Terminal Resistors
Two 120 Ohm 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 10 kOhms and 50 kOhms.