Data Bus, Checking

Data Bus, Checking


Function:
The Engine Control Module (ECM) communicates with the ABS/ESP control module, the instrument cluster, the steering angle sensor, and the transmission control module through a CAN data bus.

The bus able control modules are connected through two data bus wires which are twisted together (CAN_High and CAN_Low), and they exchange information (messages). Missing information on the data bus is recognized as an error by both the Engine Control Module (ECM) and by other CAN data bus control modules.

Trouble-free operation of the CAN data bus requires that it have a termination resistor of 66 Ohms. This central termination resistor is located in the Engine Control Module (ECM).

For more information:
See Self-study program no.186 "CAN-databus"

Test condition

- A DTC has been recognized by the CAN data bus On Board Diagnostic (OBD)

Special tools and equipment

- VAG 1598 test box
- VAG 1598/31 test box
- VAG 1598/33 test box
- VAG 1526 multimeter or VAG 1715 multimeter
- VAG 1594 connector test kit
- Wiring diagram

Vehicles with automatic transmission:
- VAG 1598/18 test box

Test sequence

- Switch ignition off.
- Disengage connector from Engine Control Module (ECM) and then pull it off.







- Connect VAG 1598/31 test box to Engine Control Module (ECM). Control module wiring harness is not connected.
- Test central termination resistor in Engine Control Module (ECM):
- To do this, measure resistance between test box

sockets 58 + 60 and
sockets 77 + 79.

Specification: 60 - 72 Ohms

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

If resistance value is in expected range:
- Remove VAG 1598/31 test box from Engine Control Module (ECM).







- Connect VAG 1598/31 test box to control module wiring harness.
- Check CAN data bus wires for shorts among one another:
- To do this, measure resistance between test box sockets 58 + 60:

Expected value: infinity Ohms

If expected value is obtained (no shorts in wiring):
- Eliminate error in wiring (see wiring diagram):

If expected value is obtained (shorts in wiring):
- Eliminate error in wiring (see wiring diagram).