Comfort Data Bus System Faults Displayed In Another Control Module

Comfort Data Bus System, Checking

• If the connector is disconnected from a control module connected to the comfort data bus system and the ignition is switched on, a fault entry can occur in the other control modules connected to the comfort data bus system. The same is true if the voltage supply is interrupted by activation of a fuse for example.

• The instrument cluster control module (J285) forms the interface between the two data bus systems. It is also the determining control module in the comfort data bus system. Via adaptation the control modules connected to the comfort data bus system in this vehicle are entered (equipment-dependent). Refer to => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "guided fault finding" function.

• The wiring diagrams can be used to determine how many control modules are connected to the comfort data bus system.

Comfort Data Bus System Faults Displayed in Another Control Module

- Check the data lines and the relevant shielding between the various control modules connected to the comfort data bus system for open circuit and line resistance. Line resistance: max. 1.5 ohms.

- Check the data lines and the relevant shielding between the various control modules connected to the comfort data bus system for short circuit to ground or positive.

• Pay particular attention to possible loose contacts in the wire connections and plug connections.

• If this fault is not displayed in all connected control modules, an open circuit at a connection point in the cabling is the cause.

If no fault is identified even though this fault is displayed for all control modules connected to the comfort data bus system, there is a fault in one of the connected control modules.

- Check the version, coding, and entered adaptation of the instrument cluster control module (J285) , => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "guided fault finding" function

- Check the version and coding of the other control modules connected to the comfort data bus system (e.g. the vehicle electrical system control module (J519), steering column electronic systems control module (J527), etc.). Refer to Vehicle Diagnosis, Testing and Information System VAS 5051 in the "guided fault finding" function

- Disconnect all control modules except for the instrument cluster control module (J285) (interface to the powertrain data bus system) and the steering column electronic systems control module (J527) from the comfort data bus system. Refer to => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "guided fault finding" function and

- Erase the DTC memory of the instrument cluster control module (J285). Refer to => Vehicle Diagnosis, Testing and Information System VAS 5051 in "guided fault finding" function.

- Query the DTC memory of the instrument cluster control module (J285) and the steering column electronic systems control module (J527) => Vehicle Diagnosis, Testing and Information System VAS 5051 in "guided fault finding" function.

If the fault in the comfort data bus system is no longer displayed, the fault is in one of the disconnected control modules.

- Connect the various control modules one-by-one.

- Query the DTC memory after connecting each control module

- Replace the control module for which the fault was displayed after reconnection.

If the fault in the comfort data bus system is displayed again, there is a fault in the instrument cluster control module (J285 ) or in the steering column electronic systems control module (J527).

- Disconnect the steering column electronic systems control module (J527) and connect another control module (e.g. the Climatronic control module (J255) A/C control head).

- Query the DTC memory of the instrument cluster control module (J285) again.

• By disconnecting the steering column electronic systems control module (J527) and connecting a different control module, the defective control module can be identified.

- Replace the defective control module