Failure Of One Of The Control Units Attached To The Can: Notes

Each control unit connected to the CAN has an integrated communication module that makes it possible for that control unit to exchange information on the CAN. Failure of a control unit normally triggers a fault code in the other control units connected to the bus.

There are instances where failure of a module may paralyze or take down the entire CAN bus. This scenario would be evident by CAN faults stored in every control unit on the bus.

In order to isolate the defective control unit, the control units can be disconnected one at a time while monitoring the status of the CAN using a Voltmeter or oscilloscope. This can be further reinforced by clearing the faults of the remaining control units and then reading them again. If the disconnected control module is the defective one, the faults will only point to communication with that interrupted module and no one else. 

As a quick check on vehicles produced after 9/97 (3/98 for the E39 528i) that have the CAN connection to the Instrument cluster, the indicators provide visual indication of whether communication is restored.

If for example the tachometer and temperature display are plausible then communication is occurring between the DME and IKE/KOMBI. Other indicators such as transmission range or the DSC light may give clues to the communication status with those control units.

Once the module has been replaced and coded or programmed, perform the CAN bus Test Module in each control unit to ensure that communication is OK.