CAN-Bus Checking

Vehicles with automatic gearbox

Function

The Engine Control Module (ECM) communicates with the transmission control module via a CAN data bus.
The CAN-Bus capable control modules are connected via two data bus wires (CAN_High and CAN_Low) which are twisted together (twisted pair) and exchange information (messages). Information missing on the data bus will be recognized as a malfunction by the Engine Control Module (ECM) and the transmission control module.
The CAN-Bus requires a matching resistor of 66 ohms to function correctly. This matching resistor is located in the Engine Control Module (ECM).

Special tools and equipment
- VAG1598/18 test box
- VAG1598/31 test box
- Multimeter (Fluke 83 or equivalent)
- VW1594 adapter set
- Wiring diagram

Test requirement
- A malfunction has been recognized by the CAN-Bus On Board Diagnostic (OBD).

Test sequence
- Remove wiper arms and plenum chamber cover.
- Connect VAG1598/31 test box to Engine Control Module (ECM) wiring harness. Engine Control Module (ECM) is not connected by this action.
- Check centralized matching resistor in Engine Control Module (ECM) by measuring resistance between test box sockets 77 + 79.
Specified value: 60-72 ohms

If the resistance measurement is outside the specified range:
- Replace Engine Control Module (ECM). Service and Repair

If the resistance measurement is within the specified range:
- Disconnect VAG1598/31 test box from Engine Control Module (ECM).







- Connect VAG1598/31 test box to Engine Control Module (ECM) wiring harness. Engine Control Module (ECM) is not connected by this action.





- Disconnect harness connector (arrow) from transmission control module.
- Check CAN-Bus wiring for short to one another.
- Perform resistance measurement again between test box sockets 77 + 79:
Specified value: Infinity ohms

If the specified value is not obtained (wires are shorted together):
- Repair wiring malfunction according to wiring diagram.
- Check DTC memory. If necessary, repair any malfunctions and erase DTC memory.
- Display readiness code.

If DTC memory has been erased or the Engine Control Module (ECM) was disconnected from permanent battery (B+) voltage, the readiness code must be generated again.

If the specified value is obtained (wires are not shorted together):

- Check wiring to CAN-data bus components for short to battery positive or Ground or open circuit using Current flow diagram:
- Check wiring to CAN-Bus for short to battery positive (B+), short to Ground (GND), or open circuit.
- Check DTC memory. If necessary, repair any malfunctions and erase DTC memory.
- Display readiness code. Monitors, Trips, Drive Cycles and Readiness Codes
If DTC memory has been erased or the Engine Control Module (ECM) was disconnected from permanent battery (B+) voltage, the readiness code must be generated again.