MOST Bus Diagnosis
The MOST network uses a ring bus for data communication between the various modules. Signal transmission is by means of a fiber-optic cable. Data transmission on the ring bus takes place in one direction only. On the the MOST ring, messages can only be transmitted provided the bus ring complete and fully functional.
If for example the power supply or the diode of a control module is defective, the MOST bus in non-operational and no communication is able to take place. Exceptions are the gateway control module (CCC or MASK) and the HUD control module.
Diagnosis Excessive Temperature
If a control module becomes too hot, this control module switches off for up to 10 minutes. This switches the ENTIRE MOST bus down for 10 minutes. After the control module cools down it (and the MOST bus) is functional again.
If this fault, "Deactivation excess temperature", is stored in the CCC (or MASK) and any of the other MOST control modules there are two possibilities
- If the fault is ONLY in the CCC (MASK), then this module is responsible for the MOST bus shut down.
- If the fault is stored in the CCC (MASK) and one or more other control modules on the MOST bus, the most likely cause of MOST bus failure is the other control module. However, this does not rule out the possibility that the CCC (MASK) is also defective.
Ring Break Diagnosis
Ring break diagnosis is already known from the E65. During ring break diagnosis on the E65, the voltage supply is interrupted directly at the battery.
Contrary to the E65, restart of the MOST users on the E60 is realized via "Transport mode." The following relays are switched off
- Relay 1 MPM.
- Terminal 30g.
These relays supply voltage to all control units in the MOST system network. On activating transport mode, the relays are switched off and the voltage supply interrupted.
After the relays have been switched on again and the control units started up, the multiaudio system controller/car communication computer can detect the control units that still have communication capabilities and evaluate the results. Consequently, the BMW diagnosis system specifies a nodal point that provides an indication of the interruption on the MOST bus.
The functional description in the BMW diagnosis system provides detailed information on determining the nodal position on the MOST bus.
Fault Symptoms E65
Particularly when carrying out troubleshooting in the MOST system network it is important to know the precise significance of visible and audible symptoms. With this information it is possible to locate and correct faults much faster.
The starting point for locating faults should always be to pose detailed questions relating to the fault to the customer. This information can then be used to formulate initial consideration before the BMW diagnosis system is connected.
The following information should be available to the technician
- When does the fault occur? E.g. already during engine start or while driving?
- Since when does the fault occur?
- Does the fault depend on temperature? When cold, hot or after longer period of vehicle operation?
- Do several functions fail simultaneously? If so, which?
- Is the fault still present after shutting down and restarting the vehicle?
- Was something switched on or off immediately before the fault occurred?
- Is it necessary to enter the telephone PIN after restarting the system?
- Does the radio/CD need to be switched on separately after restarting the system?
- How often does the system fail?
- How long does audio failure last?
- Can a loud clicking noise be heard in connection with the fault?
- Is the system OK again after switching terminal 15 off and on?
- What was shown on the main screen and in the assistance window?
- Does the control display fail?
- Is the display white or black or does it flicker in connection with the fault?
- Can a status line still be seen?
- What is shown after resetting the MOST bus (last menu, basic menu/start menu, BMW logo)?
- Does the CD changer make a noise after audio failure?
In addition, various factors that could also cause malfunctions in the system should be clarified
- What control units have already been replaced?
- Did the fault symptom change after replacing control units?
- Was the light intensity reduced for the control units?
- Were the plug connections at the MOST control units checked?
- Was another or new road DVD inserted?
- What road DVD was used?
- Was something retrofitted on the vehicle or a repair carried out?
- To what CIP status (integration stage) does the vehicle correspond?
- Were other-make parts retrofitted in the electrical systems?
Based on the information provided by the customer and after examining any changes in the system it may be possible to determine faults already at this stage.
Why are these statements important? Examples from visual applications
Diagnosis
In addition to the test options: fault code memory check, diagnosis check and component activation, this system can also be used to measure the light attenuation to ensure complete operability of the light section.