MOST Bus Diagnosis: Notes

WARNING: This page does not describe the selected car, but rather 59 other vehicles, including the 2009 BMW M5, 2009 BMW 650i, 2009 BMW 550i, 2008 BMW M5, and 2008 BMW 760Li. However, it is still accessible from the selected car via links, so may be relevant.

Due to the differences in the configuration of the MOST bus, diagnosis methods will differ slightly between the E65/E66 and the E60, E63, E64. However, there are many similarities and there are some basic rules which apply to all MOST equipped vehicles.

The following diagnosis applies to the E65/E66 configuration and the differences will be pointed out as necessary.

It is important to remember that on the MOST network, messages can only be transmitted provided the bus ring is complete and fully functional. If there is a ring fault in the MOST network, the diagnostic system only communicates with the instrument cluster and the Control Display because both of these modules are directly connected to the KCAN System Bus.

The fiber optic signals on the MOST network always travel in one direction and only in one direction. Signals always originate at the Control Display and travel to the CD changer, AVT, Logic 7 (if equipped), SVS, NAV, Multi-Media Changer (if equipped), Telephone, ASK, Kombi and back to the Control Display.

The MOST bus allows intersystem fault memory entries in the individual control modules. A feature of the system faults is that faults may be entered in a control module although the control module is OK. Conclusions may be drawn about the cause of the fault, using the fault information stored in all the control modules.

The possible system faults are:

Optical wave guide communication fault (All MOST Control Module) FC 111
A Control Module does not switch a light off (All MOST Control Modules)
Network wake-up unsuccessful (Control Display (Gateway), ASK, Telephone Only) FC E18D
Ring fault diagnosis run (Control Display (Gateway) and Kombi Only) FC E190

The Control Display functions are split between acting as a Gateway and Displaying information. The Gateway function serves as the interface between the MOST and the KCAN System buses. Although the Control Display is one control module, two control module names are displayed in the DIS Plus:

CD Control Display (MOST CAN Gateway or MCGW)
CD Control Display

The faults stored in the Control Display are distributed between the Control Display Gateway and Control Display according to the function of the fault.