MOST bus: a control unit is not switching off light

The MOST (Media Oriented Systems Transport) network uses a ring bus for data communication between the various control units. Signal transmission is by means of fibre-optic cable. Data transmission on the ring bus takes place in one direction only. On the MOST ring, messages can only be transmitted provided the bus ring is complete and fully functional. If there is a ring fault in the MOST network, however, the diagnosis system can only communicate with the instrument cluster and the Control Display. This is possible because those two control units are directly connected to the K-CAN SYSTEM bus.

If, for example, the power supply or the diode of a control unit is defective, it is no longer possible to communicate with the control units on the MOST bus. Exceptions here are of course the instrument cluster and the Control Display.

Transmission direction

Transmission of messages is from the Control Display towards the CD changer, aerial tuner, ..., instrument cluster, ... and back to the Control Display.

Control Display

The Control Display is divided into the functions CD GW Control Display (Gateway) and CD Control Display. The gateway is the interface between the MOST and K-CAN SYSTEM buses. For the remaining troubleshooting (read fault code memory, etc.) of the Control Display, only the control unit name CD GW Control Display (Gateway) needs to be selected at the tester!

Diagnosis MOST bus

For the MOST bus, there are inter-system fault code memory entries in the control units. A feature of the system faults is that they can be entered in a control unit although the control unit is OK . With the information regarding system faults in all MOST control units, conclusions can be drawn about the cause of the fault. The section below covers the system fault ”Fibre-optic cable, communication fault”. This fault can be entered in all MOST control units.

Fault code memory entry: a control unit is not switching off light

A control unit is not switching off the light although it is no longer receiving light at its input. Effect: the vehicle does not sleep, leading to slow battery discharge.

If the MOST ring is requested to ”sleep”, the Control Display switches the light off. Subsequently, the MOST control units switch the light off in MOST ring order (CD changer, aerial tuner, etc.). If a control unit does not switch the light off, all of the downstream control units in store the fault ”A control unit is not switching off light” in MOST ring order, up to and including the Control Display (Gateway).

Locating the fault:

  1. Starting in MOST ring order (see circuit diagram) with the control unit CD changer, check whether the fault ”A control unit not switching off light” is stored.
    Note:
    Following the quick test, the fault-pattern selection contains the all stored vehicle faults. This enables you to determine quickly which MOST control unit was first to store the fault ”A control unit not switching off light”.
    The fault lies before the control unit that was first to store the fault (= control unit A) ”A control unit is not switching off light”.
  2. As this fault can also occur sporadically, run the following test as a countercheck:

Example: Audio System Controller does not switch the light off

If the Audio System Controller is not switching off the light, the downstream control units instrument cluster up to and including the Control Display (Gateway) store the fault ”A control unit is not switching off light”. Subsequently, run procedure according to 1. and 2. If the fault ”A control unit is not switching off light” is again stored first in the instrument cluster, then replace the Audio System Controller.