CAN-Bus Checking

CAN-bus, checking (from MY 2000)

Function
The Engine Control Module (ECM) communicates with all control modules capable of data bus via CAN data bus.

The control modules capable of data bus are connected together via two data bus wires twisted together (CAN high and CAN low) and exchange information (messages). Missing information on the data-bus is recognized as a malfunction by the Engine Control Module (ECM) as well as by other CAN-bus control modules.

Further information: Self study program No. 186 "The CAN data bus".

The data bus requires a matching resistor to function fault free. This central matching resistor is located in the engine control module.

Note: You can read out which control units are actively connected via data bus to the engine control module in measured value blocks 125 and 126.

Special tools, test equipment and auxiliary items
- Test box VAG 1598/31
- Hand multimeter VAG 1526 or multimeter VAG 1715
- Adapter set VAG 1594
- Wiring diagram

Test conditions
A malfunction has been recognized by the CAN data bus On Board Diagnostic (OBD)

Test sequence
- Switch off ignition.
- Remove wiper arms and plenum chamber cover.
- Connect test box VAG 1598/31 to the engine control module. The control module wiring harness is not connected by this action.
- Check centralized matching resistor in engine control module.
- Perform a resistance measurement between test box sockets:
Sockets 6 + 7
Specification: 60 to 72 ohms

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

If the resistance value is within the specified range:
- Repair the malfunction in the wiring using wiring diagram: