone, navigation system, etc.). Bridging these audio control modules can pinpoint the fault.

B) If the noise only come from the central loudspeaker at the front, from the woofers under the seats or from the rear door loudspeakers :
---The MOST likely cause is the amplifier (Top-Hi-fi).

C) If it only occurs in the radio mode and NOT with any other audio sources.
---The MOST likely cause is the antenna tuner.

Monitor failures

A) With a terminal change ”ignition off” to ”terminal R”, the entire screen remains black (status bar not visible). The MOST - CAN gateway receives no terminal status from the CAN bus.
---Possible causes are the instrument cluster and the CAS.

B) During operation, the entire screen goes black (status bar not visible) and its starts again with the BMW logo. The Control Display has run a reset.
---MOST likely cause is the Control Display.

C) The part of the screen that displays the navigation information flickers white and works again.
---The MOST likely cause is the roadmap CD/DVD OR the NAVI computer.

D) The part of the screen that displays the navigation information goes black and stays dark.
---In MOST cases it's the NAVI computer.
Old 12-30-2008, 04:50 PM
M.Wong's Avatar
M.Wong M.Wong is offline
E38 E91 (R56 E30 E39)
Location: Seattle WA
Join Date: Jan 2003
Posts: 2,325
Mein Auto: The E38
Thread closed as a resource. For the related discission, please go here:
E38 2000 740iL Orient Blue
E91 2012 328xiT Alpine White

Passed on to new families:
E30 1987 325i Royal Blue
E39 2003 540i/6 Sterling Gray
R56 2010 Mini Cooper S British Racing Green

Closed Thread

See More Related BMW Stories

e65/66 most bus issues

Forum Navigation
Go Back   Bimmerfest - BMW Forums > BMW Model Discussions > 7 Series & 8 Series > 7 Series DIY
Today's Posts Search
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -7. The time now is 08:04 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
© 2001- VerticalScope Inc. All rights reserved.