Information Bus: Testing and Inspection

CAN-Bus, checking (from m.y. 2000)






1 - Engine Control Module (ECM)
2 - ABS control module
3 - Transmission control module1)
4 - Airbag control module
5 - Instrument cluster
6 - CAN-Bus

Function
The Engine Control Module (ECM) (item -1-)communicates with the ABS control module (item -2-), the transmission control module (item -3-),the airbag control module (item -4-) and the instrument cluster (item -5-) via a CAN-Bus (item -4-).

1) Only vehicles with automatic transmission.

At least two control modules capable of CAN-Bus are connected together via two CAN-Bus wires which are a twisted pair (CAN_High and CAN_Low) and exchange information (messages). Missing information on the CAN-Bus will be recognized as a malfunction by the ECM as well as by other CAN-Bus control modules.
All control modules have 120 ohm terminal resistors which make sure that the CAN-Bus can function without malfunctioning.

Special tools and equipment
- Multimeter (Fluke 83 or equivalent)
- VW1594 adapter set
- Wiring diagram
- VAG 1598/21 test box

Test conditions
- Malfunction has been recognized by CAN-Bus On Board Diagnostic (OBD)
- ABS control module coded to CAN-Bus operation

Test sequence






- Connect VAG 1598/21 test box -1- to ABS control module wiring harness -2-.
- Check terminal resistor in Engine Control Module (ECM).
- To do this, perform resistance measurement between test box sockets 10 + 11:
Specified value: 115-135 ohms

If the resistance value is outside the specified range:
- Replace Engine Control Module (ECM).

If the resistance value is within the specified range:
- Check CAN-Bus wiring using wiring diagram.