Data Exchange Between ECM and Other Modules W/CAN-Bus Interface

Data Exchange, Motronic / ABS (CANbus), Checking

NOTE:
- Data exchange between individual control modules occurs via a bus system.
- The term "bus" refers to a system that transports and distributes data.
- The wire connections between the control modules, via which data is transferred, are referred to as data wires.
- Data is transferred to the connected control modules serially (one after the other) across the data wires.

Checking bus system
The DTC table suggested checking the data exchange between the Engine / Transmission / ABS control modules.
- Check whether the correct engine, transmission (if installed) and ABS control modules are installed in this vehicle (part numbers) and whether the coding is appropriate for the control modules. (Refer to the indicated ID codes for the control modules). If the correct control modules are installed:
- Check DTC memory of the Transmission Control Module (TCM) (if installed).
- Check DTC memory of ABS control module.

If a malfunction involving " . . . databus" or " . . . communication" is indicated:
- Switch ignition off.
- Connect VAG1598/31 test box at wiring harness to ECM, do not connect ECM. Wiring and Component Testing Using VAG1598/31 Test Box
- Check data wires from Engine Control Module (ECM) pin 60 and pin 58 to Transmission Control Module (TCM) (if installed) and to ABS control module for open circuit and short circuit.
- If wire connections are OK, replace Engine Control Module (ECM), Transmission Control Module (TCM) (if installed) and ABS control module one at a time as a test.

The following systematic procedure is useful for troubleshooting if three or more control modules are communicating via a "two-line bus system":
- Analyze the DTCs stored in the memories of the control modules.

NOTE: This analysis will help you locate the cause of the line malfunction.






Example 1:
From the DTCs stored in the memories, you have determined that control module 1 is not communicating with control modules 2 and 3.

Control module DTCs stored in DTC memories:
1 Missing signal from control module 2
Missing signal from control module 3
2 Missing signal from control module 1
3 Missing signal from control module 1

- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 1 if no malfunctions can be found in the bus wires.






Example 2: From the DTCs stored in the memories, you have determined that control module 2 is not communicating with control modules 1 and 3.

Control module DTCs stored in DTC memories:
1 Missing signal from control module 2
2 Missing signal from control module 1
Missing signal from control module 3
3 Missing signal from control module 2

- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 2 if no malfunctions can be found in the bus wires.

Example 3:
Using the DTCs stored in the memories, you have determined that none of the control modules are sending or receiving signals.

Control DTCs stored in DTC
module memories:
1 Powertrain databus faulty
2 Powertrain databus faulty
3 Powertrain databus faulty

- Switch ignition off.






- Disconnect the control modules connected across the bus wires and check for a short circuit between the bus wires.






- Check the bus wires for short circuit to B+ or Ground (GND).

If you cannot determine a cause for the DTC "Powertrain databus malfunction," check whether the DTC is caused by one of the control modules

All control modules that use the CAN-bus are still disconnected. The ignition is switched off.

- Connect one of the control modules.
- Connect VAG 1551 scan tool. Switch ignition on, and erase the DTC memory of the control module you just connected. End scan tool output using the "End Output" function 06.
- Switch ignition off and then on.
- Leave ignition on for 10 seconds. Then, read the DTC memory of the control module you just connected.
- If the DTC "Powertrain databus malfunction" is displayed, replace the control module you just connected.
- If the DTC "Powertrain databus malfunction" is not displayed, connect the next control module and repeat the procedure.