The MOST BUS is the communication BUS for entertainment modules in Mercedes-Benz vehicles. When the MOST BUS ring fails or a module doesn’t relay the message, the other modules can fail to function correctly.
Our subject vehicle had the following fault codes present.
- U1100 MOST master open
- U1102 MOST Component at Position 1 reports open ring
- U1104 MOST Component at Position 4 reports open ring
- U1105 MOST Component at Position 5 reports open ring
Begin by identifying the vehicle you are working on
Select Information and Coms
Select (Audio video, Navigation, Telematics)
The Control modules in each slot will be checked
On our subject vehicle, each component position is identified:
- Audio master AGW (Audio Gateway)
- Component 1 Voice Control System
- Component 2 Navigation Processor
- Component 3 CD Changer
- Component 4 is skipped
- Component 5 Comand or Audio
- Component 6 Sound System
Select Actual Values
Select Actual Configuration of MOST Components
Actual configuration of Most components is the pre-programmed fiber optic ring order set by the factory. Unless any aftersales components have been retrofitted, this list should determine the ring order and prepare you for which modules have communication and what the last node to receive the wakeup command would be. Consider this as a “roll call”.
Once the roll call is complete, identify the components noted as vacant in red text. These components are expected to be present, indicated by the specified value configuration. When highlighted in red, they did not communicate during the roll call. Components listed vacant with black text, are not fitted to the vehicle.
On our subject vehicle, we have one vacant slot with red text and three components with red text. The vacant slot is a component expected to be on bus not not present. The components in red denote out of order but present components. This happens when a component in the roll call doesn’t communicate
The missing component here is the navigation unit. Which caused the order reshuffle of the present components. With this info, we know to begin our testing at the navigation unit.
Note: MOST control units have the ability internally to repeat the roll call even when there is no power to the actual control unit at fault. If the actual fiber optic connector has been compromised such as bends, burns or breaks, as well as being disconnected, this will display in actual values as complete “Vacant” because the roll call never makes it back to AGW to report findings. For this scenario a MOST ring bypass would be needed to find the failure or Ring Break.
Gain access to the CD changer and Navigation unit
Turn the key ON and the radio ON
Locate the MOST Bus connector at the navigation module
Press the tab and pull the MOST connector straight out of the navigation module
Check the MOST Connector for a red light. This indicates the MOST Bus light is getting to the navigation module
To check if the Navigation module is the module not relaying the data, reconnect the connector
Then working at the CD Changer, press the tab and pull the MOST connector straight out of the CD Changer
Check the MOST Connector for a red light. No red light is present on subject vehicle. This means the Navigation modules is not relaying the data.