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Mercedes-Benz MOST Bus Diagnosis

In this article & video, see how to diagnose a fault or break in the MOST bus ring is shown.

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
The fault codes have information that can help isolate the possible component or location of the open in the ring. For example with our subject vehicle, component 1, 4 and 5 are all reporting the open. The first step would be to identify all components and their corresponding positions on the MOST Bus 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 AGW

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.

Press the tab and pull the MOST connector straight out of the navigation module

Then, disassemble the MOST Bus connector. Release the plastic tabs and separate the connector

Press the blue retainer and pull out of housing

Release the plastic tabs to unlock MOST fiber optic cables. Pull straight out while holding tabs.

Install a MOST Bus test loop in place of the module.

Cycle the key OFF wait 30 seconds, then back ON. Turn the Radio ON.

Working back at the CD changer connector, check if the red light is now preset. If it is, the Navigation module is likely faulty.

Confirm power and ground to the navigation module are present. If so, the module should be replaced.

A quick test for hard to access fiber optic cables is to use a laser to shoot light from one end to the other of the cable. This can be used to test the integrity of a cable that runs through the interior of the vehicle. Disconnect the cable at both ends.

Next, shoot the laser into the fiber optic cable.

The laser will be seen as a red light at the other side of that leg in the fiber optic cable. This test can confirm the integrity of the cable from one component to the next.