Data Exchange Between Control Modules, Checking
Data Exchange Between Control Modules, Checking
NOTE:
- Data exchange between individual control modules occurs via a bus system.
- The term "CAN-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 indicated that data exchange via the bus should be checked:
- Check to be sure the multi-pin connectors of the control modules are securely connected.
- Connect the VAS5051 tester or VAG1551 scan tool. Refer under "Scan Tool Testing/Scan Tool Connecting And Initial Checks/Connecting and Selecting Functions VAS5051 Tester/VAG1551 Scan Tool", Under "Lighting and Horns/Headlamp". Testing and Inspection
- Switch ignition on
- Switch printer on via the PRINT button (indicator lamp in button lights up).
- Press button 1 to select "Rapid data transfer" operating mode 1.
Indicated on display
Rapid data transfer HELP
Insert address word XX
NOTE: The address word 00 initiates the automatic test sequence, i.e. it results in a DTC check on all OBD capable systems in the vehicle via rapid data transfer
When a control module responds with its identification, the display indicates the number of stored DTCs or indicates "no malfunctions recognized."
DTCs stored in the system are indicated sequence and printed out. Then the VAG1551 scan tool sends the next address word.
Automatic test sequence is over when the following indication appears on the display:
V.A.G - ON BOARD DIAGNOSTIC HELP
1 - Rapid data transfer*
2 - Blink code output*
If a malfunction involving "... databus" or "... no communication" is indicated:
- Check whether the correct control module for this vehicle is installed (part number and coding).
If the correct control modules are installed, check the CAN bus system.
Checking a "two-line bus system"
Three or more control modules are communicating across a "two-line bus system."
- Analyze the DTCs stored in the memories of the control modules.
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 lines and check the bus lines for an open circuit.
- If you cannot find a malfunction in the bus lines, replace control module 1.
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 lines and check the bus lines for an open circuit.
- If you cannot find a malfunction in the bus lines, replace control module 2.
Example 3
Using the DTCs stored in the memories, you have determined that none of the control modules are sending or receiving signals.
Control module DTCs stored in DTC memories
1 - e.g. powertrain data-BUS faulty
2 - e.g. powertrain data-BUS faulty
3 - e.g. powertrain data-BUS faulty
- Switch ignition off.
- Disconnect the control modules connected across the bus lines and check for short circuits between the bus lines.
- Check the bus lines for short circuit to B+ or Ground (GND).
If you cannot determine a cause for the malfunction, e.g. "power train data-BUS faulty", check whether one of the control modules may be responsible for the DTC
All control modules that use the CAN-bus are still disconnected. Ignition is switched off.
- Connect one of the control modules.
- Connect VAG1551 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 "power train data-BUS faulty" is displayed, replace the control module you just connected.
- If the DTC "power train data-BUS faulty" is not displayed, connect the next control module and repeat the procedure.