CAN-Bus, Checking Terminal Resistance

Diagnostic Procedures

CAN-Bus, Checking Terminal Resistance

The Engine Control Module (ECM) communicates with all databus-capable control modules via a CAN databus.

These databus-capable control modules are connected via two data bus wires which are twisted together (CAN_High and CAN_Low), and exchange information (messages). Missing information on the databus is recognized as a malfunction and stored.

Trouble-free operation of the CAN-bus requires that it have a terminal resistance. This central terminal resistor is located in the Engine Control Module (ECM).

Special tools, testers and auxiliary items required

• Multimeter (VAG1526) or multimeter (VAG1715)

• Connector test kit (VAG1594)

• Wiring diagram

Test requirements

• A CAN-bus malfunction was recognized via On Board Diagnostic (OBD) , Diagnostic mode 3: Check DTC memory.

• Ignition switched off.

Test sequence

- Connect test box to Engine Control Module (ECM) , connect test box for wiring test. During this, wiring harness to Engine Control Module (ECM) is not connected.

- Check the central termination resistor in the Engine Control Module (ECM):

- To do this, measure resistance between test box sockets 58 + 60:

Specified value: 120 to 144 ohms

If resistance value is not in specified range:

- Replace Motronic Engine Control Module (ECM) (J220) --> [ Engine Control Module, Replacing ] Engine Control Module, Replacing.

If resistance value is in specified range:

- Eliminate malfunction in Data-Bus wires or in other Data-Bus components according to wiring diagram: