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 and workshop equipment required 

• Multimeter VAG 1526 or Multimeter VAG 1715
• Connector test kit VAG 1594
• 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), connecting test box for wiring test. During this, wiring harness to Engine Control Module (ECM) is not connected.

-- Check 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 Ω

If resistance value is not in specified range:

-- Replace Motronic Engine Control Module (ECM) -J220-.

If resistance value is in specified range:

-- Eliminate malfunction in Data-Bus wires or in other Data-Bus components according to wiring diagram: Refer to appropriate SYSTEMS WIRING DIAGRAM

Final Procedures 

After the repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to DIAGNOSTIC MODE 03 - READ DTC MEMORY .
2. If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
3. If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .