Data Exchange Motronic / ABS (CAN-Bus) Checking

Data Exchange, Motronic / ABS (CAN-bus), 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 the bus system

The DTC table suggested checking the data exchange between the Engine / Transmission / ABS control modules.

- Check whether correct engine, transmission (if installed) and ABS control modules are installed in this vehicle (part numbers) and whether coding is appropriate for control modules. (Refer to indicated ID codes for control modules).

To determine if the correct control modules are installed:
- Connect VAS 5051 and select vehicle system "01 -Engine electronics" in selection.

When doing this the ignition must be switched on.







Display on VAS 5051:
- Under -1- select diagnostic function "08 - Read measured value block".







Display on VAS 5051:
1 - Enter display group

NOTE: The measured value blocks 125 and 126 indicate CAN drive users.

- Enter "125" for "display group number 125" in field -2- and confirm entry by pressing -Q- button.







Display on VAS 5051:
- Check displays in fields -1- to -4-.

CAN-bus compatible control modules are displayed with the Engine Control Module:
- No display: Engine Control Module not CAN-compatible
- Display 1: CAN-compatible control module is a data bus user
- Display 0: CAN able control module is not user of the Databus

- To switch to display group 126 press (up arrow) button.
- Check in same way with display group 126.
- To switch to display group 127 press (up arrow) button.
- Check in same way with display group 127.
- Exit function "08 - Read measured value block" by pressing <- button.







Display on VAS 5051:
- Under -1- select diagnostic function "06 - End output".







Display on VAS 5051:
- Under -1- select diagnostic function "02 - Check DTC memory - Total system".
- The DTC memory is then checked for all systems in the vehicle which are capable of On Board Diagnostic (OBD).

If a control module answers with its identification, the number of DTCs stored or "No DTCs detected" appears on the display.

Any system DTCs that are stored are displayed successively. The VAS 5051 will then transmit the next address word.

If a DTC is displayed relating to "Data bus drive" or "...CAN bus":
- Check that Engine Control Module (ECM) and other CAN-compatible control modules installed are suitable for this vehicle (part no. and code).

If the correct control modules are installed:
- Check that multiple connectors for control modules are properly seated.

If the multi-pin connectors are firmly seated:
- Check CAN bus system.

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

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

Example 1:







The DTCs stored in the DTC memory indicate that the control module 1 does not communicate with control modules 2 and 3.

Control module DTCs stored in DTC memory
1 - No message from control module 2
- No message from control module 3
2 - No message from control module 1
3 - No message from control module 1

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

Example 2:







From the DTCs stored in the memories, it can be determined that control module 2 is not communicating with control modules 1 and 3.

Control module DTCs stored in DTC memory
1 - No message from control module 2
2 - No message from control module 1
- No message from control module 3
3 - No message from control module 2

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

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

Control module DTCs stored in DTC memory
1 - Powertrain data bus malfunction
2 - Powertrain data bus malfunction
3 - Powertrain data bus malfunction

- Switch ignition off.







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

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

Requirements
- Vehicle diagnosis, measurement and information system VAS 5051 connected and vehicle self-diagnosis selected.

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







Connect VAG 1551.
- Switch ignition on.
- Press appropriate vehicle system in selection -1-.
- Check DTC memory of control module just connected and erase it.
- Press <- button.
- Press diagnostic function "06 - End Output" in selection -1-.
- Switch ignition off and on again.
- Leave ignition switched on for 10 seconds. Then read DTC memory of control module just connected using Scan Tool (ST).
- If DTC "Powertrain CAN-bus malfunction" is displayed, replace control module just connected.
- If DTC "Powertrain CAN-bus malfunction" is not displayed, connect next control module and repeat procedure.