Failure Of The CAN Bus Cables

The following faults can occur to the CAN bus wiring:

• CAN-H/L interrupted
• CAN-H/L shorted to battery voltage
• CAN-H/L shorted to ground
• CAN-H shorted to CAN-L
• Defective plug connections (damaged, corroded, or improperly crimped)

In each instance, the connected control units will store a fault due to the lack of information received over the CAN bus.

The voltage of the CAN bus is divided between the two data lines: CAN-High and CAN-Low for an average of 2.5V per line. The voltage measurement is taken from each data line to ground. Each module on the CAN contributes to this voltage.

The fact that 2.5V are present does not mean that the CAN bus is fault free, it just means that the voltage level is sufficient to support communication.

Terminal resistors:  are used in the CAN bus circuit to establish the correct impedance to ensure fault free communication. A 120 Ohm resistor is installed in two  control units of the CAN between CAN-H and CAN-L. Because the CAN is a parallel circuit, the effective resistance of the complete circuit is 60 Ohms. On some vehicles there is a jumper wire that connects the two parallel branches together, others have an internal connection at the instrument cluster.

The resistance is measured by connecting the appropriate adapter to any of the modules on the CAN and measuring the resistance between CAN-L and CAN-H. The resistance should be 60 Ohms. The CAN bus is very stable and can continue to communicate if the resistance on the CAN bus is not completely correct; however, sporadic communication faults will occur.

The terminal resistors are located in the ASC/DSC control unit and either the instrument cluster or in the DME.

Early 750iL vehicles that used the star connector have a separate external resistor which connect CAN-H and CAN-L together.

Modules which do not have the terminal resistor can be checked by disconnecting the module and checking the resistance directly between the pins for CAN-H and CAN-L. The value at these control units should be between 10k Ohms and 50k Ohms.

If there are CAN communication faults that use the term "Timeout"  this refers to a module not being able to communicate with another on the bus. Each module on the CAN bus will attempt communication several times. If unsuccessful, the module will store a "Timeout" or "CAN bus" fault and determine that there is a problem with either the bus line or the module that it is trying to communicate with.

These types of faults may indicate a problem with the bus wiring, interference, missing data or failure of the communication module of an individual control unit.

Checking the CAN lines is carried out just like any other wiring. Perform continuity tests between the connections of different modules (all modules disconnected) without forgetting to make sure that the two CAN lines have not shorted to ground or to each other. It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check. 

If Voltage level and the wire test are O.K, then looking at the communication signal may be useful.

The following are some examples of scope patterns that may be observed when checking the CAN bus.