With Manufacturer's Scan Tool
Exchange of data between connected control modules, checking
NOTE:
- Data is exchanged between individual control modules by means of a bus system.
- "CAN bus" is used to describe a system that transports and distributes data
- The wires between the control modules that are used to transfer the data are known as signal wires.
- Data is transmitted via data wires in sequence, i.e. in a specific order to the connected control modules (e.g. engine rpm and accelerator pedal position).
Testing the bus system
The Diagnostic Trouble Code (DTC) table includes instructions to check the data exchange between engine control module, transmission (gearbox) control module and the ABS control module and/or instrument cluster (dash insert).
- Check that multiple connectors for control modules are properly seated.
- Connect vehicle diagnostic, testing and information system VAS 5051 or V.A.G 1551 Scan Tool and select engine electronics control module 1 with "Address word" 01. For this purpose, engine must be running at idle speed.
Connecting VAS 5051 Scan Tool
Indicated on display
Rapid data transfer HELP
Select function XX
- Press keys 0 and 8 for function "Read measured value block" and confirm entry with Q key.
Indicated on display
Read measured value block 08
Enter display group number XXX
- Press keys 1, 2 and 5 for "display group number 125" and confirm entry with Q key.
Indicated on display
Read measured value block 125 ->
1 2 3 4
Display Group 125:
Automatic test sequence
- Press key 0 twice for address word "Automatic test sequence" and confirm entry with Q key. V.A.G 1551 transmits all known address words in sequence.
If a control module answers with its identification, the number of malfunctions stored or "No malfunction detected" appears on the display.
Any system DTCs stored will be displayed in sequence and printed out. The V.A.G 1551 will then transmit the next address word.
The automatic test sequence has ended when this display appears
V.A.G - ON BOARD DIAGNOSTIC HELP
1 - Rapid data transfer*
2 - Blink code output*
If malfunction relating to "Data bus..." or "...no message" is displayed:
- Check to make sure that vehicle is installed with correct engine control module, transmission control module, ABS control module and instrument cluster (check part numbers and code).
If the correct control modules are installed:
- Check that multiple connectors for control modules are properly seated.
When multiple connectors are tight:
- Check CAN bus system.
Checking a "two-line bus system"
The communication between three or more control modules is performed via a "two-line bus system".
- Evaluate malfunctions stored in control modules.
NOTE: This helps to localize line malfunctions.
Example 1:
The malfunctions stored in the memories indicate that control module 1 does not communicate with control modules 2 and 3.
Control module Malfunctions stored in 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 to each other via bus lines and investigate whether there is a short circuit between bus lines.
If no malfunction can be detected in the bus lines:
- Replace control module 1.
Example 2:
The malfunctions stored in the memories indicate that the control module 2 does not communicate with control modules 1 and 3.
Control module Malfunctions stored in 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 to each other via bus lines and investigate whether there is a short circuit between bus lines.
If no malfunction can be detected in the bus lines:
- Replace control module 2.
Example 3:
With the malfunctions stored in the memory it can be seen that there is no sending or receiving operation possible in any of the control modules.
Control module Malfunctions stored in memory
1 Data bus drive faulty
2 Data bus drive faulty
3 Data bus drive faulty
- Switch ignition off.
- Disconnect control modules that are linked by bus wires and check for a short circuit between bus wires
- Check bus wires for short to positive and short to Ground.
If cause of malfunction "Data bus drive faulty" cannot be found in bus lines check whether one of the control modules is responsible.
- All the control modules that communicate via the CAN data bus are still disconnected.
- The ignition is switched off.
- Connect one control module.
- Connect V.A.G 1551 Scan Tool. Switch ignition on and erase Diagnostic Trouble Code (DTC) memory of control module that has just been connected
- Terminate DTC memory readout by selecting function 06 "End output".
- Switch ignition off and then on again.
- Leave ignition switched on for 10 seconds. Check DTC memory of control module that has just been connected, using Scan Tool.
- If "Data bus drive faulty" is now indicated, replace control module which has just been connected.
- If "Signal wire defect" is not read out, connect next control module and repeat procedure.