Component Tests and General Diagnostics

Terminal resistance for CAN-Bus, checking

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

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 CAN-Bus 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).

Recommended special tools and equipment
- VAG1526 multimeter or VAG1715 multimeter
- VAG1594 connector test kit
- Wiring diagram

Test requirements
- A CAN-bus malfunction was recognized via On Board Diagnostic (OBD), Diagnostic mode 3: Check DTC memory. 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. Test Box, Connecting For Wiring Test
- Check the central termination resistor in the Engine Control Module (ECM).
- To do this, perform a resistance measurement between test box sockets 58 + 60 and 77 + 79:
Specified value: 120...144 Ohms

If resistance value is not in expected range:
- Replace Motronic Engine Control Module (ECM) -J220-.

If resistance value is in expected range:
- Eliminate malfunction in wires or in other Data-Bus components according to wiring diagram.