Exchange Between ECM ABS/Edl Transmission or Inst. Cluster
Data exchange between engine control module, ABS/EDL control module, transmission control module or instrument cluster, checking
NOTE:
- The data exchange between the various control modules is carried out via a data bus.
- The term "CAN data bus" refers to a system for transmitting and distributing data.
- The wires which connect the control modules and convey the data between them are called data wires.
- Data is transmitted via data wires in serial, i.e. in a specific order to the connected control modules (e.g. engine rpm and accelerator pedal position).
Checking bus system
The DTC table indicates that the data exchange between the engine control module, ABS control module and transmission control module or instrument cluster should be tested.
- Make sure that multi-pin connectors for control modules are properly connected
- Connect vehicle diagnostic, testing and information system VAS 5051 or VAG1551 scan tool and select engine electronics control module by entering address word "01". When doing this the engine must be at idle. Refer to "Scan Tool Testing/Scan Tool Connecting and Initial Checks/Connecting VAS5051 or VAG1551" Testing and Inspection
Indicated on display
Rapid data transfer HELP
Select function XX
- Press buttons -0- and -8- to select function "Read Measuring Value Block" and confirm entry with -Q- button.
Indicated on display
Read Measuring Value Block
Input display group number XXX
- Press buttons -1-, -2- and -5- to select "Display Group 125" and confirm entry with -Q- button.
Indicated on display
Read Measuring Value Block 125 ->
1 2 3 4
Display Group 125:
Automatic Test Sequence
- Press button -0- twice for address word "Automatic Test Sequence" and confirm entry with -Q- button. The VAG1551 scan tool transmits all known address words one after the other.
When a control module answers with its identification the number of stored malfunctions appears on the display or "No DTC recognized."
Any system malfunctions that are stored will be displayed one after the other and printed out. The VAG1551 scan tool will then transmit the next address word.
The automatic test sequence has ended when the following appears on the display:
VAG -On Board Diagnostic HELP
1 - Rapid data transfer*
2 - Blink code output*
If malfunctions have been stored relating to the "Data bus drive" or to data transmission: "No message from..."
- Check Part No. and coding to make sure that the vehicle is equipped with the correct engine control module, transmission control module, ABS control module, instrument cluster and AICC control module (if installed).
If the correct control modules are installed, test the CAN data bus system.
Checking two-wire bus system
Three or more control modules communicating via a "two-wire bus system"
- Read out malfunctions stored in the control modules.
This will help to trace a malfunction in the wiring.
Example 1
From the malfunctions present in the DTC memories, you can see that control module 1 has no connection to control modules 2 or 3.
Control module Malfunctions present 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 the control modules which are linked by the bus wires and check whether there is an open circuit in one of the bus wires
- If no malfunction can be identified in the bus wires replace control module 1.
Example 2
From the malfunctions present in the DTC memories, you can see that control module 2 has no connection to control modules 1 or 3.
Control module Malfunctions present 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 the control modules which are linked by the bus wires and check whether there is an open circuit in one of the bus wires
- If no malfunction can be identified in the bus wires replace control module 2.
Example 3
From the malfunctions present in the DTC memories, you can see that none of the control modules are able to transmit or receive signals.
Control module Malfunctions present in DTC memory:
1 Data Bus Powertrain Malfunction
2 Data Bus Powertrain Malfunction
3 Data Bus Powertrain Malfunction
- Switch ignition off.
- Disconnect the control modules which are linked by the bus wires and check for a short circuit between the bus wires
- Check bus wires for short to B+ and short to Ground (GND).
If the cause of the malfunction "Data Bus Powertrain Malfunction" cannot be found in the data bus wires, check whether one of the control modules is causing the fault
At this stage all the control modules which communicate via the CAN data bus are disconnected. The 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 which has just been connected. End output from scan tool with function 06 "End Output."
- Switch ignition off and then on again.
- Leave ignition switched on for 10 seconds. Then check DTC memory of the control module that has just been connected, using the scan tool.
- If the malfunction "Data Bus Powertrain Malfunction" is now indicated, replace the control module which has just been connected.
- If malfunction "Data Bus Powertrain Malfunction" is not indicated, connect next control module, and repeat above procedure.